STS51C-143-0027 Mississippi River Delta and Coastal Louisiana, U.S.A. January 1985

NASA PHOTO

78

STS61A-42-0051 Mississippi River Delta, Louisiana, U.S.A.October 1985

NASA PHOTO

20 mi

N

2

Outline

Petroleum systems Geologic principles and geologic time Rock and minerals, rock cycle, reservoir

properties Hydrocarbon origin, migration and accumulation Sedimentary environments; stratigraphic traps Plate tectonics, structural geology Structural traps Geophysical methods Importance to Schlumberger

4

Cross Section Of A Petroleum System

Overburden Rock

Seal or Cap[Rock

Reservoir Rock

Source Rock

Underburden Rock

Basement Rock

Top Oil Window

Top Gas Window

Geographic Extent of Petroleum System

Petroleum Reservoir (R)

Fold-and-Thrust Belt(arrows indicate relative fault motion)

EssentialElements

ofPetroleum

System

(Foreland Basin Example)

(modified from Magoon and Dow, 1994)

Reservoir

Sed

imen

tary

Bas

in F

ill

R

Stratigraphic Extent of

PetroleumSystem

ActiveSource Rock

Extent of Play

Petroleum System A Petroleum System requires timely convergence of certain geologic factors and geologic events.

These Include:

Mature source rockMigrationReservoir rockSeal or cap rock

Petroleum Geology

•Law of cross-cutting relationships. In the figure above, the igneous dike (F) is younger than layers A-E but older than layer G, because a geologic feature is younger than any other geologic feature that it cuts. This is an important law for determining the relative ages of geologic features.

• According to the “Law of Superposition,” layer “I” is older than layer “J,” and the rocks beneath the unconformity are older from right to left. From the “Principle of Original Horizonality,” we infer that layers “A” through “F” have been deformed.

•Sedimentary rock are deposited in successive layers that record the history of their time, much like the pages in history book. However, the rock record is never complete. Missing layers (gaps in time) result in unconformities.

• An unconformity is a surface of non-deposition or erosion that separates younger rocks from older rocks. The slide shows an angular unconformity. A nonconformity is an unconformity in which younger sedimentary rocks overlie older metamorphic or intrusive igneous rocks

The following are basic principles or laws are used to evaluate the relative ages and the relations among rock layers.

Uniformitarianism - “The present is the key to the past.” By studying modern geologic processes, we can interpret past geologic events and rock-forming processes.

Original Horizonality - “Sedimentary layers are deposited in a horizontal or nearly horizontal position.” If sedimentary layers are tilted or folded, they have been subjected to deforming stresses.

Superposition - “Younger sedimentary beds occur on top of older beds, unless they have been overturned or faulted.”

Cross-Cutting Relations - “Any geologic feature that cuts another geologic feature is younger than the feature that it cuts.”

6

Cross-Cutting Relationships

Angular Unconformity

Igneous Sill

A

B

C

D

EF

GHIJ

K

IgneousDike

7

0

50

100

150

200

250

300

350

400

450

500

550

600

0

10

20

30

40

50

60

Cry

pto

zoic

(Pre

cam

bri

an)

Phanerozoic

Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

Mill

ion

s o

f ye

ars

ago

Mill

ion

s o

f ye

ars

ago

Bill

ion

s o

f ye

ars

ago

0

1

2

3

4

4.6

Paleocene

Eocene

Oligocene

Miocene

Pliocene

PleistoceneRecent

Qu

ate

rnar

yp

eri

od

Te

rtia

ryp

eri

od

Eon Era Period Epoch

Geologic Time Chart

Pa

leoz

oic

Mes

ozo

ic

Cen

ozoi

c E

ra

Geologic Time Scale - Biostratigraphy

Triassic period

Jurassic periodPermian period

Pennsylvanian period

Mississippian period

vonian period

rian period

Evolution of cellswith nucleus

Oldestfossilcells

Oldest rocksdated on Earth

4 b.y4.6 billionyears ago

3 b.y

2 b.y

1 b.y

146 m.y208 m.y 245 m.y 290 m.y

323 m.y

363 m.y

65 m.y

57 m.y

35 m.y

23 m.y

5 m.y

0.01 millionyears ago

ERA

PERIOD

EPOCHHolocene

epoch

570 m.y

510 m.y

439 m.y

409 m.y

5

Basic Geologic Principles

Uniformitarianism - “The present is the key to the past.”

Original Horizonality - “Sedimentary layers are deposited in a horizontal or nearly horizontal position.”

Superposition - “Younger sedimentary beds occur on top of older beds, unless they have been overturned or faulted.”

Cross-Cutting Relations - “Any geologic feature that cuts another geologic feature is younger than the feature that it cuts.”

10

Classification of RocksSEDIMENTARY

Ro

ck-f

orm

ing

pro

ces

sS

ou

rce

of

ma

teri

al

IGNEOUS METAMORPHIC

Molten materials in deep crust andupper mantle

Crystallization(Solidification of melt)

Weathering anderosion of rocks

exposed at surface

Sedimentation, burial and lithification

Rocks under high temperatures

and pressures in deep crust

Recrystallization due toheat, pressure, or

chemically active fluids

The Rock Cycle

Magma

MetamorphicRock

SedimentaryRock

IgneousRock

Sediment

Heat and Pressure

Weathering,Transportationand Deposition

Weathering, Transportation,

and Deposition

Cooling and

Solidification

Mel

ting

(Crystalization)

He

atA

nd

Pre

ssur

e

(Met

amor

phi

sm)

Weath

ering,T

ransp

ortation

An d

Dep

osition

Cementation andCompaction

(Lithification)

12

Igneous RocksComprise 95% of the Earth's crust.

Originated from the solidification of molten material from deep inside the Earth.

There are two types:

•Volcanic - glassy in texture due to fast cooling.

•Plutonic - slow-cooling, crystalline rocks.

13

Igneous Rocks and Reservoirs

Igneous rocks can be part of reservoirs. Fractured granites form reservoirs in some parts of the world. Volcanic tuffs are mixed with sand in some reservoirs.

Example: Granite Wash - Elk City, Okla., Northern Alberta,CA

17

Siltstone, mudand shale

~75%

Sedimentary Rock Types

• Relative abundanceSandstone

and conglomerate~11%

Limestone anddolomite

~13%

18

Depositional EnvironmentsThe depositional environment can be: Shallow or deep water. Marine (sea) and lake or continental. This environment determines many of the reservoir

characteristics

Frigg Gas Field - North Sea

19

Depositional Environments Continental deposits are usually dunes. A shallow marine environment has a lot of turbulence hence varied grain

sizes. It can also have carbonate and evaporite formation. A deep marine environment produces fine sediments.

21

Clastic Reservoirs

Consolidated and unconsolidate sands

Porosity• Determined mainly by the packing and mixing of grains.

Permeability• Determined mainly by grain size and packing, connectivity

and shale content.

Fractures may be present.

ExampleExample

BrecciaBreccia

SandstoneSandstone

ConglomerateConglomerate

ShaleShale

Clastic Sedimentary Rocks

•Some sedimentary rock types•Breccia - Coarse-grained, angular fragments - little transport; •Conglomerate - Coarse-grained, mixture of rounded pebbles and sand ranging widely in size; well rounded pebbles imply some transport in a high energy system•Sandstone - commonly quartz, feldspar, or rock fragments; deposited in many environments•Shale - very fine grained; composed primarily of clay; deposited in low-energy environments such as lakes, bays, lagoons, of deep marine settings

24

Clastic Rocks

Clastic rocks are sands, silts and shales. The difference is in the size of the grains.

Size ??

23

Average Detrital Mineral Composition of Shale and Sandstone

Mineral Composition Shale (%) Sandstone (%)

Clay Minerals

Quartz

Feldspar

Rock Fragments

Carbonate

Organic Matter,Hematite, andOther Minerals

60

30

4

<5

3

<3

5

65

10-15

15

<1

<1

(modified from Blatt, 1982)

25

Sedimentation

27

Environment Agent Of TransportationDeposition

Sediments

Alluvial Rivers Sand, gravel, mud

Lake Lake currents, waves Sand, mud

Desert Wind Sand, dust

Glacial Ice Sand, gravel, mud

Delta River + waves, tides Sand, mud

Beach Waves, tides Sand, gravel

Shallow shelf Waves, tides Sand, mud

Deep sea Ocean currents, settling Sand, Mud

Clastic Sedimentary Environments

Organic Material =

28

Depositional Environment - Delta Sediments are transported to the basins by rivers. A common depositional environment is the delta where the river empties into the sea. A good example of this is the Mississippi (Miocene and Oligocenesands)

29

Rivers

Some types of deposition occur in rivers and sand bars. The river forms a channel where sands are deposited in

layers. Rivers carry sediment down from the mountains which is then deposited in the river bed and on the flood plains at either side.

Changes in the environment can cause these sands to be overlain with a shale, trapping the reservoir rock.

31

Sandstone Composition Framework Grains

Qtz

Qtz

Qtz

Qtz

Ankerite

Quartz

Quartz

Quartz

Qtz

Framework

Cement

Pores

Matrix

32

Scanning Electron MicrographNorphlet Formation, Offshore Alabama, USA

Pores Provide theVolume to ContainHydrocarbon Fluids

Pore Throats RestrictFluid Flow

PoreThroat

Porosity in Sandstone

33

Secondary Electron Micrograph

Jurassic Norphlet SandstoneHatters Pond Field, Alabama, USA (Photograph by R.L. Kugler)

Illite

SignificantPermeabilityReduction

Negligible PorosityReduction

Migration ofFines Problem

High IrreducibleWater Saturation

Clay Minerals in Sandstone ReservoirsFibrous Authigenic Illite

34

Secondary Electron Micrograph

Occurs as ThinCoats on DetritalGrain Surfaces

Occurs in SeveralDeeply BuriedSandstones WithHigh Reservoir Quality

Iron-Rich Varieties ReactWith Acid

Clay Minerals in Sandstone ReservoirsAuthigenic Chlorite

35

Secondary Electron Micrograph

Carter SandstoneNorth Blowhorn Creek Oil UnitBlack Warrior Basin, Alabama, USA

Significant PermeabilityReduction

High Irreducible WaterSaturation

Migration of FinesProblem

(Photograph by R.L. Kugler)

Clay Minerals in Sandstone ReservoirsAuthigenic Kaolinite

36

100

10

1

0.1

0.01 0.01

0.1

1

10

100

1000

2 6 10 14 2 6 10 14 18

Per

mea

bili

ty (

md

)

Porosity (%)

Authigenic Illite Authigenic Chlorite

(modified from Kugler and McHugh, 1990)

Effects of Clays on Reservoir Quality

37

Carbonate Reservoirs

Carbonates (limestone and dolomite) normally have a very irregular structure.

Porosity:• Determined by the type of shells, etc. and by depositional

and post-depositional events (fracturing, leaching, etc.).

Permeability:• Determined by deposition and post-deposition events,

fractures.

Fractures can be very important in carbonate reservoirs.

38

Carbonate types Chalk is a special form of limestone (CaCO3) and is

formed from the skeletons of small creatures (cocoliths).

Dolomite (CaMg(CO3)2) is formed by the replacement of some of the calcium by a lesser volume of magnesium in limestone. Magnesium is smaller than calcium, hence the matrix becomes smaller and more porosity is created. • ???

Evaporites such as Salt (NaCl) and Anhydrite (CaSO4) can also form in these environments.• ??

39

Depositional Environment Carbonates

Carbonates are formed in shallow seas containing features such as:• Reefs.• Lagoons.• Shore-bars.

40

Diagenesis The environment can also involve subsequent alterations of the rock

such as:• Chemical changes.• Diagenesis is the chemical alteration of a rock after burial. An example is the

replacement of some of the calcium atoms in limestone by magnesium to form dolomite.

• Mechanical changes - fracturing in a tectonically-active region.

42

Source Rocks Hydrocarbon originates from minute organisms in seas and

lakes. When they die, they sink to the bottom where they form organic-rich "muds" in fine sediments.

These "muds" are in a reducing environment or "kitchen", which strips oxygen from the sediments leaving hydrogen and carbon.

The sediments are compacted to form organic-rich rocks with very low permeability.

The hydrocarbon can migrate very slowly to nearby porous rocks, displacing the original formation water.

43

Hydrocarbon Migration

Hydrocarbon migration takes place in two stages:Primary migration - from the source rock to a porous rock. This is a complex process and not fully understood.It is probably limited to a few hundred metres.Secondary migration - along the porous rock to the trap.This occurs by buoyancy, capillary pressure and hydrodynamics through a continuous water-filled pore system.It can take place over large distances.

Structural Hydrocarbon Traps

SaltDiapir

Oil/WaterContact

GasOil/GasContact

Oil

ClosureOilShale Trap

Fracture Basement

(modified from Bjorlykke, 1989)

Fold Trap

Seal

OilSalt

Dome

44

Organic Matter in Sedimentary Rocks

Reflected-Light Micrographof Coal

Vitrinite

KerogenDisseminated Organic Matter inSedimentary Rocks That is Insolublein Oxidizing Acids, Bases, andOrganic Solvents.

VitriniteA nonfluorescent type of organic materialin petroleum source rocks derived primarily from woody material.

The reflectivity of vitrinite is one of thebest indicators of coal rank and thermalmaturity of petroleum source rock.

45

Interpretation of Total Organic Carbon (TOC)(based on early oil window maturity)

HydrocarbonGenerationPotential

TOC in Shale(wt. %)

TOC in Carbonates(wt. %)

Poor

Fair

Good

Very Good

Excellent

0.0- 0.5

0.5- 1.0

1.0- 2.0

2.0- 5.0

>5.0

0.0- 0.2

0.2- 0.5

0.5- 1.0

1.0- 2.0

>2.0

Magma risingMagma rising

AsthenosphereAsthenosphere

Magma Magma formingforming

• Distribution ofearthquakes

Lithosphere

Oceanic crustMid-ocean ridge

DIVERGENT BOUNDARY:Seafloor spreading

Volcanism

CONVERGENT BOUNDARY:Plate subduction

Mountain building

Continental crust

Basic Elements of Plate Tectonics

48

Wrench fault

Sedimentary Basin andStress Fields

Pull-apart Basin(Lateral Stress)

Normal fault

Thrust fault

Foreland Basin(Compressive Stress)

Rift Related Basin(Extensional Stress)

Fault Types Basin Geometries

Sedimentary Fill

50

Folded Structures

Anticline Syncline

Convex upward

?? Age

• Definitions–A fold is a bend in the strata.–An anticline is a fold that is convex upward. The oldest beds occur in the center of an anticline.

–A syncline is a fold that is concave upward. The youngest beds occur in the center of a syncline.

–A monocline (not shown) is composed of strata that dip in one direction and are not known to form a flank of an anticline.

51

Fold Terminology

Anticline

Syncline

Oldest rock

Youngest rock

Modified from xxx)

Limb

Limb

N

Limb

Faulting

54

Strike Slip Fault(Left Lateral)

N

Fault Plane

Stri

ke Dip Angle

52

H.W.

FaultScarp

Dow

nthrown

Upthrow

n

Strike direction

F.W.

Fault plane

Dip angle

Normal Fault

Key bed

Fault scarp

Dow

nthrown

Upthrow

n

F.W.

H.W.

Fault plane

Dipangle

Strike direction

Reverse Fault

Faults

Fracture: Joint and Fault

56

Geologic Reservoir Heterogeneity

57

Scales of Geological Reservoir Heterogeneity

Fie

ld W

ide

Inte

rwel

lW

ell-

Bo

re

(modified from Weber, 1986)

Hand Lens orBinocular Microscope

Unaided Eye

Petrographic orScanning Electron

Microscope

DeterminedFrom Well Logs,Seismic Lines,

StatisticalModeling,

etc.

10-100'sm

10-100'smm

1-10'sm

100'sm

10'sm

1-10 km

100's m

Well WellInterwell

Area

ReservoirSandstone

58

Hydrocarbon Traps

Structural traps

Stratigraphic traps

Combination traps

59

Traps General

Ghawar Oilfield - Saudi Arabia- Ls - 145 mi x 13 mi wide x260 ftproduces 11,000 b/d total 82B bblsGasharan Oilfield - Iran - Ls - 6000ft. Net pay total 8.5 B bbls

60

Structural Hydrocarbon Traps

SaltDiapir

Oil/WaterContact

GasOil/GasContact

Oil

ClosureOilShale Trap

Fracture Basement

(modified from Bjorlykke, 1989)

Fold Trap

Seal

OilSalt

Dome

61

Fault Traps Faults occur when the rock shears due to stresses. Reservoirs

often form in these fault zones. A porous and permeable layer may trap fluids due to its location

alongside an impermeable fault or its juxtaposition alongside animpermeable bed.

Faults are found in conjunction with other structures such as anticlines, domes and salt domes.

Normal Faults - Nigeria,Hibenia (E. Canada), VicksburgTrends (Victoria, TX)

Drag Faults - Wyoming,most Rocky Mountains

Normal or Reverse Fault???

62

Stratigraphic Traps

Point Bars - Powder River Basin, WY, Clinton SS in Western Ok,

Michigan - Belle River Mills

Devonian reefs (Barriers and Atolls) -Alberta CA. (Leduc & Redwater)

Midland Basin &Delaware Basin of West TX - Barrier Reefs

64

Petroleum Exploration-Geophysical Methods

Gravity methods

Magnetic surveys

Seismic surveys

65

-3-2-1+1

Salt2.1 gm/cm3

Corrected Gravity(Bouguer Anomaly)

UncorrectedGravity

Clastics2.4 gm/cm3

Meter

GravityValue (mgal)

Principle of Gravity Surveys

66

Principle of Magnetic Surveys

+

-

Basement

Sedimentary Basin

Magnetization

Measured

(from xxx, 19xx)

67

Seismic Surveys

The seismic tools commonly used in the oil and gas industry are 2-D and 3-D seismic data

Seismic data are used to:– Define and map structural folds and faults– Identify stratigraphic variations and map sedimentary

facies– Infer the presence of hydrocarbons

68

Pre-Drilling Knowledge Exploration

Structural information obtained from surface seismic data. Rough geological information can be provided by nearby

wells or outcrops. Approximate depths estimated from surface seismic data.

69

Sea bed

Boat

Cable with hydrophones

Sea Surface

Source(Airguns)

Sedimentary Layers

Incidentwaves Reflected

waves

Marine Acquisition System

70

Crossline 470 (East)

N S

Source

Reservoirs

Seal (unconformity)

71

Applications of Seismic Data

Make a structural model of the reservoir Delineate and map reservoir-quality rocks Establish gas/water contacts

72

VL

E 4

00

Fa

ult

- 128

00

- 12800

00621

-

- 12600

- 11400

- 12600

00821-

00821-

- 12400

-1

2000

-11800

00611-

-11600

-12400

- 126

00

- 124

00

- 130

00

00421-

00621-

00621-

- 128

00

- 12800

-124

00

-12

20

0

-11

60

0- 1

24

00

-12

40

0

-13

00

0

-12

80

0

-1260 0

W

N

m

0 3000

0 1000

ft

-11,600-12,000

-12,000-12,400

11,400-11,600

Top Misoa C-4 SandElevation (ft)Sea-level datum

-12,400-12,800

-12,800-13,200

OW

OW

O

Structural Map, VLE 196 Field

N

Structural interpretationbased on 3-D seismic and well log data

73

SeismicAmplitude

Mapof a

Horizon

Channels

Modified from Brown, 1996

3-D Seismic datadefine reservoir-quality,channel-fillsand deposits

74

Fluid Level Boundaries on 3-D Data

Modified from Brown, 1996

Not Interpreted Interpreted

Flat spot on seismic line indicates petroleum / water contact

Fault

Exercise 11. Oil forms at lower temperatures than gas. T_____ F ______

2. The law of (original horizontality, uniformitarianism, superposition) states that, in a normal sedimentary sequence, younger layers occur on top of older layers.

3. The largest division of geologic time is the (era, eon, period, epoch).

4. Hydrocarbons are most abundant in (metamorphic, igneous, sedimentary) rocks.

5. The most abundant sedimentary rock type is shale. T____ F ______

6. Name 3 clay minerals common in sandstone reservoirs

A. _____________________ B.____________________ C. _________ ___________

7. Clastic rocks are formed from the materials of older rocks by the actions of erosion, transportation and __________________.

8. Clastic rocks are sedimentary. T___ F____

9. Name two non-clastic sedimentary rocks. A.______________ B.________________

10.Alluvial, desert, delta, beach and shallow shelf sediment make the best reservoirs

T_______ F_______

81

Exercise 21. 1. Diagenesis is the chemical alteration of a rock after burial. T___ F ___

2. (Magnesium, Iron, or Sulfate) must be in the formation water in order to convert limestone to dolomite.

3. Limestone is (CaCO3 or Ca(CO3)2).

4. Dolomite is MgCaCO3 or MgCa(CO3)2.

5. Reef deposits are classified as (clastic, carbonate) sedimentary rocks.

6. The source rock must contain (organic material, coal, methane).

7. Fault and anticline traps occur only in gas wells. T___ F___

8. The oil water contact can be observed using seismic T___ F___

9. (Historical, structural, tectonic) geology addresses the occurrence and origin of smaller scale deformational features, such as folds and faults, that may be involved in hydrocarbon migration or which may form structural hydrocarbon traps.

10. Good quality sandstone reservoirs normally contain ~ (1-10 or 25-30% silt and clay).

82

N

a b

c d

23

1

4

34 4

Well

Exercise 3

Well

• Circle the correct answer or label the drawing as directed.

• 1. Figure “a” is a (normal, strike-slip, lateral) fault.• 2. If a well is drilled as shown on block “a” the target

sandstone will most likely be missing. T ___ F ___• 3. Figure “b” is a(n) (lateral fold, anticline, syncline).• 4. In Figure “b,” layer 1 = salt, 2 = sandstone, 3 = shale,

and 4=limestone. On the figure, indicate the layer that is most likely have trapped hydrocarbon.

• 5. Figure “c” is a(n) (right, left) lateral fault.• 6. On Figure “d,” the structure is a (normal fault, reverse,

strike-slip) fault.• 7. A well drilled at the location shown on Figure “d” will find

strata (repeated, missing).

83

Exercise 41. Hydrocarbons reservoirs are normally in (igneous, metamorphic,

sedimentary) rocks.

2. Fluorescence of drill cuttings or core indicates (oil, gas, water) is present.

3. Reservoir traps are (very impermeable, highly permeable).

3. What are 2 uses of seismic data in petroleum exploration and development?

1. ________________________________________________

2. _________________________________________________

4. In inclined reservoir rocks, what is the significance of a “flat spot” in seismic sections?

5. What is a 4-D seismic evaluation?

5

Basic Geologic Principles

Uniformitarianism - “The present is the key to the past.”

Original Horizonality - “Sedimentary layers are deposited in a horizontal or nearly horizontal position.”

Superposition - “Younger sedimentary beds occur on top of older beds, unless they have been overturned or faulted.”

Cross-Cutting Relations - “Any geologic feature that cuts another geologic feature is younger than the feature that it cuts.”

•The following are basic principles or laws are used to evaluate the relative ages and the relations among rock layers.

•Uniformitarianism - “The present is the key to the past.” By studying modern geologic processes, we can interpret past geologic events and rock-forming processes.•Original Horizonality - “Sedimentary layers are deposited in a horizontal or nearly horizontal position.” If sedimentary layers are tilted or folded, they have been subjected to deforming stresses.•Superposition - “Younger sedimentary beds occur on top of older beds, unless they have been overturned or faulted.”•Cross-Cutting Relations - “Any geologic feature that cuts another geologic feature is younger than the feature that it cuts.”

KLASIFIKASI BATUAN Batuan merupakan agregat padat yang terdiri dari mineral atau mineraloid, kebanyakan batuan terdiri atas beberapa jenis mineral (mineral, gelas, ubahan mineral organik, dan kombinasi dari komponen-komponen tersebut) (Ernest G. Ehlers & Harvey Blatt, 1980).

Batuan didefinisikan juga sebagai kumpulan mineral alamiah yang terkristalkan oleh ‘proses pembentukan batuan’ (Huckenholz, 1982).

• BERDASARKAN GENESA DAN KOMPOSISI

– Batuan Beku– Batuan Piroklastik– Batuan Sedimen– Batuan Metamorf

Distribusi batuan di bumi :

• Batuan beku di kerak bumi bagian atas

• Batuan sedimen di permukaan

• Batuan metamorf di inti dalam, mantel, kerak bumi bagian bawah

BATUAN BEKU

• Batuan beku adalah batuan yang terbentuk akibat membekunya magma pada waktu perjalannya menuju ke permukaan bumi.

• Hasil dari pembekuan magma tersebut membentuk berbagai jenis mineral yang mengikuti aturan tingkat diferensiasi dari magma.

• Magma adalah cairan silikat yang panas dan pijar yang terdiri atas unsur-unsur O, Si, Al, Fe, Mg, Ca, Na, K dan sebagainya.

• Komposisi batuan beku dapat dibedakan dari komposisi secara mineralogi.:

– Mineral utama (olivin, piroksen, felspar, kuarsa, plagioklas, dsb)

– Mineral tambahan: mineral yang terbentuj dari kristalisasi magma tapi kehadirannya sedikit (contoh: apatit, rutil, mineral bijih, dsb)

– Mineral sekunder: mineral hasil ubahan dari mineral-mineral primer (contoh: klorit, epidot, dll)

Kimiawi

• Unsur utama (major element): seperti unsur oksoda SiO2, Al2O3, dll.

• Unsur jejak (trace element): seperti Sr, Rb, Ba, dll.

• Unsur tanah jarang (rare earth element): seperti La, Ce, Pr, dll.

Klasifikasi batuan beku:Pada dasarnya klasifikasi batuan beku didasarkan pada

tekstur dan mineralogi.

• a. Berdasarkan tekstur:• IUGS (International Union of Geological

Sciences) membagi batuan beku berdasarkan pada besar butir:

• Batuan fanerik diklasifikasikan sebagai batuan plutonik, dimana butirannya kasar, sehingga secara individu dapat dibedakan, berbutir kasar-sedang (> 1 mm). Kristal-krital yang lebih besar (fenokris) tertanam dalam masa dasar yang lebih halus (Gambar 2). Klasifikasi batuan fanerik dilakukan oleh IUGS, 1973 (Gambar 3).

• Batuan afanitik diklasifikasikan sebagai batuan vulkanik, dimana ukuran mineralnya terlalu kecil untuk dibedakan, umumnya berbutir haus (< 1mm). Klasifikasi batuan ini dapat dilihat pada Gambar 4.

• Berdasarkan mineralogi (Gambar 5)• Dasar klasifikasi:• Komposisi (%) mineral utama• Kimiawi:

• silika (% SiO2) : ultrabasa (SiO2 < 45%)

• basa (SiO2 45 – 52%)• intermediate (SiO2 52 – 66%)• asam (SiO2 > 66%)

• alumina saturation– peralumina : jenuh terhadap alumina (Al2O3 > Na2O + K2O +CaO)– peralkaline : oksida alkalin > oksida alumina– subalumina : oksida alumina =/> oksida alkalin (Na2O + K2O)– metalumina : oksida alumina =/> Na2O + K2O +CaO

• color index proporsi mineral felsik dan mafik

• Batuan Piroklastik adalah batuan hasil letusan gunungapi. Terdiri atas material-material piroklastik, yaitu pecahan gelas/abu/debu gunungapi, kristal, lithik.

• Klasifikasi batuan piroklastik:• Pada dasarnya pembagian batuan piroklastik

didasarkan pada ukuran butir. Penamaan: tuf, tuf lapili, breksi piroklastik atau breksi vulkanik (Gambar 6). Untuk yang berbutir halus (<4 mm): tuff gelas, tuf kristal, tuf lithik (Tabel 1).

UKURAN CLAST

(PECAHAN)

PIROKLAS ENDAPAN PIROKLASTIK

NAMA BATUAN

> 64 mm Bomb Block

Lapisan bom/blok atau tefra bom/blok

Aglomerat, breksi piroklastik

2 - 64 mm Lapillus / Lapili Lapisan lapili atau tefra lapili

Lapillitone / tuf lapili

< 2 – 1/16 mm Butiran debu kasar

Debu kasar Tuf debu kasar

< 1/16 mm Butiran debu halus

Debu halus Tuf debu halus

• Selain batuan piroklastik ini juga dikenal batuan epiklastik, yaitu batuan yang terbentuk dari campuran atau rombakan material-material batuan piroklastik (vulkanik) (Gambar 7). Contoh: batupasir vulkanik, tuf pasiran, dll.

BATUAN SEDIMEN

• Batuan sedimen adalah batuan yang berasal dari rombakan batuan yang telah ada yang telah mengalami siklus sedimentasi (pelapukan-transportasi-sedimentasi-diagenesa) (Gambar 9).

• Komposisi batuan sedimen:– Fragmen mineral/batuan hasil rombakan (terigen)– Material hasil proses kimiawi (material auttigenik), contoh:

karbonat, fosfat.– Material allochem (rombakan hasil presipitasi terdahulu),

contoh: fosil, mineral organik, dll.

• Penggolongan batuan sedimen

• Batuan sedimen dapat diklasifikasikan berdasarkan beberapa cara:

» Berdasarkan proses pembentukannya (Gambar 10):» Sedimentasi mekanis, contoh batulanau,

batulempung, batupasir, dll.» Sedimentasi organis, contoh batubara, batugamping

terumbu, batugamping bioklastik, dll» Sedimentasi kimiawi, contoh batugamping kristalin,

dolomit, batugamping oolith, gips, anhidrit, dll.

? Berdasarkan asal-usulnya: Klastik terigenous Endapan biokimia –

biogenik – organik Pengendapan

kimia Volkaniklastik

Rudit, arenit, lutit Batugamping, dolomit, rijang, fosfat, batubara

Ironstones, evaporit

Tufa, aglomerat

Batuan Sedimen Berdasarkan Tekstur

• Berdasarkan teksturnya dibagi menjadi dua, yaitu yang bertekstur klastik (berdasarkan mekanisme pengendapan), dan batuan yang bertekstur non klastik (kristalin).

• Batuan Sedimen Klastik

• Terdiri atas material detritus (hasil rombakan / pecahan), memperlihatkan tekstur klastik. Ukuran butir halus – kasar (Gambar 11), dibagi berdasarkan skala yang dinyatakan oleh Wentworth (Gambar 12).

Unsur-unsur tekstur batuan sedimen klastik:

• Butiran (grain) : klastik yang tertransport yang disebut sebagai fragmen.• Matriks (masa dasar) : lebih halus dari fragmen/butiran, mengisi rongga antar fragmen,

diendapkan bersama-sama dengan fragmen.• Semen : berukuran halus, mengikat butiran/fragmen dan matriks, diendapkan ditempat

sedimentasi setelah fragmen dan matriks.• Pemilahan (sorting) : derajat kesamaan atau keseragaman butir. Dinyatakan dalam skala baik,

sedang, atau buruk.• Porositas : perbandingan volume pori terhadap volume batuan secara keseluruhan. Biasanya

dinyatakan dalam % atau dalam kualitas (baik, sedang atau buruk). Batuan dengan butir yang seragam (terpilah baik) akan mempunyai porositas yang relatif lebih besar dari batuan dengan pemilahan buruk. Clay memiliki porositas yang paing besar, lalu batupasir dan kemudian breksi atau konglomerat.

• Kebundaran : menyatakan kebundaran atau ktajaman butiran yang mencerminkan tingkat abrasi selama transportasi. Merupakan sifat permukaan dari butiran yang disebabkan oleh pengaruh transportasi terhadap butiran.

• Kemas (fabric) : merupakan sifat hubungan antar butir sebagai fungsi orientasi atau packing. Dinyatakan dalam skala terbuka (kontak antar butiran tidak bersentuhan) dan tertutup (kontak antar butiran saling bersentuhan).

• Permeabilitas : kemampuan batuan meloloskan fluida, yang mencerminkan poriyang saling berhubungan. Batupasir merupakan batuan dengan permeabilitas yang baik, sedangkan clay walaupun memiliki porositas baik tapi permeabilitasnya yang buruk. Karena mineral dalam clay termasuk kedalam minera pirosilika yang bersifat konduktif, sehingga clay ini mengikat kation yang akan mengikat OH. Oleh karena itu clay memiliki sifat swelling (dapat mengembang bila terkena air), yang menyebabkan resistivity dari clay ini sangat rendah (Gambar 13).

• Struktur sedimen : penyimpangan dari bidang perlapisan. Struktur sedimen ini mencerminkan mekanisme yang mempengaruhi pengendapan batuan sedimen. Contoh: strutur sedimen pada mekanisme arus turbidit yang dinyatakan oleh Bouma dalam Sikuen Bouma.

Batuan Sedimen Non-Klastik

• Umumnya tersusun atas mineral autigenik (terbentuk di tempat sedimentasi). Pada P dan T tertentu seringkali memperlihatkan gejala diagenesa, akibatnya porositas batuan menjadi sangat rendah atau bakhan tidak ada. Porositas primer rendah dan memperlihatkan tekstur mozaik (contoh batugamping). Kadang-kadang terdapat butiran yang amorf (seperti kalsedon dan opal) sebagai semen.

Batuan Sedimen Kimiawi

• Terbentuk akibat peranan/pengaruh proses-proses kimia dari larutan. Terdiri atas batuan karbonat dan batuan evaporit.

Batuan Karbonat

• Batuan karbonat adalah batuan sedimen yang mempunyai komposisi garam-garam karbonat yang dominan (> 50%). Proses pembentukannya dapat secara insitu, berasal dari larutan yang mengalami proses kimiawi maupun biokimiawi.

• Komposisi kimia dan mineralogi batuan karbonat:

» Aragonit (CaCO3 orthorombik)» Kalsit (CaCO3 hexagonal)» Dolomit (CaMg(CO3)2)» Magnesit (Mg CO3)

Porositas batuan karbonat:

• Ada dua macam klasifikasi porositas dalam batuan karbonat:

• menurut Murray (1960) merupakan klasifikasi berdasarkan pada genesa, dibagi menjadi:– Porositas primer : terbentuk pada saat sedimentasi

berlangsung. Terdiri atas porositas kerangka frame-work porosity), porositas lumpur (mud porosity), dan porositas pasir (sand porosity).

– Porositas sekunder : terbentuk setelah pengendapan, akibat pelarutan, rekahan atau perubahan yang terjadi setelah proses sedimentasi.

– Sucrose dolomite porosity : terbentuk sebagai akibat adanya penggantian kalsit oleh dolomit.

• menurut Choquette anfd Pray (1970) merupakan klasifikasi deskriptif dan genetik. Unsur-0unsurnya terdiri atas:

– Basic porosity types:• fabric selective : interpartikel, intrapartikel,

interkristalin, moldic, fenestral, shelter, growth framework.

• Non fabric selective : fracture, channel, vuggy, cavern

• Fabric selective or not : breccia, boring, burrow, shrinkage.

– Modifying terms : genetic modifiers, size modifiers, abundance modifiers.

Klasifikasi batuan karbonat

• Klasifikasi dalam batuan karbonat antara lain dikemukakan oleh Grabau (1913), Folk (1953), Pettijohn (1957), Dunham (1962), Embry and Klovan (1972), dll.

• Klasifikasi yang banyak digunakan dalam penggolongan batuan karbonat adalah klasifikasi menurut Dunham, dan Embry and Klovan, karena klasifikasi ini cukup sederhana dan mudah dalam pemnakaiannya.

Klasifikasi Dunham (1962)

• Klasifikasi ini didasarkan pada tekstur pengendapan (Gambar 17). Faktor yang penting dalam klasifikasi ini adalah:

• Butiran didukung lumpur (mud supported)• Butiran saling menyangga (grain

supported)• Sebagian butiran didukung lumpur,

sebagian butiran saling menyangga (parteil)

Klasifikasi Embry and Klovan (1972)

• Merupakan modifikasi dari klasifikasi Dunham, didasarkan pada terdapatnya lumpur diantara kerangka atau pecahan kerangka (Gambar 14).

Batuan Evaporit

• Merupakan batuan garam yang terbentuj jarena evaporasi air laut.. Mineral penyusunnya bersifat monomineralik, antara lain: garam (CaSO4 2H2O), anhidrit (CaSO4), dan halit (NaCl)

BATUAN METAMORF

• Batuan metamorf adalah batuan yang terbentuk akibat proses perubahan tekanan (P) dan temperatur (T) atau keduanya, dimana batuan memasuki kesetimbangan baru tanpa adanya perubahan komposisi kimia (isokimia) dan tanpa melalui fasa cair (dalam keadaan padat) dengan temperatur berkisar 200-800º C.

Perubahan yang terjadi dalam proses metamorfosa: perubahan tekstur dan struktur (yang merefleksikan sejarah pembentukkannya); dan asosiasi mineral.

Struktur batuan metamorf:• Struktur foliasi (schistosity) struktur paralel

yang ditimbulkan oleh mineral pipih/mineral prismatik, seringkali terjadi pada metamorfosa regional dan metamorfosa kataklastik.

• Struktur non foliasi struktur yang dibentuk oleh mineral-mineral yang equidimensional, seringkali terjadi pada metamorfosa termal.

Beberapa struktur batuan metamorf:

Yang bersifat foliasi:

• Slaty cleavage planar, dijumpai bidang belah batu sabak/slate.• Filitik rekristalisasi lebih kasar dari slaty cleavage.• Shistose struktur perulangan dari mineral pipih dan mineral

granular dimana mineral pipih orientasinya menerus (tidak terputus).• Gneisose struktur perulangan dari mineral pipih dan mineral

granular dimana mineral pipih orientasinya terputus, sering disebut close schistosity.

• Milonitik menunjukan goresan-goresan akibat penggerusan yang kuat.

• Filonitik gejala dan kenampakan sama dengan milonitik, hanya disini butirannya lebih halus.

Yang bersifat non foliasi:

• Granulose terdiri atas mineral granular• Hornfelsik identik dengan granoblastik, tapi mineral

equidimensional. Lepidoblastik terdiri atas mineral pipih/tabular

• Nematoblastik terdiri atas mineral prismatik• Granoblastik terdiri atas mineral granular• Homeoblastik terdiri atas satu tekstur saja• Heteroblastik terdiri atas beberapa tekstur• Relic (sisa) tekstur sisa yang terbentuk sebelum

metamorfosa• Kristaloblastik setiap tekstur yang terbentuk pada saat

metamorfosa• Awalan “meta” bila masih dikenali sifat batuan

asalnya, seperti metasedimen, metavolkanik, dll.

CONTINENTAL SEDIMENTARY ENVIRONMENTS Copyright 1998 Pamela J . W. Gore

ALLUVIAL FAN FLUVIAL LACUSTRINE DESERT (DUNES) PALUDAL

Rock Type Breccia, conglomerate, arkose

Conglomerate, sandstone, siltstone, shale

Siltstone, shale, limestone, or evaporites (gypsum)

Quartz arenite (sandstone) or gypsum

Peat, coal, black shale, siltstone

Composition Terrigenous Terrigenous Terrigenous, carbonate, or evaporite

Terrigenous or evaporite

Terrigenous

Color Brown or red Brown or red Black, brown, gray, green Yellow, red, tan, white

Black, gray, or brown

Grain Size Clay to gravel Clay to gravel (Fining upward)

Clay to silt or sand (Coarsening upward)

Sand Clay to silt

Grain Shape Angular Rounded to angular

--- Rounded ---

Sorting Poor Variable Variable Good Variable

Inorganic Sedimentary Structures

Cross-bedding and graded bedding

Asymmetrical ripples, cross-bedding, graded bedding, tool marks

Symmetrical ripples, lamination, cross-bedding, graded bedding, mudcracks, raindrop prints

Cross-bedding Laminated to massive

Organic or Biogenic Sedimentary Structures

--- Tracks, trails,burrows

Tracks, trails, burrows, rare stromatolites

Tracks, trails Root marks, burrows

Fossils --- Rare freshwater shells, bones, plant fragments

Freshwater shells, fish, bones, plant fragments

--- Plant fossils, rare freshwater shells, bones, fish

MARINE SEDIMENTARY ENVIRONMENTS Copyright 1998 Pamela J . W. Gore

REEF CONTINENTAL SHELF

CONTINENTAL SLOPE AND RISE

ABYSSAL PLAIN

Rock Type Fossiliferous limestone

Sandstone, shale, siltstone, fossiliferous limestone, oolitic limestone

Litharenite, siltstone, and shale (or limestone)

Shale, chert, micrite, chalk, diatomite

Composition Carbonate Terrigenous or carbonate

Terrigenous or carbonate Terrigenous or carbonate

Color Gray to white Gray to brown Gray, green, brown Black, white red

Grain Size Variable, frameworks, few to no grains

Clay to sand Clay to sand Clay

Grain Shape --- --- --- ---

Sorting --- Poor to good Poor Good

Inorganic Sedimentary Structures

--- Lamination, cross-bedding

Graded bedding, cross-bedding, lamination, flute marks, tool marks (turbidites)

Lamination

Organic or Biogenic Sedimentary Structures

--- Trails, burrows Trails, burrows Trails, burrows

Fossils Corals, marine shells

Marine shells Marine shells, rare plant fragments Marine shells (mostly microscopic)

TRANSITIONAL SEDIMENTARY ENVIRONMENTS Copyright 1998 Pamela J . W. Gore

DELTA BARRIER BEACH LAGOON TIDAL FLAT

Rock Type Sandstone, siltstone, shale, coal

Quartz arenite, coquina Siltstone, shale, limestone, oolitic limestone or gypsum

Siltstone, shale, calcilutite, dolostone or gypsum

Composition Terrigenous Terrigenous or carbonate

Terrigenous, carbonate, or evaporite

Terrigenous, carbonate, or evaporite

Color Brown, black, gray, green, red

White to tan Dark gray to black Gray, brown, tan

Grain Size Clay to sand (Coarsening upward

Sand Clay to silt Clay to silt

Grain Shape --- Rounded to angular --- ---

Sorting Poor Good Poor Variable

Inorganic Sedimentary Structures

Cross-bedding, graded bedding

Cross-bedding, symmetrical ripples

Lamination, ripples, cross-bedding

Lamination, mudcracks, ripples, cross-bedding

Organic or Biogenic Sedimentary Structures

Trails, burrows Tracks, trails, burrows Trails, burrows Stromatolites, trails, tracks, burrows

Fossils Plant fragments, shells Marine shells Marine shells Marine shells