HYDROCARBONS

Tekst voor de cursus Grondstoffen en het Systeem Aarde (HD 698)H.E.Rondeel, december 2001

Teksten gebaseerd op:Blackbourn, G.A. (1990) Cores and core logging for geologists. Whittles Publ.,Caithness. 113 pp.Shauer Langstaff, C. & D. Morrill (1981) Geologic cross sections. IHRDC, Boston. 108 pp.Stoneley, R. (1995) An introduction to petroleum exploration for non-geologists. Oxford University Press,Oxford. 119 pp.Waples, D. (1981) Organic geochemistry for exploration geologists. Burgess Publ. Co., Mineapolis. 151pp.Waples, D.W. (1985) Geochemistry in petroleum exploration. Reidel Publ. Co, Dordrecht & IHRDC,Boston. 232 pp.

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HYDROCARBONS

CONTENTS

1 - INTRODUCTION............................................................................................................................. 5

FORMATI0N OF 0IL AND GAS......................................................................................................... 5

2 - ORGANIC FACIES.......................................................................................................................... 6

THE CARBON CYCLE ....................................................................................................................... 6FACTORS INFLUENCING ORGANIC RICHNESS............................................................................ 7

PRODUCTIVITY .............................................................................................................................. 7PRESERVATION.............................................................................................................................. 8DILUTION ..................................................................................................................................... 11

SUMMARY ....................................................................................................................................... 12

3 - ORGANIC CHEMISTRY .............................................................................................................. 13

INTRODUCTION.............................................................................................................................. 13NAMES AND STRUCTURES........................................................................................................... 13

HYDROCARBONS ......................................................................................................................... 13NONHYDROCARBONS ................................................................................................................. 15

4 - KEROGEN...................................................................................................................................... 17

INTRODUCTION.............................................................................................................................. 17KEROGEN FORMATION................................................................................................................. 17KEROGEN COMPOSITION ............................................................................................................. 18KEROGEN MATURATION.............................................................................................................. 20

INTRODUCTION ........................................................................................................................... 20EFFECTS OF MATURATION ON KEROGENS ............................................................................. 21HYDROCARBON GENERATION................................................................................................... 22

SUMMARY ....................................................................................................................................... 23

5 - BITUMEN, PETROLEUM, AND NATURAL GAS...................................................................... 24

INTRODUCTION.............................................................................................................................. 24COMPOUNDS PRESENT IN BITUMEN AND PETROLEUM ......................................................... 24

GENERAL CLASSES OF COMPOUNDS ....................................................................................... 24SPECIFIC COMPOUNDS.............................................................................................................. 25

FACTORS AFFECTING COMPOSITION OF BITUMEN AND PETROLEUM................................ 25SOURCE AND DIAGENESIS ......................................................................................................... 25RESERVOIR TRANSFORMATIONS............................................................................................... 26COMPARISON OF BITUMEN AND PETROLEUM ....................................................................... 27NATURAL GAS .............................................................................................................................. 28

SUMMARY ....................................................................................................................................... 28

6 - MIGRATION.................................................................................................................................. 29

DEFINITIONS................................................................................................................................... 29PRIMARY MIGRATION................................................................................................................... 29

MECHANISMS............................................................................................................................... 29DISTANCE AND DIRECTION ....................................................................................................... 30

SECONDARY MIGRATION............................................................................................................. 31MECHANISM................................................................................................................................. 31

Contents

DISTANCE AND DIRECTION ....................................................................................................... 31ACCUMULATION............................................................................................................................ 32

INTRODUCTION ........................................................................................................................... 32CLASSICAL TRAPS........................................................................................................................ 33KINETIC TRAPS ............................................................................................................................ 33TAR-MAT TRAPS ........................................................................................................................... 34GAS HYDRATES ............................................................................................................................ 34

EFFECTS ON OIL AND GAS COMPOSITION ................................................................................ 34SIGNIFICANCE FOR EXPLORATION ............................................................................................ 35

7 - PETROLEUM TRAPS ................................................................................................................... 36

THE REPRESENTATION OF TRAPS .............................................................................................. 36STRUCTURAL TRAPS ..................................................................................................................... 37STRATIGRAPHIC TRAPS................................................................................................................ 41COMBINATION TRAPS................................................................................................................... 42HYDRODYNAMIC TRAPS .............................................................................................................. 43THE RELATIVE IMPORTANCE OF TRAPS ................................................................................... 43EXERCISES ...................................................................................................................................... 45

8 - SOURCE-ROCK EVALUATION.................................................................................................. 49

DEFINITION OF SOURCE ROCK.................................................................................................... 49PRINCIPLES OF SOURCE-ROCK EVALUATION .......................................................................... 49

QUANTITY OF ORGANIC MATERIAL .......................................................................................... 49MATURITY OF ORGANIC MATERIAL.......................................................................................... 49CONTAMINATION AND WEATHERING....................................................................................... 52ESTIMATION OF ORIGINAL SOURCE CAPACITY ...................................................................... 52

INTERPRETATION OF SOURCE-ROCK DATA ............................................................................. 53QUANTITY OF ORGANIC MATERIAL .......................................................................................... 53TYPE OF ORGANIC MATTER....................................................................................................... 53MATURITY..................................................................................................................................... 54COALS AS SOURCE ROCKS ......................................................................................................... 54

SUMMARY ....................................................................................................................................... 55EXERCISES ...................................................................................................................................... 56

9 - PREDICTING THERMAL MATURITY ...................................................................................... 60

INTRODUCTION.............................................................................................................................. 60CONSTRUCTION OF THE GEOLOGICAL MODEL ....................................................................... 60

BURIAL-HISTORY CURVES.......................................................................................................... 61TEMPERATURE HISTORY............................................................................................................ 61SPECIAL CONSIDERATIONS ABOUT BURIAL-HISTORY CURVES ............................................ 62

CALCULATION OF MATURITY..................................................................................................... 63FACTORS AFFECTING THERMAL MATURITY............................................................................ 64POTENTIAL PROBLEMS WITH MATURITY CALCULATIONS..................................................... 65

EXERCISES ...................................................................................................................................... 66

10 - QUANTITATIVE ASSESSMENT ............................................................................................... 69

OIL IN PLACE .................................................................................................................................. 69RESERVES........................................................................................................................................ 69

DISCOVERED RESERVES............................................................................................................. 70UNDISCOVERED RESERVES ....................................................................................................... 72ULTIMATE RESERVES.................................................................................................................. 73

Organic Facies - 5

1 - Introduction

FORMATI0N OF 0IL AND GASProponents of the organic origin of oil and gas have given us a general picture of how organic matterderived from dead plants is converted to hydrocarbons. Although the transformation process is verycomplex, with many details still poorly understood, it is known that organic debris derived fromplants and algae is best preserved in fine-grained sediments deposited in the absence of oxygen.Low-temperature chemical and biological reactions (called diagenesis) that occur during transportto and early burial in the depositional environment modify this organic matter. Many of the chemicalcompounds present in sediments are in fact derived from bacteria, and were formed as dead organicmatter was converted to microbial tissues.Most of this organic matter is transformed during diagenesis info very large molecules, the largest ofwhich are called kerogen. These play a key role as the precursors for oil and much natural gas.The earliest stage of hydrocarbon generation occurs during diagenesis. Certain microorganisms,called methanogens, convert some of the organic debris to biogenic methane. Formation of biogenicmethane has been recognized for a long time, but only within the last few years have we realized thatin many areas a large portion of the natura!-gas reserves are biogenic.As burial depth increases, porosity and permeability decrease, and temperature increases. Thesechanges lead to a gradual cessation of microbial activity, and thus eventually bring organicdiagenesis to a halt. As temperature rises, however, thermal reactions become increasinglyimportant. During this second transformation phase, called catagenesis, kerogen begins todecompose into smaller, more mobile molecules. In the early stages of catagenesis most of themolecules produced from kerogen are still relatively large; these are the precursors for petroleum,and are called bitumen . In the late stages of catagenesis and in the final transformation stage, calledmetagenesis, the principal products consist of smaller gas molecules.In recent years this relatively simple picture of hydrocarbon generation has been complicated slightlyby our growing awareness that kerogens formed from different kinds of organic matter, or underdifferent diagenetic conditions, are chemically distinct from each other. These differences can have asignificant effect on hydrocarbon generation.Once formed, oil and gas molecules can be expelled from the source rock into more permeablecarrier beds or conduits. Migration through these conduits often leads to traps, where hydrocarbonmovement ceases and accumulation occurs.

Organic Facies - 6

2 - Organic Facies

THE CARBON CYCLEBecause oil and gas are generated from organic matter in sedimentary rocks, we need tounderstand how this organic matter came to be preserved in the rocks. Preservation of organicmaterial is actually a rare event. Most organic carbon is returned to the atmosphere through thecarbon cycle; less than 1% of the annual photosynthetic production escapes from the carboncycle and is preserved in sediments. Oxidative decay of dead organic matter is a highly efficientprocess mediated largely by microorganisms.Preservation of organic matter begins with photosynthesis. Some of the organic material insediments consists of fragments of plants or algae that derived their energy from the sun. A largefraction, however, comprises microbial tissue formed within the sediments by the bacterialtransformation of plant and algal debris. Zooplankton and higher animals contribute relativelylittle organic matter to sediments. The recently discovered deep-sea ecosystems in the PacificOcean that derive their energy from oxidation of sulfides in hydrothermal vents are interesting

but volumetrically unimportant.Despite the great imbalance in biomass between terrestrial plants (450 billion metric tons [t]) andaquatic phytoplankton (5 billion t), the yearly productivity of both groups is about equal, as aconsequence of the much more rapid reproduction of simple aquatic organisms. Because of

Organic Facies - 7

extensive oxidation of land-plant debris in soils, however, much of the terrestrial organic materialis already highly oxidized when it arrives in the sediments.Although some destruction of organic material occurs during transport to the depositionalenvironment, a great deal of the oxidation of organic matter occurs within the sedimentsthemselves. Total Organic Carbon (TOC) values decrease monotonically through the first 300meters of burial before levelling out at about 0.1%, suggesting that either depth or organic-carbon content eventually limits diagenesis. Depth could interfere with microbial diagenesis whencompaction reduces pore sizes and nutrient fluxes in interstitial waters. On the other hand, thelow TOC values could indicate that the remaining organic matter has no more nutritional value,and that the microbes have given up trying to digest it. Each factor may be dominant underdifferent conditions.Although oxidative decay destroys most of the yearly production, over vast amounts of geologictime the small fraction that escaped the carbon cycle has built up extremely large quantities oforganic matter (20,000,000 billion t) dispersed in fine-grained sedimentary rocks. Only a smallfraction of this (10,000 billion t, or about 0.05%) occurs in economic deposits of fossil fuels.When we consider inefficiencies in discovery and recovery, only one molecule out of about everyone million successfully negotiates the journey from living organism to the gasoline pump.

FACTORS INFLUENCING ORGANIC RICHNESSIn order for organic-rich rocks to be formed, significant amounts of organic matter must bedeposited and protected from diagenetic destruction. The three primary factors influencing theamount of organic matter in a sedimentary rock are productivity, preservation, and dilution.Productivity is the logical place to begin our analysis, because without adequate productivity,accumulation of organic-rich sediments cannot occur.

PRODUCTIVITYA partial listing of the many factors influencing productivity would include nutrient availability,light intensity, temperature, carbonate supply, predators, and general water chemistry. Each ofthese categories could in turn be further subdivided. For example, nutrient availability woulddepend on such factors as water circulation patterns, orogeny and erosion, volcanism,paleoclimate, and recycling by organic decay.Nutrient availability is, in fact, one of the critical parameters governing productivity. Shallow-marine environments, where there is local recycling of nutrients from decaying organisms andinflux of fresh nutrients from terrestrial sources, are therefore much more productive than theopen ocean.In relatively unrestricted marine environments, watercirculation patterns are particularlyimportant for supplying nutrients and thus controlling productivity. Bodies of water naturallydevelop density stratification, with a preference for horizontal water movement within eachdensity layer. Nutrients dissolved in waters below the photic zone therefore go unutilized,because under normal circumstances they cannot move upward into the zone of photosynthesis.Only where there is upwelling of subsurface waters can these nutrients return to the photic zone.Upwelling occurs where bulk movement of surface water away from a particular area allowsdeeper water to ascend to replace it. If this deeper water is enriched in nutrients, highphotosynthetic productivity will occur at the site of upwelling. In the modern world there arezones of intense seasonal upwelling off the west coasts of California, Peru, Namibia, andNorthwest Africa that result from the movement of surface waters away from these coasts. Thereis another zone of seasonal upwelling off the Horn of Africa in the Indian Ocean as a result of

Organic Facies - 8

monsoonal winds that drive surface waters away from the coast. All these areas exhibit highproductivity when upwelling occurs.Theoretical models have been developed to predict upwelling (and consequent productivity) inancient seas from input data on continental configurations, landmasses, wind and watercirculation patterns, and paleoclimates.Such models are interesting, and may in fact prove useful in future exploration efforts. There are,however, some problems associated with their application. First, productivity is probably not asimportant a factor as preservation. There are many more organic-rich facies resulting fromexcellent preservation than from extremely high productivity. After all, if on the average only 1%of organic matter is preserved, increasing preservation rates is a very efficient way to increaseorganic richness. Secondly, the accuracy with which we can reconstruct continental positions,paleoclimatic conditions, and all the other factors that influence upwelling loci is severelylimited, especially in the Palaeozoic.

PRESERVATIONThe principal control on organic richness is the efficiency of preservation of organic matter insedimentary environments. Three factors affect the preservation (or destruction) of organicmatter: the concentration and nature of oxidizing agents, the type of organic matter deposited,and the sediment-accumulation rate. Of these, oxidizing agents are probably the most crucialfactor.

ANOXIA. Because most of the oxidation occurring in the water column, soils, and sediments isbiological, and because most biological oxidation processes require molecular oxygen, thesimplest way to limit oxidation is to limit the supply of oxygen. All large organisms requireoxygen in order to live, although some species can tolerate extremely low oxygen levels (0.5milliliters (mL) per liter (L)). At lower levels of dissolved oxygen, many species disappear; theremaining individuals often become dwarfed in an effort to survive in a hostile environment. Atdissolved oxygen levels below about 0.2 mL/L, essentially the only viable organisms are thosethat we call anaerobes, microorganisms that utilize materials like sulfate or nitrate ions insteadof molecular oxygen as electron acceptors in their metabolic processes.We call the zone in which oxygen contents are high the oxic zone; the zone where oxygen fallsbelow 0.2 mL/L is called the anoxic zone. Processes that occur in these two zones are calledaerobic and anaerobic, respectively. The term dysaerobic has been used to describe processesoccurring in the transitional zone (0.2-0.5 mL/L), and we could coin the term dysoxic to describethe zone itself. The term "anoxic" literally means "having no oxygen," hut because of the radicalchange in biota that occurs at about 0.2 mL/L, its use in practice has been expanded to includevery low oxygen levels as well.Anoxia is of tremendous importance in the preservation of organic matter in sediments, becausewhen the availability of oxygen is limited, diagenesis is restricted to anaerobic processes. Theseanaerobic processes are inefficient compared with aerobic diagenesis, and are usually limited inscope by the availability of sulfate or nitrate. Thus if anoxia can develop, preservation of organicmatter will be much enhanced.Anoxic sediments are not always easy to recognize, because some of the commonly usedindicators of anoxia may be misleading. Anoxic sediments always contain elevated TOC values(generally above 2% and always above 1% ). However, much oxic sediment also contains largeamounts of organic matter, especially of woody origin. TOC values alone must therefore be usedwith caution. The presence of undegraded marine organic material is a strong indication ofanoxia, because marine organic matter is consumed preferentially by organisms. Its presence in

Organic Facies - 9

rocks therefore indicates that diagenesis was stopped prematurely, most likely by absence ofoxygen.Color is not a reliable indicator. All anoxic sediments will be very dark gray or black whendeposited. Many black rocks, however, are not rich in organic carbon; they often owe their darkcolor to finely divided pyrite or to particular chert phases. Color should be used mainly as anegative criterion: If a rock is not very, very dark, it cannot represent an anoxic facies.The presence of pyrite itself can also be deceptive. Although pyrite does indeed form underanoxic conditions, and its presence indicates that the anaerobic reduction of sulfate ion did occur,there is no guarantee that anoxia was present at the sea floor; it may well have developed afterburial. Furthermore, anoxia can be very local; intense pyritization of benthic bivalves istestimony to the fact that pyrite is not a good indicator of bottom-water anoxia at the time ofdeposition.Finally, anoxic sediments show preserved depositional laminae on a millimeter or submillimeterscale. The laminae prove that burrowing fauna were absent, and therefore that dissolved-oxygenlevels were below 0.2 mL/L. Conversely, the presence of bioturbation indicates that the bottomwaters were not anoxic, although stunted burrows can be used as evidence of dysoxia.The ultimate implications of anoxia for petroleum exploration are great; it has been estimated, infact, that most of the world's oil was generated from source beds deposited under anoxicconditions. It therefore behoves us to understand the conditions under which anoxia develops.

STAGNANT BASINS. Truly stagnant basins are actually quite rare; slow circulation orturnover of the water column occurs almost everywhere. Nevertheless, it is instructive toconsider complete stagnation, particularly in understanding lacustrine beds. If an isolated body ofwater is deep enough, and if the climate is subtropical or tropical, then permanent densitystratification will arise as a result of temperature differences within the water column. Depths inexcess of 200 m are required to prevent mixing during storms, and warm climates are necessaryto avoid overturn caused by freeze-thaw cycles. The cooler, denser waters remain at the bottom,leading to the eventual development of a pycnocline (density interface) which preventsinterchange between the two layers. Lack of communication between the layers prohibitsreplenishment of oxygen in the bottom layer. Therefore, once the original oxygen has beenconsumed in oxidizing organic matter, no more oxygen can enter, and both the waters in thebottom layer and the underlying sediments will become anoxic.Marine basins are seldom isolated enough to fit well into the stagnant-basin model, but limnicenvironments often are. Among the ancient lake beds thought to have been deposited inpermanently stratified waters are the well-known Green River Shale (middle Eocene, Wyoming),the Elko Formation (Eocene/Oligocene, Nevada), and strata from several basins in China. Lakedeposits associated with continental rifting, especially during the Triassic along the margins ofthe developing Atlantic Ocean, are anoxic in some of the places where they have been penetrated.Lakes in failed rifts can also contain organic-rich, anoxic sediments. Lakes of the Rift Valley ofEast Africa are excellent modern analogs receiving much attention from both researchers andexplorationists at the present time.

OXYGEN-MINIMUM LAYER (OML). The oxygen-minimum layer is a layer of subsurfacewater that has a lower dissolved-oxygen content than the water layers either above or below.This oxygen minimum develops when the rate of consumption of oxygen within that layerexceeds the rate of influx of oxygen to it. Consumption of oxygen results from decay of deadorganisms that have sunk from the photic zone above. The oxygen minimum layer usually beginsimmediately below the photic zone, where photosynthesis and turbulence can no longercontribute oxygen to the water. The supply of fresh oxygen is therefore limited to horizontal

Organic Facies - 10

movement of oxygen-bearing waters. However, because these horizontally moving waters also liewithin the oxygen minimum layer, the oxygen they can contribute is limited. Below the OMLoxygen levels again increase, as a result of diminished oxygen demand, since most organic matterwas destroyed within the overlying OML.Although an oxygen-minimum layer exists virtually everywhere in the ocean, its intensity variesgreatly. Intensely developed OMLs occur in areas of high productivity and, to a lesser extent, inareas of poor circulation. Wherever an intensely developed OML intersects the sediment-waterinterface, sediments will be deposited under low-oxygen conditions. Any organic matter arrivingin those sediments will have an excellent chance to escape oxidation.Bottomset beds associated with prograding delta systems can be rich in organic matter if they arelaid down within a well-developed oxygen-minimum layer. In contrast, foreset beds within thesame system are leaner in organic matter because they are deposited above the OML.There are other ancient and modern examples of organic-rich rocks deposited under anoxic ornear-anoxic conditions associated with OMLs. These include the modern Peru-Chile shelf (highproductivity associated with upwelling) and occurrences of black sediments of Aptian toTuronian age in the North Atlantic.It has been proposed that at certain times in the past (e.g., mid-Cretaceous, Late jurassic, LateDevonian) the world oceans were severely depleted in dissolved oxygen. This depletion wasprobably the result of the complex interplay of several factors, including paleoclimate and watercirculation. During those times the OML expanded both upward and downward because of poorsupply of oxygen to subsurface waters. In times like the mid-Cretaceous, when a majortransgression had greatly increased the continental shelf area, an upward expansion of the OMLled to a tremendous increase in the surface area covered by anoxic bottom waters. It is notcoincidental that these were times of deposition of large amounts of organic-rich rocks in manyparts of the world.

RESTRICTED CIRCULATION. Settings in which circulation is restricted are much morecommon than stagnant basins. Furthermore, because of their connection with the open-marinerealm, those environments can also incorporate the features of an oxygen-minimum-layer model.Shallow Silling. Circulation is often restricted by the presence of a sill, the point of connectionbetween the restricted area and the open-marine environment. Where the sill is shallow, thewaters entering or leaving the basin are near surface. In an evaporitic environment (Karabogaz inthe Caspian Sea) there is a net flow of water into the basin, whereas in a fluvially dominatedsystem (Black Sea) the net flow of surface water is out over the sill. In either case, if the basin isdeep enough, permanent density stratification will develop, with the bottom layer almost isolatedfrom the open-marine waters. In actuality there is a lazy turnover of the bottom waters, but it istoo slow to disturb the anoxia which develops in the bottom layer.Shallowly silled basins often yield evaporites, which could be excellent hydrocarbon sourcerocks. Evaporitic environments combine the opportunity for abundant growth of algae with idealconditions for preservation. Nutrients are concentrated by evaporation, and grazers andpredatory organism are eliminated by the high salinities. High productivity reduces oxygenlevels, and high hydrogen-sulfide concentrations create conditions poisonous to predators. Theresult is often deposition of organic-rich laminae within evaporites, or as lateral faciesequivalente thereof.Coal Swamps. Large amounts of organic material are preserved in coal swamps as a result ofthe combined effects of poor water circulation, high influxes of organic matter, and diminishedbacterial activity. Coal swamps can develop under a variety of conditions in both marine andnon-marine environments. Although circulation in coal swamps is generally sluggish, theshallowness of the swamps prevents the waters themselves from becoming anoxic. Anoxia

Organic Facies - 11

develops within the sediments rather than in the water column. Phenolic bactericides derivedfrom lignin hinder bacterial decay in the water and throughout the sediment column. Lack ofsulfate in non-marine swamps further prevents anaerobic microbial destruction of the organicmatter.Coals are important source rocks for gas accumulations, but their supposedly low potential forgenerating oil is to be reconsidered.Oxic Settings. Most depositional settings not specifically catalogued above will be more or lesswell oxygenated, and therefore wi11 contain primarily oxidized organic matter. Near-shoreoxidizing facies sometimes have high TOC values, but the organic material is almost invariablywoody. Abyssal sediments are notoriously low in organic carbon as the result of the combinedeffects of high oxygen levels in abyssal waters, very slow sedimentation rates, and lowproductivity in the overlying pelagic realm. The hydrocarbon-source potential of all of theseoxidizing facies is low, and more favorable for gas than for oil.

TYPE OF ORGANIC MATTER. Organic matter of algal (phytoplanktonic) origin isconsumed more readily by organisms than are other types of organic material, because itschemical components are digestible and provide precisely the nutrients required by scavengersand predators. Nitrogen and phosphorus are in particular demand; their virtual absence in muchterrestrial organic material, especially in structural (woody) material, renders it of littlenutritional value. Furthermore, the phenolic components present in lignin-derived terrestrialmaterial are toxic to many micro-organism, thus preventing extensive diagenesis of suchmaterial.Any extensive organic diagenesis is therefore likely to eliminate algal organic matter first. Thatmaterial which remains is dominantly of terrestrial origin, and may include woody, cellulosic,lignitic, cuticular, or resinous material, all of which are chemically quite distinct from each other.It may also contain very resistent organic debris derived from erosion of ancient rocks, forestfires, and other oxidative processes.

RAPID SEDIMENTATION AND BURIAL. Rapid sedimentation and burial con also enhancepreservation. TOC values increase as sediment-accumulation rates increase, as a result of morerapid removal of organic material from the zone of microbial diagenesis.Rapid burial is accomplished by a high influx of inorganic detritus, biogenic inorganic sediment, ororganic material. Rapid deposition of inorganic detritus is common in turbidites and in prodeltashales. The extremely high accumulation rates for biogenic carbonates and siliceous sediments inzones of high productivity promote preservation of the associated algal protoplasm. Coals alsoaccumulate very rapidly and, with their high concentrations of organic matter, provide an idealmeans of maintaining low-oxygen conditions.Rapid settling of organic debris through the water column is also important, because extensivedecomposition occurs during its fall to the ocean floor. In fact, much of the organic material thatdoes reach the bottom in deep waters arrives in relatively large fecal pellets, which settle severalorders of magnitude faster than individual phytoplankton.

DILUTIONAlthough high sediment-accumulation rates enhance preservation of organic matter, at very highaccumulation rate dilution may become a more important factor than increased preservation.Dilution does not reduce the total amount of organic matter preserved, but it does spread thatorganic material through a larger volume of rock. The net result is a reduction in TOC values.

Organic Facies - 12

Dilution effects depend upon rock lithology. Biogenic sediments, in which the organic andinorganic materials arrive together, are not as strongly affected by dilution. Shales, in contrast,show strong dilution effects when accumulation rates are very high. Facies changes fromcarbonates to shales may create large dilution effects that can be wrongly interpreted asindicating changes in oxygen levels.

SUMMARYThere are three principal factors that affect the amount of organic matter in sedimentary rocks:primary photosynthetic productivity, effectiveness of preservation, and dilution by inorganicmaterial. Of these, preservation is generally the most important.Productivity can be predicted by locating ancient sites of marine upwellings. Our ability to makeaccurate predictions is limited, however, by uncertainties about exact continental positions andconfigurations in the past, lack of knowledge of seawater chemistry and nutrient availability atthose times, and a very imperfect understanding of oceanic- and atmospheric-circulation patterns.Consequently, such models are not yet of much practical value for the distant past.Preservation is best accomplished where oxygen is excluded from bottom waters. There are anumber of mechanisms by which oxygen depletion may be fostered and maintained, includingstagnancy or near-stagnancy, a strongly developed oxygen-minimum layer, and rapid burial. It isoften very difficult to separate the influences of these various factors in a particular depositionalenvironment.Rapid accumulation of sediment shortens the residence time of organic matter in the zone ofdiagenesis and thus promotes preservation. If the rapidly accumulating sediment is mainlyclastic, however, dilution effects may lead to lower TOC values in spite of enhanced preservationrates. In biogenic sediments or coals, in contrast, where sediment-accumulation rates are directlyproportional to organic-carbon-accumulation rates, dilution is far less marked.Because of its role in creating rocks with excellent hydrocarbon-source potential, anoxia inbottom waters is a phenomenon whose effects we should learn to recognize in ancient rocks.Some of the commonly applied criteria are apt to be misleading, however. It is important to beable to distinguish local anoxia or anoxia developed deep within sediments from anoxia inducedby anoxic bottom waters. The most reliable criteria for bottom-water anoxia are the preservationof fine depositional laminae, and the presence of high TOC values coupled with the occurrence ofundegraded marine organic matter.Anoxic events in the past were probably not as large in scale or as long lasting as some workershave suggested. Although certain periods undeniably contain more than their share of anoxicrocks, anoxic sediments were deposited discontinuously through time and space. Direct control ofthe anoxia was thus probably local, as a result of high productivity or sluggish circulation. As inthe modern oceans, such events were often interrupted for long periods before anoxia wasreinduced.Models that integrate the concepts of organic richness with depositional cycles and faciesanalysis will be valuable tools for understanding hydrocarbon systems in basins. To derivemaximum value from our analyses, we should always strive to place the organic rich rocks in thelarger context of basin evolution through time and space.

Organic Chemistry - 13

3 - Organic Chemistry

INTRODUCTIONAnyone who uses petroleum geochemistry must be familiar with basic chemical terminology. Theobjective of this chapter is to acquaint the reader with the names of common compounds and withseveral different conventions for drawing their structures. This objective is very different trom thatof a normal course in organic chemistry, in which one must also learn all the reactions of manyclasses of compounds. The chemical reactions of interest to us are very few and are discussed onlybriefly. All compounds containing carbon atoms, except carbon dioxide, carbonates, and metalcarbides, are termed organic. This usage is historical and does not imply that all such compoundsare necessarily derived from living organisms. Organic chemistry is thus the study of carbon-containing compounds, and organic geochemistry the study of organic compounds present ingeological environments.

NAMES AND STRUCTURES

HYDROCARBONSIn chemical terms a hydrocarbon is a compound containing only the elements carbon and hydrogen.Petroleum and natural gas are themselves often referred to as "hydrocarbons," but that usage isincorrect trom the chemist's point of view because those materials often contain substantial amountsof nitrogen, sulfur, oxygen, trace metals, and other elements. In this chapter we restrict the usage ofthe term hydrocarbon to the standard chemical one; elsewhere in this text usage will vary, as it doesin the real world.

Examples of hydrocarbons are methane, ethane, and cyclohexane, whose structures are shownbelow.

In each of these compounds, and indeed in every carbon compound (except a few highly unstableones created only in laboratories), every carbon atom forms four bonds. Similarly, hydrogen alwaysforms one bond; oxygen and sulfer, two bonds; and nitrogen, three bonds. Carbon atoms like toform bonds with each other, creating long chains and ring structures. This unique property ofcarbon is responsible for the existence of literally millions of different organic compounds.Writing the detailed structure of a simple molecule like methane is no problem, especially if one hasto do it only occasionally. If one wants to draw large molecules, however, the explicit inclusion ofevery atom and every bond becomes extremely tedious. Several different types of shorthand havetherefore developed to facilitate drawing organic molecules.One common convention is to represent all the hydrogen atoms attached to a given carbon atom bya single H, using a subscript on the H to denote the total number of hydrogens around that atom.The structures of methane and ethane are thus represented by CH4 and CH3CH3 respectively.We can make other logical simplifications for longer carbon chains. The following representationsof n-pentane are equivalent: CH3CH2CH2CH2CH3 or CH3(CH2)3CH3.

Organic Chemistry - 14

An even quicker shorthand that uses no letters at all has evolved. Each carbon atom is representedby a point, and carbon-carbon bonds are shown as lines connecting those points. Hydrogen atomsand bonds to hydrogen atoms are not shown at all. Because we know that each carbon atom formsfour bonds and each hydrogen atom forms one bond, simple inspection shows how mant' hydrogenatoms each carbon atom must have. For example, n-pentane and cyclohexane are represented by theline structures shown below.

The zigzag configuration illustrated for n-pentane isadopted to show clearly each carbon atom.The simplest series of hydrocarbons has linear structures;these molecules are called n-alkanes or nparains. Theletter n stands for normal, and indicates that there is nobranching in the carbon chain. We have ahreadyencountered n-pentane; the names of the other ninesimplest n-alkanes are given in the following table. Note

that the name of each compound ends in -ane, as in "alkane." The first four names are irregular, butthe prefixes denoting the number of carbon atoms in the other alkanes are derived from Greeknumbers.

Names and formulas of the ten smallest n-alkanesMethane CH4 CH4Ethane C2H6 CH3CH3Propane C3H8 CH3CH2CH3Butane C4H10 CH3 (CH2)2 CH3Pentane C5H12 CH3 (CH2)3 CH3Hexane C6H14 CH3 (CH2)4 CH3Heptane C7H16 CH3 (CH2)5 CH3Octane C8H18 CH3 (CH2)6 CH3Nonane C9H20 CH3 (CH2)7 CH3Decane C10H22 CH3 (CH2)8 CH3

Carbon atoms need not always bond together in a linear arrangement. Branching can occur, givingrise to a vast number of possible structures.The term methyl, which we used earlier, is the adjectival form of the word methane. In the case of 2-methylhexane (C7H16) the basic structure is hexane; a CH3 (methyl) group is attached to the secondcarbon atom. Other adjectival forms are made by dropping the -ane ending and adding yl (forexample, ethyl and propyl).Among the most important branched hydrocarbons in organic geochemistry are the isoprenoids.Regular isoprenoids consist of a straight chain of carbon atoms with a methyl branch on everyfourth carbon. Isoprenoids ranging in length from six to forty carbon atoms have been found inpetroleum and rocks.

We have also seen that carbon atoms can be arranged in rings. These cyclic compounds (callednaphthenes) are named by counting the number of carbon atoms in the ring and attaching the prefixcyclo.All the compounds mentioned above are called saturated hydrocarbons or saturates, because theyare saturated with respect to hydrogen. That is, no more hydrogen can be incorporated into themolecule without breaking it apart.Another important group of hydrocarbons is the unsaturates, which, in contrast, are able tocombine with additional hydrogen. Many unsaturated compounds have carbon-carbon double

Organic Chemistry - 15

bonds; these compounds are called alkenes. Examples are ethene (C2H4) , propene (C3H6), andcyclohexene (C6H10), the structures of which are shown below. They are named in a similar mannerto the alkanes, except that the ending -ene indicates the presence of a double bond.

Because alkenes are highly reactive, they do not long persist in geologic environments. In thelaboratory they are readily converted to alkanes by the addition of hydrogen in the presence of acatalyst. By hydrogenation ethene thus reacts to form ethane.

A variety of reactions, including hydrogenafion, converts alkenes to alkanes and cyclic compoundsduring diagenesis.Aromatics form an extremely important class of unsaturated hydrocarbons. At first glancearomatics appear to be nothing more than cyclic alkenes containing several double bonds, but theyactually have completely different chemical properties from alkenes and are unusually stable.Although they are unsaturated, they do not add hydrogen easily. Their stability permits aromatics tobe important constituents of oils and sediments.Aromatics possess a system of alternating single and double bonds within a cyclic structure. Asimplified notation for drawing these molecules permits us to represent the double-bond system by acircle within the ring. The circle indicates that the electrons in the double bonds are delocalized;that is, they are free to move throughout the cyclic system instead of being held between twoparticular carbon atoms. It is this delocalization of electrons which makes aromatic compoundsvery stable.Some aromatic molecules are very large. Polycyclic aromatic hydrocarbons having fused ringstructures are quite common. The extreme case is graphite, which is an almost-endless sheet ofaromatic rings.The hydrocarbons we discussed so far are relatively simple molecules. Although they are veryimportant constituents of petroleum, these compounds are quite different trom the majority of theorganic molecules found in living organisms. Most biological molecules are larger and morecomplex than the simple hydrocarbons; the majority contain oxygen, nitrogen, phosphorus, sulfur,or other elements. The hydrocarbons present in petroleum are mostly the end products of extensivedegradation of biogenic molecules. In fact, some complex hydrocarbons that are found in fossilorganic material can be related directly to individual biological precursors.

NONHYDROCARBONSAtoms other than hydrogen and carbon that occur in petroleum, bitumen, and kerogen are calledheteroatoms; the compounds in which they occur are called heterocompounds. Heterocompoundsare also called NSO compounds, because the most common heteroatoms are nitrogen, sulfur, andoxygen. Fossil organic matter often contains a vide variety of heterocompounds, of which some arebiogenic and others are formed during diagenesis. Many of the heterocompounds present inorganisms are converted to hydrocarbons during diagenesis and catagenesis.

Many common NSO compounds are not directly related to biogenic precursors. Among the mostimportant NSO compounds are the asphaltenes, which are large, highly aromatic materials of

Organic Chemistry - 16

varying structure. They have many characteristics in common with kerogen, but asphaltenemolecules are smaller and more aromatic than most kerogens.

Many nonhydrocarbon molecules common to living organisms are also present in sediments. Amongthese are lignin, carbohydrates, and amino acids. Lignin is an important component of wood,providing much of the structural support for large land plants. It is a polymer consisting of manyrepetitions and combinations of three basic aromatic subunits.Lignin monomers are linked topether to form molecules having molecular weights from 3000 to10,000 atomic mass units. Upon decomposition lignin forms phenolic compounds, which arearomatics having a hydroxyl group (OH) attached. Because phenols are potent bactericides, ligninis rather resistant to degradation, and thus tends to become concentrated as other organic matter isdecomposed.Carbohydrates include starch, sugars, and cellulose; the latter is the most abundant organiccompound in the biosphere. Like lignin, it is an important constituent of terrestrial organic matter.Although cellulose is quite resistant to decomposition under some conditions, most carbohydratesare attacked readily by microorganisms. Lignin and cellulose are major constituents of humic coals.Amino acids are the building blocks of proteins. They are rapidly metabolized by virtually allorganisms, however, and thus are seldom preserved in sediments (exceptions occur in shell materialand in bones, where small amounts of preserved amino acids can be used to date specimens)

Kerogen - 17

4 - Kerogen

INTRODUCTIONKerogen is normally defined as that portion of the organic matter present in sedimentary rocks thatis insoluble in ordinary organic solvents. The soluble portion, called bitumen, will be discussed in afollowing chapter. Lack of solubility is a direct result of the large size of kerogen molecules, whichhave molecular weights of several thousand or more. Each kerogen molecule is unique, because ithas patchwork structures formed by the random combination of many small molecular fragments.The chemical and physical characteristics of a kerogen are strongly influenced by the type ofbiogenic molecules from which the kerogen is formed and by diagenetic transformafions of thoseorganic molecules.Kerogen composition is also affected by thermal maturation processes (catagenesis and metagenesis)that alter the original kerogen. Subsurface heating causes chemical reactions that break off smallfragments of the kerogen as oil or gas molecules. The residual kerogens also undergo importantchanges, which are reflected in their chemical and physical properties.Kerogen is of great interest to us because it is the source of most of the oil and some of the gas thatwe exploit as fossil fuels. Diagenetic and catagenetic histories of a kerogen, as well as the nature ofthe organic matter from which it was formed, strongly influence the ability of the kerogen togenerate oil and gas. A basic understanding of how kerogen is formed and transformed in thesubsurface is therefore important in understanding how and where hydrocarbons are generated,whether these hydrocarbons are mainly oil or gas, and how much oil or gas can be expected.The term kerogen was originally coined to describe the organic matter in oil shales that yielded oilupon retorting. Today it is used to describe the insoluble organic material in both coals and oilshales, as well as dispersed organic matter in sedimentary rocks. The amount of organic matter tiedup in the form of kerogen in sediment is far greater than that in living organisms or in economicallyexploitable accumulations of coal, oil, and natural gas.Coals are a subcategory of kerogen. Humic coals are best thought of as kerogens formed mainlyfrom landplant material without codeposition of much mineral matter. Algal (boghead) coals areformed in environments where the source phytoplankton lack both calcareous and siliceous skeletalcomponents. Oil shales, in contrast, have more mineral matter than algal coals, with some of theinorganic matrix often being contributed by the algae themselves. Coals and oil shales shouldtherefore be viewed merely as sedimentary rocks containing special types of kerogens in very highconcentrations.

KEROGEN FORMATIONThe process of kerogen formation actually begins during senescence of organisms, when thechemical and biological destruction and transformation of organic tissues begin. Large organicbiopolymers of highly regular structure (proteins and carbohydrates, for example) are partially orcompletely dismantled, and the individual component parts are either destroyed or used to constructnew geopolymers, large molecules that have no regular or biologically defined structure. Thesegeopolymers are the precursors for kerogen but are not yet true kerogens. The smallest of thesegeopolymers are usually called fulvic acids; slightly larger ones, humic acids; and still larger ones,humins. During the course of diagenesis in the water column, soils, and sediments, the geopolymersbecome larger, more complex, and less regular in structure. True kerogens, having very highmolecular weights, develop after tens or hundreds of meters of burial.The detailed chemistry of kerogen formation need not concern us greatly. Diagenesis results mainlyin loss of water, carbon dioxide, and ammonia from the original geopolymers. If anaerobic sulfate

Kerogen - 18

reduction is occurring in the sediments, and if the sediments are depleted in heavy-metal ions (whichis often the case in nonclastic sediments but is seldom true in shales), large amounts of sulfur maybecome incorporated into the kerogen structure. The amount of sulfur contributed by the originalorganic matter itself is very small. Carboncarbon double bonds, which are highly reactive, areconverted into saturated or cyclic structures.Kerogen formation competes with the destruction of organic matter by oxidative processes. Mostorganic oxidation in sedimentary environments is microbially mediated. Microorganisms prefer toattack small molecules that are biogenic, or at least look very much like biogenic molecules.Geopolymers are more or less immune to bacterial degradation, because the bacterial enzymesystems do not know how to attack them. In an oxidizing environment many of the small biogenicmolecules will be attacked by bacteria before they can form geopolymers. In a low-oxygen(reducing) environment, in contrast, the subdued level of bacterial activity allows more time for theformation of geopolymers and, therefore, better organic preservation.Kerogens formed under reducing conditions will be composed of fragments of many kinds ofbiogenic molecules. Those kerogens formed under oxidizing conditions, in contrast, contain mainlythe most resistant types of biogenic molecules that were ignored by microorganisms duringdiagenesis.

KEROGEN COMPOSITIONBecause each kerogen molecule is unique, it is somewhat fruitless to attempt a detailed discussionof the chemical composition of kerogens. Even if such a description were possible, it would not beof great and direct significance to exploration geologists. What is within our reach, and ultimatelyof much greater practical value, is developing a general method of describing gross kerogencomposition and relating it to hydrocarbon-generative capacity. One way that we can begin is byclassifying kerogens into a few general types.About a decade ago workers at the French Petroleum Institute developed a useful scheme fordescribing kerogens that is still the standard today. They identified three main types of kerogen(called Types I, II, and III) and have studied the chemical characteristics and the nature of theorganisms from which all types of kerogens were derived. Subsequent investigations have identifiedType IV kerogen as well.

The four types of kerogen, the macerals that they arecomposed of, and their organic precursors

Transformation of organic material in sediments andsedimentary rocks.

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Type I kerogen is quite rare because it is derived principally from lacustrine algae. The best-knownexample is the Green River Shale, of middle Eocene age, from Wyoming, Utah, and Colorado.Extensive interest in those oilshale deposits has led to many investigations of the Green River Shalekerogens and has given Type I kerogens much more publicity than their general geologicalimportance warrants. Occurrences of Type I kerogens are limited to anoxic lakes and to a fewunusual marine environments. Type I kerogens have high generative capacities for liquidhydrocarbons.Type II kerogens arise from several very different sources, including marine algae, pollen andspores, leaf waxes, and fossil resin. They also include contributions from bacterial-cell lipids. Thevarious Type II kerogens are grouped together, despite their very disparate origins, because they allhave great capacities to generate liquid hydrocarbons. Most Type II kerogens are found in marinesediments deposited under reducing conditions.Type III kerogens are composed of terrestrial organic material that is lacking in fatty or waxycomponents. Cellulose and lignin are major contributors. Type III kerogens have much lowerhydrocarbon-generative capacities than do Type II kerogens and, unless they have small inclusionsof Type II material, are normally considered to generate mainly gas.Type IV kerogens contain mainly reworked organic debris and highly oxidized material of variousorigins. They are generally considered to have essentially no hydrocarbon-source potential.Hydrogen contents of immature kerogens (expressed as atomic H/C ratios) correlate with kerogentype. In the immature state, Type I (algal) kerogens have the highest hydrogen contents becausethey have few rings or aromatic structures. Type II (liptinitic) kerogens are also high in hydrogen.Type III (humic) kerogens, in contrast, have lower hydrogen contents because they containextensive aromatic systems. Type IV kerogens, which mainly contain polycyclic aromatic systems,have the lowest hydrogen contents.Heteroatom contents of kerogens also vary with kerogen type. Type IV kerogens are highly oxidizedand therefore contain large amounts of oxygen. Type III kerogens have high oxygen contentsbecause they are formed from lignin, cellulose, phenols, and carbohydrates. Type I and Type IIkerogens, in contrast, contain far less oxygen because they were formed from oxygen-poor lipidmaterials.

Van Krevelen diagram showing maturationpathways for Types 1 to IV kerogens astraced by changes in atomic HIC and OICratios. The shaded areas approximatelyrepresent diagenesis, catagenesis, andmetagenesis, successively.

Kerogen - 20

Sulfur and nitrogen contents of kerogens are also variable and, in some cases, interrelated. Nitrogenis derived mainly from proteinaceous material, which is destroyed rapidly during diagenesis. Mosthigh-nitrogen kerogens were therefore deposited under anoxic conditions where diagenesis wasseverely limited. Because lignins and carbohydrates contain little nitrogen, most terrestriallyinfluenced kerogens are low in nitrogen.Kerogen sulfur, in contrast, is derived mainly from sulfate that was reduced by anaerobic bacteria.High-sulfur kerogens (and coals) are almost always associated with marine deposition, because freshwaters are usually low in sulfate. Sulfur is only incorporated into kerogens in large quantities wheresulfate reduction is extensive and where Fe +2 ions are absent (organic-rich, anoxic, marine,nonclastic sediments). Many high-sulfur kerogens are also high in nitrogen.The division of kerogens into Types I-IV on the basis of chemical and hydrocarbon-generativecharacteristics has been supported by another independent scheme for classifying kerogens usingtransmitted-light microscopy. Kerogen types are defined by the morphologies of the kerogenparticles. In many cases the original cellular structure is still recognizable, proving the origin of theparticle. In others the original fabric has disappeared completely, forcing us to make assumptionsabout the source organisms. Microscopic organic analysis has reached a fairly high level ofrefinement and is often capable of assessing kerogen type with good accuracy.The different types of kerogen particles are called macerals, a term taken trom coal petrology.Macerals are essentially organic minerals; they are to kerogen what minerals are to a rock. Thekerogen in a given sedimentary rock includes many individual particles that are often derived from avariety of sources. Thus few kerogens consist of a single maceral type.Maceral names were developed by coal petrologists to describe, wherever possible, the materialsfrom which a maceral was derived. A list of the most common macerals and their precursors isgiven in the table presented earlier in this chapter.It is possible to make a reasonably good correlation between kerogen type based on chemicalcharacteristics and kerogen type based on visual appearance. The correspondence is not perfect,however, because there is not a perfect biological separation of the various types of living organicmatter. The biggest problem comes in identifying Type III kerogen. What appears to be vitrinite(Type III kerogen) by visual analysis may have chemical characteristics intermediate between TypeII and Type III kerogens because of the presence of small amounts of resin or wax.

KEROGEN MATURATION

INTRODUCTIONVery important changes, called maturation, occur when a kerogen is subjected to high temperaturesover long periods of time. Thermal decomposition reactions, called catagenesis and metagenesis,break off small molecules and leave behind a more resistant kerogen residue. The small moleculeseventually become petroleum and natural gas.By convention the term catagenesis usually refers to the stages of kerogen decomposition duringwhich oil and wet gas are produced. Metagenesis, which occurs after catagenesis, represents dry-gas generation. Despite its name, metagenesis is not equivalent to "metamorphism." Metagenesisbegins long before true rock metamorphism, but it also continues through the metamorphic stage.Although the terms catagenesis and oil generation are often used synonymously, they are notprecisely equivalent. Catagenesis and hydrocarbon generation occur concurrently, but they reallyrepresent different aspects of the same process. Catagenesis refers to transformations of kerogenmolecules, whereas hydrocarbon generation focuses on the production of hydrocarbon molecules. Inthis text we shall use the terms somewhat interchangeably, especially when we are discussing bothaspects simultaneously. In principle, however, they represent fundamentally different perspectives.

Kerogen - 21

This chapter will focus on those changes in the residual kerogen that accompany catagenesis. Thecomposition of the products (bitumen, oil, and gas) will be discussed in a following chapter.Kerogen maturation is not a reversible process-any more than baking a cake is reversible.Furthermore, the chemical process of maturation never stops completely, even if drastic decreasesin temperature occur. Chemical reaction-rate theory requires that the rates of reactions decrease astemperature decreases, but it also states that at any temperature above absolute zero reactions willbe occurring at some definable rate. For practical purposes, however, the rates of catagenesis aregenerally not important at temperatures below about 70 C. Furthermore, in most cases decreasesof temperature in excess of about 20-30 C due to subsurface events or erosional removal willcause the rates of catagenesis to decrease so much that it becomes negligible for practical purposes.It is impossible to set precise and universal temperature limits for catagenesis, because time alsoplays a role. Old rocks will often generate hydrocarbons at significantly lower temperatures thanyoung rocks, simply because the longer time available compensates for lower temperatures. Thiscomplex interplay between the effects of time and temperature on maturity is discussed in a laterchapter.

EFFECTS OF MATURATION ON KEROGENSKerogen undergoes important and detectable changes during catagenesis and metagenesis. Some ofthese changes can be measured quantitatively, thus allowing us to judge the extent to which kerogenmaturation has proceeded. The real reason for following kerogen catagenesis, of course, is tomonitor hydrocarbon generation. Although it is obvious that many measurable changes in kerogensare related to hydrocarbon generation, it is also true that other changes in kerogen properties havelittle or nothing to do with it, and thus are not necessarily valid indicators of hydrocarbongeneration. We shall look now at the various techniques for estimating the extent of hydrocarbongeneration from kerogen properties and see how closely each of them is related to hydrocarbongeneration.As we saw earlier, the cracking of any organic molecule requires hydrogen. The more hydrogen akerogen contains, the more hydrocarbons it can yield during cracking. Because many of the lightproduct molecules are rich in hydrogen, the residual kerogen gradually becomes more aromatic andhydrogen poor as catagenesis proceeds. Thus the steady decrease in hydrogen content of a kerogen(usually measured as the atomic hydrogen/carbon ratio) during heating can be used as an indicatorof both kerogen catagenesis and hydrocarbon generation, provided that the hydrogen content of thekerogen was known prior to the onset of catagenesis.Nitrogen and sulfur are also lost from kerogens during catagenesis. Nitrogen loss occurs primarilyduring late catagenesis or metagenesis, after hydrogen loss is well advanced. In contrast, much ofthe sulfur is lost in the earliest stages of catagenesis, as evidenced by low maturity, high-sulfur oilsfound in a number of areas, including the Miocene Monterey Formation of southern California.The most important implication of these chemical changes is that the remaining hydrocarbon-generative capacity of a kerogen decreases during catagenesis and metagenesis. All kerogensbecome increasingly aromatic and depleted in hydrogen and oxygen during thermal maturation. Inthe late stages of maturity, Types I, II, and III kerogens will therefore be very similar chemically,possessing essentially no remaining hydrocarbon generative capacity.Kerogen particles become darker during catagenesis and metagenesis, much as a cookie brownsduring baking. There is a steady color progression yellow-goldenorange-light brown-dark brown-black as a result of polymerization and aromatization reactions. These reactions are intimatelyrelated to important changes in the chemical structure of kerogen, but they are not necessarilyidentical with hydrocarbon generation. There is therefore no necessary cause-and-effect relationship

Kerogen - 22

between kerogen darkening and hydrocarbon generation, and no guarantee that a particular kerogencolor always heralds the onset of oil generation.As kerogen matures and becomes more aromatic, its structure becomes more ordered, because theflat aromatic sheets can stack neatly. These structural reorganizations bring about changes inphysical properties of kerogens. One property that is strongly affected, and which can be used togauge the extent of molecular reorganization, is the ability of kerogen particles to reflect incidentlight coherently. The more random a kerogen's structure, the more an incident light beam will bescattered, and the less it will be reflected.Half a century ago coal petrologists discovered that the percentage of light reflected by vitriniteparticles could be correlated with coal rank measured by other methods.Because coal rank is merely a measure of coal maturity, and because vitrinite particles also occur inkerogens, the technique, called vitrinite reflectance, has been widely and successfully applied inassessing kerogen maturity.Cracking often produces free radicals, which are unpaired electrons not yet involved in chemicalhonds. Kerogens, especially highly aromatic ones, contain large numbers of unpaired electrons. Theconcentration of free radicals in a given kerogen has been found to increase with increasingmaturity. Free-radical concentrations can be measured by electron-spin resonance.Kerogens often fluoresce when irradiated. The intensity and wavelength of the fluorescente arefunctions of kerogen maturity.Some properties of kerogen change very little during catagenesis. For example, carbon-isotopiccompositions of kerogens are affected little by maturation. Except for darkening, the visualappearance of kerogen also does not change during catagenesis: kerogen types are generallyrecognizable until the particles become black and opaque, somewhat beyond the oil-generationwindow.

Plot of bitumen generation as afunction of maturity (dashed fine)compared to bitumen remaining inrock (solid line). The differencebetween the two curves representsbitumen expelled from the rock orcracked to light hydrocarbons.

HYDROCARBON GENERATIONAs kerogen catagenesis occurs, small molecules are broken off the kerogen matrix. Some of theseare hydrocarbons, while others are small heterocompounds. These small compounds are much moremobile than the kerogen molecules and are the direct precursors of oil and gas. A general name torthese molecules is bitumen.Bitumen generation occurs mainly during catagenesis; during metagenesis the chief product ismethane. If neither expulsion from the source rock nor cracking of bitumen occurred, there wouldbe a large and continuous build-up of bitumen in the rock as a result of catagenetic decompositionof kerogen. What actually occurs, however, is that some of the bitumen is expelled from the sourcerock or cracked to gas, resulting in lower bitumen contents in the source. Both curves are highly

Kerogen - 23

idealized, however, because natural variations among samples cause much scatter in experimentaldata.It has become apparent in recent years that not all kerogens generate hydrocarbons at the samecatagenetic levels, as measured by parameters such as vitrinite reflectance. Given the significantchemical differences among the various types of kerogens, this result is hardly surprising.Resinite and sulfur-rich kerogens are able to generate liquid hydrocarbons earlier than otherkerogens because of the particular chemical reactions occurring in those two materials. Resiniteconsists of polymerized terpanes (ten-carbon isoprenoids) that can decompose easily by reversingthe polymerization process. Sulfur-rich kerogens decompose easily because carbon-sulfur hbondsare weaker than any bonds in sulfur-poor kerogens.Effective generation of hydrocarbons requires that the generated products be expelled from thesource-rock matrix and migrated to a trap. Timing and efficiency of expulsion depend on a numberof factors, including rock physics and organic-geochemical considerations. We shall consider thelatter briefly here.Many workers now believe that microfracturing of source rocks is very important tor hydrocarbonexpulsion. Microfracturing is related to overpressuring, which in turn is partly attributed tohydrocarbon generation itself. Rich rocks will become overpressured earlier than lean ones and thuswill also expel hydrocarbons earlier. In very lean rocks expulsion may occur so late that cracking ofthe generated bitumen is competitive with expulsion. In such cases the expelled products will bemainly gas.

SUMMARYKerogen begins to form during early diagenesis, when large geopolymers are created frombiological molecules. The chemical composition and morphology of kerogen macerals depend bothon the type of original organic matter and on diagenetic transformations. Numerous methods existfor tracing the history of a kerogen and determining its original chemical and physicalcharacteristics.Catagenesis of kerogen produces a more aromatic, hydrogen-poor, residual kerogen as well assmall molecules that are the direct precursors for petroleum and natural gas. Several methods existfor estimating the extent to which hydrocarbon generation has occurred in a given kerogen, butnone of these measurements is closely linked to the actual process of hydrocarbon generation.Thus, although we know that oil generation does occur during the phase we call catagenesis, wecannot always define the limits of hydrocarbon generation with great confidence.The chemical composition of a kerogen controls the timing of hydrocarbon generation and the typeof products obtained. Kerogens formed from lipid-rich organic material are likely to generate liquidhydrocarbons, whereas those kerogens that contain few lipids will generate mainly gas. Kerogensformed from resinite will generate condensates or light oils quite early. High-sulfur kerogensgenerate heavy, high-sulfur oils at low levels of maturity. Other kerogens usually follow a moretraditional model.Source rocks that generate large amounts of hydrocarbons early are likely to expel thosehydrocarbons early. Candidates for early expulsion would be very organic rich rocks and thosecontaining resinite or high-sulfur kerogens. Conversely, those rocks that generate few hydrocarbonsmay not expel them until they have been cracked to gas.

Bitumen, Petroleum, and Natural Gas -24

5 - Bitumen, Petroleum, and Natural Gas

INTRODUCTIONPetroleum obtained from reservoir rocks and bitumen extracted from fine-grained rocks have manysimilarities, but they also exhibit many important differences. There is no doubt that they arerelated; indeed, bitumen is almost universally accepted as the direct precursor for petroleum.However, many unanswered questions remain about the processes that transform bitumen intopetroleum. Major compositional changes occur in going from bitumen to petroleum, but we are notcertain whether they occur mainly within the source rock or during migration through the reservoirrock. We also do not know how much of the change involves chemical reactions, and how much isdue to physical separation of chemical compounds having very different properties. The influence ofthe lithologies of source and reservoir rocks on these compositional changes is poorly understood.Both bitumens and petroleums exhibit a wide range of compositions. Much of this variety is relatedto source-rock facies and the composition of the kerogens that generated the bitumens. Maturityalso exerts control over bitumen and petroleum composition. Reservoir transformations in somecases greatly affect oil composition and properties.Bitumen and petroleum compositions can also be used as tools in correlating samples with eachother. Such correlations can be particularly useful in establishing genetic relationships amongsamples. In order to understand bitumen and petroleum compositions and to use them forexploration, however, we must separate the characteristics related to kerogen composition fromthose related to the transformation of bitumen to petroleum and from those related to changesoccurring in reservoirs. This chapter will compare and contrast bitumen and petroleumcompositions and examine the factors responsible for the observed differences.

COMPOUNDS PRESENT IN BITUMEN AND PETROLEUM

GENERAL CLASSES OF COMPOUNDSBoth bitumen and petroleum contain a very large number of different chemical compounds. Some ofthese are present in relatively large quantities, while others are only trace contributors. In order toinvestigate the individual compounds present, we first separate a crude oil or a bitumen into severalfractions having distinct properties.Each of the fractions contains certain types of chemical compounds. One fraction consists mainly ofsaturated hydrocarbons; n-alkanes, branched hydrocarbons (including isoprenoids), and cyclics.Saturated hydrocarbons are the most thoroughly studied of the components of petroleum andbitumen because they are the easiest to work with analytically.A second fraction consists of aromatic hydrocarbons and some light sulfur-containing compounds.Light aromatic hydrocarbons, like benzene and toluene, have been studied in petroleums, but thesecompounds are lost from bitumens during evaporation of the solvent used in extracting the bitumenfrom the rock. Heavier aromatic and naphthenoaromatic hydrocarbons, particularly those derivedfrom diterpanes, triterpanes, and steranes, are more commonly studied.Most of the NSO compounds appear in the remaining two fractions. The lighter of these fractions,variously called polars, NSOs, and resins, contains a wide variety of small and medium-sizedmolecules with one or more heteroatoms. Few of these heterocompounds have been studiedcarefully.The final fraction contains very large, highly aromatic asphaltene molecules that are often rich inheteroatoms. Asphaltenes tend to aggregate into stacks because of their planarity, and formcomplexes with molecular weights of perhaps 50,000. The large sizes of asphaltene units render

Bitumen, Petroleum, and Natural Gas - 25

them insoluble in light solvents. Asphaltenes can thus be removed from oils or bitumens in thelaboratory or refinery by adding a light hydrocarbon, such as pentane or propane. Because of theirmolecular complexity and heterogeneity, asphaltene molecules have not been studied in detail.

SPECIFIC COMPOUNDSBiomarkers. Many of the compounds and classes of compounds that we find in crude oils andbitumens are called biomarkers, an abbreviation for biological markers. These compounds, whichare derived from biogenic precursor molecules, are essentially molecular fossils. The most usefulbiomarkers serve as indicators of the organisms from which the bitumen or petroleum was derived,or of the diagenetic conditions under which the organic matter was buried. In a few cases specificprecursor organisms or molecules can be identified, whereas in other instances we may be able tolimit the possible precursors to only a few species. In most cases, however, although we know forcertain that the biomarker molecule is biogenic, we are unable to use it as an "index fossil" forspecific organisms.Other compounds. Many other types of organic compounds in crude oils and bitumens are notconsidered to be biomarkers because they cannot be related directly to biogenic precursors. Theyare, however, of biological origin, but their sources are simply no longer recognizable due todiagenetic and catagenetic transformations.

FACTORS AFFECTING COMPOSITION OF BITUMEN AND PETROLEUM

SOURCE AND DIAGENESISBiomarkersn-Alkanes were among the first biomarkers to be studied extensively. Their high concentration inbitumens and oils is best explained by their existence in plant and algal lipids, and by theircatagenetic formation from long-chain compounds such as fatty acids and alcohols.Another important indication of the origin of n-alkanes is the distribution of individual homologs, ormembers of the n-alkane series. For the most part n-alkanes present in terrestrial plants have oddnumbers of carbon atoms, especially 23, 25, 27, 29, and 31 atoms.In contrast, marine algae produce n-alkanes that have a maximum in their distribution at C-17 or C-22, depending upon the species present. The distributions are quite sharp, and no preference foreither odd- or even-carbon homologs is evident.Many sediments, of course, receive contributions of n-alkanes from both terrestrial and marinesources. Their n-alkane distributions reflect this mix.Sediments are also known that exhibit a strong preference for n-alkanes having an even number ofcarbon atoms. These n-alkanes are believed to be formed by hydrogenation (reduction) of long-chain fatty acids and alcohols having even numbers of carbon atoms. (Among the acids andalcohols present in living organisms, even-carbon homologs predominate as strongly as do the odd-carbon homologs among the n-alkanes.) Even-carbon preferences occur principally in evaporiticand carbonate sediments, where input of terrestrial n-alkanes is minimal and diagenetic conditionsare highly reducing.Carbon Preference Index, or CPI, was developed as a measure of the strength of the odd-carbonpredominance in n-alkanes over the even alkanes (in the series from 23 upwards).The average of two ranges is taken to minimize bias produced by the generally decreasing n-alkaneconcentrations with increasing number of carbon atoms. If the number of odd- and even-carbonmembers is equal, the CPI is 1.0. If odd-carbon homologs predominate, the CPI is greater than 1.0.However, because the concentration of n-alkanes often decreases with increasing carbon number,the lower-carbon homologs are given more weight in the calculation. CPI values can therefore

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deviate from 1.0 even when no preference is distinguishable by visual inspection of the distributioncurve.n-Alkane distributions are greatly modified by thermal maturity. Chain lengths gradually becomeshorter, and the original n-alkanes present in the immature sample are diluted with new n-alkanesgenerated during catagenesis. Because the newly generated n-alkanes show little or no preferencefor either odd- or even-carbon homologs, CPI values approach 1.0 as maturity increases.n-Alkane distributions in bitumens and oils derived from algae do not show the influences ofmaturity as clearly because the original CPI values are already very close to 1.0. It is thereforeoften difficult to estimate maturity levels in pelagic rocks on the basis of n-alkane data.Parameters other than Biomarkers. Sulfur contents are also strongly influenced by diageneticconditions. For economic and environmental reasons, oils having more than about 0.5% sulfur aredesignated as high-sulfur. Many high-sulfur oils contain 1% sulfur or less, but in some areas sulfurcontents can reach 7% (Monterey oils from the onshore Santa Maria area, southern California, forexample). A few oils contain more than 10%.These high-sulfur bitumens and crude oils are derived from high-sulfur kerogens. As we sawearlier, sulfur is incorporated into kerogens formed in nonclastic sediments that accumulate whereanaerobic sulfate reduction is important. Most oils and bitumens derived from lacustrine orordinary clastic marine source rocks will be low in sulfur content, whereas those from euxinic oranoxic marine source rocks will be high-sulfur.Sulfur occurs predominantly in the heavy fractions of oils and bitumens, particularly in theasphaltenes. High-sulfur oils therefore have elevated asphaltene contents.

RESERVOIR TRANSFORMATIONSIntroduction. There are two main types of reservoir transformations that can affect crude oils(reservoir transformations are not applicable to bitumen because, by definition, the material in areservoir is petroleum). Thermal processes occurring in reservoirs include cracking anddeasphalting. Nonthermal processes are water washing and biodegradation. Of these, cracking andbiodegradation are by far the most important.Cracking and Deasphalting. Cracking, which breaks large molecules down into smaller ones, canconvert a heavy, heteroatom-rich off into a lighter, sweeter one. Waxy oils become less waxy. APIgravities increase, and pour points and viscosities decrease. When cracking is extreme, the productsbecome condensate, wet gas, or dry gas.Cracking is a function of both time and temperature, as well as of the composition of the oil and thecatalytic potential of the reservoir rock. It is therefore impossible to state that cracking alwaysoccurs at a certain depth or reservoir temperature. Most oils, however, will be reasonably stable atreservoir temperatures below about 90 C, regardless of the length of time they spend there. On theother hand, a reservoir above 120 C will contain normal oil only if the oil is a recent arrival.Although the role of catalysis in hydrocarbon cracking in reservoirs has not been proven, manyworkers suspect that clay minerals are important facilitators of hydrocarbon breakdown. Catalyticeffectiveness varies greatly from one clay mineral to another, however, and our partialunderstanding of this difficult subject is not of much practical use at the present time.Cracking also brings about deasphalting, because asphaltene molecules become less soluble as theoil becomes lighter. Precipitation of asphaltenes in the reservoir will lower sulfur content andincrease API gravity appreciably.Biodegradation and water washing. Water washing involves selective dissolution of the mostsoluble components of crude oils in waters that come in contact with the oils. The smallesthydrocarbon molecules and the light aromatics, such as benzene, are the most soluble. The effectsof water washing are rather difficult to determine because they do not affect the oil fractions that

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are most frequently studied. Furthermore, in most cases the effects are quite small because of thelow solubilities of all hydrocarbons in water. Finally, water washing and biodegradation often occurtogether, with the more dramatic effects of biodegradation obscuring those of water washing.Biodegradation is a transformation process of major importance. Under certain conditions somespecies of bacteria are able to destroy some of the compounds present in crude oil, using them as asource of energy. The bacteria responsible for biodegradation are probably a mixture of aerobic andanaerobic strains. Only aerobic bacteria are believed to actually attack hydrocarbons, but anaerobesmay consume some of the partially oxidized byproducts of initial aerobic attack.Because biodegradation changes the physical properties of oils, it can have serious negativefinancial implications. Heavily biodegraded oils are often impossible to produce (Athabasca TarSands of Alberta, Canada, and the Orinoco heavy oils of Venezuela, for example). If production isphysically possible, it may be expensive or uneconomic. It is therefore important to understandwhere and why biodegradation occurs, and what its effects are on oil composition.Biodegradation may actually start during oil migration (provided required temperature and oxygenconditions are met), because oil-water interactions are maximized then. Most biodegradationprobably occurs within reservoirs, however, since the length of time an oil spends in a reservoir isusually much longer than its transit time during migration.Biodegradation can vary in intensity from very light to extremely heavy. Because the chemical andphysical properties of an oil change dramatically in several predictable ways during biodegradation,biodegraded oils are easily recognized. Many basins have at least a few biodegraded oils, and insome areas they are epidemic.Bacteria that consume petroleum hydrocarbons have strong preferences. Hydrocarbons are not theirvery favorite foods, and they eat them only because there is nothing else available. The preferredhydrocarbons are n-alkanes, presumably because their straight-chain configurations allow thebacterial enzymes to work on them most efficiently. Also attractive to the "bugs" are long, alkylside-chains attached to cyclic structures.After the n-alkanes and alkyl groups are consumed, the bacteria begin to destroy compounds havingonly a single methyl branch or those having widely spaced branches. Then they move on to more-highly branched compounds, such as the isoprenoids.In the last stages of biodegradation, polycyclic alkanes are attacked.Because the hierarchy of bacterial attack on crude oils is well known, it is possible to assess thedegree of biodegradation by observing which compounds have been destroyed.Sulfur contents of crude oils also increase as a result of biodegradation. In a heavily biodegraded oilthe sulfur content may increase by a factor of two or three. Sulfur is undoubtedly concentrated inthe oil by selective removal of hydrocarbons, and may also be added by bacterially mediated sulfatereduction.

COMPARISON OF BITUMEN AND PETROLEUMAlthough bitumens and crude oils contain the same compounds, the relative amounts are quitedifferent. In the process of converting bitumen to petroleum, either the NSO compounds are lost inlarge quantities, or they are converted to hydrocarbons. In actuality, both processes probably occur,although selective loss of nonhydrocarbons during expulsion is probably most effective inconcentrating the hydrocarbons.Bitumen composition depends strongly on the lithology of the host rock. Carbonates containbitumens that are much richer in heterocompounds than are shales, and their hydrocarbon fractionsare more aromatic. These differences are the result of the higher sulfur contents of kerogens incarbonates. Oils derived from carbonate sources are also richer in heterocompounds than oilssourced from shales.

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NATURAL GASNatural gas contains many different compounds, although most of them are present only in tracequantities. The principal components with which we shall be concerned are