KU Geology 2

8/14/2019 KU Geology 2

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Department of Petroleum Technology, University of Karachi

Maturation and expulsion


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Conversion of Kerogene to Oil and Gas



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With increasing burial by later sediments and increase in

temperature, the kerogen within the rock begins to breakdown.

This thermal degradation orcracking releases shorter

chain hydrocarbons from the original large and complexmolecules found in the kerogen.

The hydrocarbons generated from the source rock are

expelled, along with other pore fluids, due to thecontinuing effects ofcompaction and start moving

upwards towards the surface, a process known as

migration.


Maturation and expulsion
http://wiki.healthhaven.com/Cracking_%28chemistry%29http://wiki.healthhaven.com/Moleculeshttp://wiki.healthhaven.com/Compaction_%28geology%29http://wiki.healthhaven.com/Compaction_%28geology%29http://wiki.healthhaven.com/Moleculeshttp://wiki.healthhaven.com/Cracking_%28chemistry%29


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Rock Pyrolysis S1 = the amount of free

hydrocarbons (gas and oil)in the sample (in milligramsof hydrocarbon per gram of

rock).


S2 = the amount of

hydrocarbons generated

through thermal cracking of

nonvolatile organic matter.

S3 = the amount of CO2 (in

milligrams CO2 per gram of

rock) produced during pyrolysis

of kerogen.

Oxygen bearing volatile

compounds are passed to a

separate detector, which

produces as S3 response.


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Rock Pyrolysis

Espitalie developed a standard procedure for the pyrolysis ofrock samples known as ROCK-EVAL PYROLYSIS.

Method: About 100 mg finely ground rock sample is placed into

a furnace at 250 degree C in an inert atmosphere than raised toa temperature of 550 degree C.

The amount of Hydrocarbon products evolved is recorded by a

Flame Ionization Detector(FID) as a function of time.

Three Peaks are typically, Known as S1, S2 and S3 peaks are

evolved and recorded.



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Example of Rock Eval trace. HC = hydrocarbon IfS1 >1 mg/g, it may be indicative of an oil show.

S1 normally increases with depth. Contamination of samples by drilling fluids and

mud can give an abnormally high value forS1.


S2 is an indication of the quantity of hydrocarbonsthat the rock has the potential of producing.

The burial and maturation should continue at this

stage.

This parameter normally decreases with burial

depths >1 km.


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S3 = the amount of CO2 (in milligrams CO2 per gram of

rock) produced during pyrolysis of kerogen.

Oxygen bearing volatile compounds are passed to a

separate detector, which produces as S3 response.

S3 is an indication of the amount of oxygen in the kerogen

and is used to calculate the oxygen index.

Contamination of the samples should be suspected ifabnormally high S3 values are obtained.

Example of Rock Eval trace. HC = hydrocarbon



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Hydrogen index/oxygen index plot from Rock Eval pyrolysis

data. TOC


HI = hydrogen index(HI = [100 x S2]/TOC).

HI is a parameter used to characterizethe origin of organic matter.

Marine organisms and algae, ingeneral, are composed oflipid-

and protein-rich organic matter,where the ratio of H to C is higherthan in the carbohydrate-richconstituents of land plants.

. PC = pyrolyzable carbon (PC = 0.083

x [S1 + S2]).PC corresponds to carbon content of

hydrocarbons volatilized andpyrolyzed during the analysis.


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Hydrogen index/oxygen index plot from Rock Eval pyrolysis

data. TOC


. OI = oxygen index

(OI = [100 x S3]/TOC).

OI is a parameter that correlates with

the ratio of O to C, which is high for

polysacharride-rich remains of land

plants and inert organic material(residual organic matter)

encountered as background in

marine sediments.

PI = production index

(PI = S1/[S1 + S2]).

PI is used to characterize the evolution

level of the organic matter.


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EXPULSION EFFICIENCY



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PGIandPEE

Mackenzie and Quigley (1988) has classified source rocks into

three end member classes on the basis of initial kerogene

concentration and kerogene type.

These parameters determine the timing and composition of

petroleumexpelled.

PGI : (Petroleum generation Index)

is the fraction of petroleum prone organic matter that has been

transformed into petroleum, and is thus a measure of source

maturity.

PEE: (Petroleum expulsion efficiency)

is the fraction of petroleum fluids formed in the source rock that

have been expelled



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Class I:

Predominantly Labile Kerogen at concentration of10Kg/ton and generationstart at about 100 degree C.

This rapidly saturates the source rock and between 120-150 degree C 60%-90%is expelled as oil with dissolved gas.

The remaining fluid crack to gas at higher temperature and expelled as gas.

Class II: This is linear version of Class I with initial Keregen concentration < 5Kg/ton.

Expulsion is inefficient up to 150 degree C because insufficient oil-richpetroleum generated.

Petroleum is expelled mainly as gas condensate formed by cracking above

150degree C followed by some Dry Gas. Class III:

Source rocks contain mostly Refractory Kerogen. Generation and expulsiontake place only above 150 degree C and petroleum fluid is a relatively dry Gas

PGI and PEE



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The Origin of PetroleumThe Origin of Petroleum

Organic-richOrganic-richSource RockSource Rock

Thermally MaturedThermally MaturedOrganic MatterOrganic Matter OilOil



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GeneratioGenerationn

MigrationMigrationAccumulationAccumulation

andandPreservationPreservation

Petroleum System: Timing is CriticalPetroleum System: Timing is Critical

Processes:

Elements:

SourceSourceRockRock

MigrationMigrationAvenueAvenue

ReservoirReservoirand Sealand Seal

Trap Must Be Available Before/During MigrationTrap Must Be Available Before/During Migration

For accumulations to occur, a trap must exist either before or coincident with the time ofmigration. The petroleum system events chart helps capture these critical aspects of timing.


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Spill PointSpill Point

Seal Rock(Mudstone)Reservoir Rock

(Sandstone)Migration fromKitchen

1) Early Generation

2) Late Generation

Gas displaces all

oil

Gas beginning todisplace oil

Displaced oil

accumulates

Petroleum SystemPetroleum System



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Several specific forms of hydrocarbons-

Dry gas-contains largely methane, specifically

contains less than 0.1 gal/1000ft3 of condensable(at surface T and P) material.

Wet gas-contains ethane propane, butane. Up to

the molecular weight where the fluids are alwayscondensed to liquids

Condensates- Hydrocarbon with a molecular

weight such that they are gas in the subsurfacewhere temperatures are high, but condense toliquid when reach cooler, surface temperatures.



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Liquid hydrocarbons- commonly known as oil, or crude oil, to distinguish it from

refined hydrocarbon products.

Plastic hydrocarbons- asphalt Solid hydrocarbons- coal and kerogen- (kerogen strictly

defined is disseminated organic matter in sediments that isinsoluble in normal petroleum solvents.

Gas hydrates- Solids composed of water molecules surrounding gas

molecules, usually methane, but also H2S, CO2, and otherless common gases.

Several specific forms of hydrocarbons-



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Hydrocarbon Generation Stages


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Cross section through part of a sedimentary basin in which ahydrocarbon source rock layer has been buried to different depths.

Due to increasing temperatures with increased burial depth, organicmatter within this source rock `cooks', resulting in partialdecomposition and petroleum generation (mature source rock).

With further burial, organic matter decomposes to generate natural gas(over-mature source rock).

Generated petroleum and natural gas are expelled from the sourcerock and migrate upward into porous overlying rock layers.

If appropriate conditions exist, petroleum and natural gas are trappedand accumulate.

If appropriate conditions do not exist, natural gas is eventuallyreleased to the atmosphere and petroleum seeps at the surface to formasphalt (tar) deposits.



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OIL AND GAS MIGRATION

Traditionally (Illing, 1933), the process ofpetroleum migration is divided into two parts:

primary migrationwithin the low-permeabilitysource rocks

secondary migrationin permeable carrier bedsand reservoir rocks.

It is now recognized that fractured source rockscan also act as carrier beds and reservoir rocks somore modern definitions are:



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OIL AND GAS MIGRATION

Primary migrationof oil and gas is movementwithin the fine-grained portion of the maturesource rock.

Secondary migrationis any movement in carrierrocks or reservoir rocks outside the source rockor movement through fractures within the sourcerock.

Tertiary migrationis movement of a previouslyformed oil and gas accumulation.



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Mechanisms of Migration With regard to the mechanisms involved in migration there are seven main

questions to answer.

When did migration take place?

What form were the hydrocarbons in when they migrated?

What moved the hydrocarbons?

If water was involved: where did the water come from?

What caused the water to move?

In which direction did the water move?

Have much water moved?



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PRIMARY MIGRATION

Type III kerogens are the

most likely source.

Migration can also occur in

aqueous solution for the

smallest and most soluble

molecules (methane,

ethane, benzene, toluene).

Migration by diffusion is not

significant.

Primary oil migration within a fine-

grained mature source rock with> 2% total organic carbon (TOC)occurs initially as a bitumen thatdecomposes to oil and gas andmigrates as a hydrocarbon (HC)phase or phases.

The process of HC generation

causes expulsion of petroleum

and is often a more potentialmechanism for migration thanmechanical compaction.

Generation and expulsion of light

oil, and gas can come from low

(< 2%) TOC source rocks without

a bitumen intermediate.


PET 631 Mi ti


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PET 631 Migration

There are two types of migrationwhen discussing the movement ofpetroleum, primary and secondary.

Primary migration refers to themovement of hydrocarbons fromsource rock into reservoir rock andit is this type that the following

discussion refers to.

Secondary migration refers to thesubsequent movement ofhydrocarbons within reservoir

rock; the oil and gas has left thesource rock and has entered thereservoir rock.



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A problem in close relation

to the later stage of theproject is the expulsion of

hydrocarbons from source

rock (primary migration).

The chemical aspects of

this process has been

extensively studied, but the

physical aspects are poorly

understood.

Primary Migration from shale source Rocks

http://www.fys.uio.no/faststoff/sup/Research/FluidExpulsion/lina1.gif


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Primary Migration

Fig.Generalized view of oilmigration using invasion

percolation concepts

(from Carruthers and Ringrose,

1998).


Mi i


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Micro-pressuring. Mompers [1978] clearly outlines

the characteristics of a source

rock which are important in thedevelopment of micro-pathwayswith the rock.At some point the pressureincrease causes micro-fracturingin the rock, and the hydrocarbonsmigrate into the micro-fractureswhich lead out of the source rock.

This concept allows thehydrocarbons to migrate in aliquid phase.

This is regarded as the mainmechanism for primary migrationout of the source rock.


GENERATION MIGRATION AND TRAPPING OF


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Seal

Reservoirrock

Seal

Migration route

Oil/watercontact (OWC)

Hydrocarbon

accumulationin thereservoir rock

Top of maturity

Source rock

Fault(impermeable)

GENERATION, MIGRATION, AND TRAPPING OF

HYDROCARBONS

Seal



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Migration through Fractures



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Mechanics of Secondary Hydrocarbon Migration

The mechanics of secondary hydrocarbon migration andentrapment are well-understood physical processes thatcan be dealt with quantitatively in hydrocarbonexploration.

The main driving force for secondary migration ofhydrocarbons is buoyancy.

If the densities of the hydrocarbon phase and the waterphase are known, then the magnitude of the buoyant forcecan be determined for any hydrocarbon column in thesubsurface.



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Hydrocarbon and water densities vary significantly.

Subsurface oil densities range from 0.5 to 1.0 g/cc;subsurface water densities range from 1.0 to 1.2 g/cc.

When a hydrodynamic condition exists in the subsurface,the buoyant force of any hydrocarbon column will bedifferent from that in the hydrostatic case.

This effect can be quantified if the potentiometric gradient

and dip of the formation are known.

The main resistant force to secondary hydrocarbonmigration is capillary pressure.



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The factors determining the magnitude of the resistantforce are the radius of the pore throats of the rock,hydrocarbon-water interfacial tension, and wettability.

For cylindrical pores, the resistant force can be quantified

by the simple relation: , where Pd is the hydrocarbon-water displacement pressure or the resistant force, isinterfacial tension, is the wettability term, and R is radiusof the largest connected pore throats.

Radius of the largest connected pore throats can bemeasured indirectly by mercury capillary techniques usingcores or drill cuttings.



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Mechanics of Secondary Hydrocarbon Migration Subsurface hydrocarbon-water interfacial tensions range

from 5 to 35 dynes/cm for oil-water systems and from 70 to30 dynes/cm for gas-water systems.

Migrating hydrocarbon slugs are thought to encounter

water-wet rocks.

The contact angle of hydrocarbon and water against thesolid rock surface as measured through the water phase, ,

is thus assumed to be 0, and the wettability term, , is

assumed to be 1.

A thorough understanding of these principles can aid both

qualitatively and quantitatively in the exploration and

development of petroleum reserves.


Dri ing forces for migration


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Driving forces for migration: Secondary migration is the movement

of hydrocarbons along a "carrier bed"from the source area to the trap.

Migration mostly takes place as one ormore separate hydrocarbons phases(gas or liquid depending on pressureand temperature conditions).

There is also minor dissolution in

waterof methane and short chainhydrocarbons.

Buoyancy (This force acts verticallyand is proportional to the densitydifference between water and thehydrocarbon so it is stronger for gas

than heavier oil)

Hydrodynamic flow (water potentialdeflect the direction of oil migration,the effect is usually minor except inover pressured zones (primary migration))


R i ti f


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Resisting forces:

Capillary pressure (opposesmovement of fluid from coarse-grainto fine- grain rock, also the capillarypressure of the water in the reservoirresists the movement of oil)

One result of hydrodynamic flow is atilted oil-watercontact (OWC) in atrap. OWC is an equipotential

surface, but if the water is flowing theequipotential surfaces are inclined inthe direction of flow, so the OWC willbe tilted too.

During migration the pressure and

temperature conditions of thehydrocarbons can change a lotaffecting the phase behavior of theoil.



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Retardatin of buoyant movement as an oil globule (X) is deformed tofit into a narrow pore throat (Y). The upward buoyant force is partlyor completely opposed by the capillary-entry pressure, the forcerequired to deform the oil globule enough to enter the pore throat. If

the capillary-entry pressure exceeds the buoyant force, secondarymigration will cease until either the capillary-entry pressure isreduced or the buoyant force is increased.


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MECHANISM

Once hydrocarbons are expelled from the source rock ina separate hydrocarbon phase into a secondary-migration conduit, subsequent movement of thehydrocarbons will be driven by buoyancy. Hydrocarbons

are almost all less dense than formation waters, andtherefore are more buoyant. Hydrocarbons are thuscapable of displacing water downward and movingupward themselves. The magnitude of the buoyant forceis proportional both to the density difference between

water and hydrocarbon phase and to the height of the oilstringer. Coalescence of globules of hydrocarbons afterexpulsion from the source rock therefore increases theirability to move upward through water-wet rocks


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Two basic types of traps:


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Two basic types of traps:

Structural traps hold oil and gas because

the earth has been bent and deformed in

some way. The trap may be a simple dome

(or big bump), just a "crease" in the rocks, orit may be a more complex fault trap like the

one shown at the right.


Stratigraphic traps are depositional in

nature.

This means they are formed in place,

usually by a sandstone ending upenclosed in shale.

The shale keeps the oil and gas from

escaping the trap.


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