Klasifikasi Batuan

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

NASA PHOTO

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

20 mi NASA PHOTO 78

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 2

Cross Section Of A Petroleum System (Foreland Basin Example) Geographic Extent of Petroleum System Extent of Play Reservoir

Stratigraphic Extent of Petroleum System

Active Source Rock

Essential Elements of Petroleum System

Overburden Rock Seal or Cap[Rock Reservoir Rock Source Rock Underburden Rock

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

Sedimentary Basin Fill

R

Basement Rock Top Oil Window Top Gas Window

(modified from Magoon and Dow, 1994) 4

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

These Include: Seal or cap rock Reservoir rock Migration Mature source 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.”

Cross-Cutting Relationships K J I H G Angular Unconformity

C

E D il S s ou e Ign

l

Igneous Dike

F B A

6

4

4.6

150

Mesozoic

100

Cretaceous

Jurassic

200

Triassic

250

Permian

300

Pennsylvanian

Recent

0 Pleistocene 10 20

Pliocene Miocene

30 Oligocene 40

Eocene

Cenozoic Era

3

Tertiary

50

50 60 Paleocene

Mississippian

350 400 450

Paleozoic

1

Millions of years ago

Phanerozoic

2

Quaternary

0

Cryptozoic (Precambrian)

Billions of years ago

0

Epoch

Tertiary period

Era Period

Millions of years ago

Eon

Quaternary period

Geologic Time Chart

Devonian Silurian

Ordovician

500 550

Cambrian

600 7

Geologic Time Scale - Biostratigraphy Triassic period

Permian period

Jurassic period

Pennsylvanian period Mississippian period

146 m.y

208 m.y

245 m.y

290 m.y

323 m.y 363 m.y

vonian period 409 m.y

rian period

439 m.y 65 m.y

1 b.y

57 m.y 510 m.y

570 m.y

Evolution of cells with nucleus

35 m.y

23 m.y

2 b.y 5 m.y

3 b.y 0.01 million years ago

4.6 billion years ago

ERA PERIOD EPOCH Holocene epoch

Oldest fossil cells

4 b.y

Oldest rocks dated on Earth

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.”

5

Classification of Rocks

Rock-forming Source of material process

IGNEOUS

SEDIMENTARY

METAMORPHIC

Molten materials in deep crust and upper mantle

Weathering and erosion of rocks exposed at surface

Rocks under high temperatures and pressures in deep crust

Crystallization (Solidification of melt)

Sedimentation, burial and lithification

Recrystallization due to heat, pressure, or chemically active fluids

10

The Rock Cycle Magma el

t

g in

M

nd

Co So oling ( C r lid i ys fic a ta liz at a

n io n) t io

(M

Sedimentary Rock

Heat and Pressure We

ath

eri n an g, T d D ran ep spo osi tion rtatio n,

Weathering, Transportation and Deposition Cem entation and Compaction

(Lithification)

Igneous Rock W eat her T ransportai ng, t i on A d D n e p o si ti o n

A nd H e at ure Press or phism) et am

Metamorphic Rock

Sediment

Igneous Rocks

Comprise 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.

12

  

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

13

Sedimentary Rock Types • Relative abundance

Sandstone and conglomerate ~11% Limestone and dolomite ~13%

Siltstone, mud and shale ~75%

17

Depositional Environments The 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 18

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.

19

Clastic Reservoirs 

Consolidated and unconsolidate sands



Porosity •



Permeability •



Determined mainly by the packing and mixing of grains.

Determined mainly by grain size and packing, connectivity and shale content.

Fractures may be present.

21

Clastic Sedimentary Rocks Conglomerate

Breccia

Example

Sandstone

Shale

•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

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

Size ?? 24

Average Detrital Mineral Composition of Shale and Sandstone Mineral Composition Shale (%)

Sandstone (%)

Clay Minerals

60

5

Quartz

30

65

4

10-15

<5

15

3

<1

<3

<1

Feldspar Rock Fragments Carbonate Organic Matter, Hematite, and Other Minerals

(modified from Blatt, 1982) 23

Sedimentation

25

Clastic Sedimentary Environments Environment

Agent Of Transportation Deposition

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

Organic Material = 27

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 Oligocene sands)

28

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. 29

Sandstone Composition Framework Grains Matrix Framework

Qtz Quartz

Qtz Quartz

Pores Qtz Qtz Qtz Quartz

Ankerite

Cement 31

Porosity in Sandstone Pore Throat

Pores Provide the Volume to Contain Hydrocarbon Fluids Pore Throats Restrict Fluid Flow

Scanning Electron Micrograph Norphlet Formation, Offshore Alabama, USA

32

Clay Minerals in Sandstone Reservoirs Fibrous Authigenic Illite Secondary Electron Micrograph Significant Permeability Reduction

Illite

Negligible Porosity Reduction High Irreducible Water Saturation Migration of Fines Problem

Jurassic Norphlet Sandstone Hatters Pond Field, Alabama, USA

(Photograph by R.L. Kugler) 33

Clay Minerals in Sandstone Reservoirs Authigenic Chlorite Secondary Electron Micrograph Iron-Rich Varieties React With Acid Occurs in Several Deeply Buried Sandstones With High Reservoir Quality Occurs as Thin Coats on Detrital Grain Surfaces

34

Clay Minerals in Sandstone Reservoirs Authigenic Kaolinite Secondary Electron Micrograph

Significant Permeability Reduction High Irreducible Water Saturation

Migration of Fines Problem

Carter Sandstone North Blowhorn Creek Oil Unit Black Warrior Basin, Alabama, USA

(Photograph by R.L. Kugler)

35

Effects of Clays on Reservoir Quality Authigenic Chlorite

Authigenic Illite Permeability (md)

100

1000 100

10

10 1 1 0.1

0.1 0.01

0.01 2

6

10

14

2

6

10

14

18

Porosity (%) (modified from Kugler and McHugh, 1990) 36

Carbonate Reservoirs 



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



Permeability: •



Determined by the type of shells, etc. and by depositional and post-depositional events (fracturing, leaching, etc.).

Determined by deposition and post-deposition events, fractures.

Fractures can be very important in carbonate reservoirs.

37

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. •

?? 38

Depositional Environment Carbonates



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

39

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.

•

40

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.

42

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. 

43

Structural Hydrocarbon Traps Shale

Oil

Trap

Sea l

Oil/Gas Contact

Gas

Closure

Oil/Water Contact Oil

Fracture Basement

Salt Dome

Fold Trap

Salt Diapir

Oil

(modified from Bjorlykke, 1989)

Organic Matter in Sedimentary Rocks Kerogen

Vitrinite

Disseminated Organic Matter in Sedimentary Rocks That is Insoluble in Oxidizing Acids, Bases, and Organic Solvents.

Vitrinite A nonfluorescent type of organic material in petroleum source rocks derived primarily from woody material. The reflectivity of vitrinite is one of the best indicators of coal rank and thermal maturity of petroleum source rock.

Reflected-Light Micrograph of Coal

44

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

TOC in Shale (wt. %)

TOC in Carbonates (wt. %)

Poor

0.0 - 0.5

0.0 - 0.2

Fair

0.5 - 1.0

0.2 - 0.5

Good

1.0 - 2.0

0.5 - 1.0

Very Good

2.0 - 5.0

1.0 - 2.0

>5.0

>2.0

Excellent

45

Basic Elements of Plate Tectonics DIVERGENT BOUNDARY: Seafloor spreading Mid-ocean ridge

CONVERGENT BOUNDARY: Plate subduction

Mountain building

Lithosphere

Magma rising

Asthenosphere Magma forming

• Distribution of earthquakes

Continental crust

Volcanism

Oceanic crust

Sedimentary Basin and Stress Fields Basin Geometries

Fault Types Rift Related Basin (Extensional Stress) Normal fault

Sedimentary Fill

Foreland Basin (Compressive Stress) Thrust fault Pull-apart Basin (Lateral Stress) Wrench fault 48

Folded Structures Convex upward

?? Age

Anticline

Syncline

50

• 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.

Fold Terminology N b

m Li

m Li

b

Li

b

m

Anticline Syncline Modified from xxx)

Youngest rock Oldest rock

51

Faulting

Strike Slip Fault (Left Lateral)

Dip Angle

St

rik e

N

Fault Plane 54

Faults Normal Fault

Reverse Fault Strike direction

Strike direction

Key bed

F.W.

H.W.

rown

F.W.

Upth

n ow thr

own nthr

arp

Dow

n row

Sc

wn Do

ult

Fa

th Up

Fault scarp

Dip angle

H.W.

Dip angle Fault plane

Fracture: Joint and Fault

Fault plane

52

Geologic Reservoir Heterogeneity

56

Scales of Geological Reservoir Heterogeneity Interwell Area

Field Wide

Well

Well

Determined From Well Logs, Seismic Lines, Statistical Modeling, etc.

100's m

Interwell

1-10 km

Reservoir Sandstone

10's m

Well-Bore

100's m

10-100's µm

Petrographic or Scanning Electron Microscope

1-10's m

10-100's mm

Hand Lens or Binocular Microscope

Unaided Eye

(modified from Weber, 1986) 57

Hydrocarbon Traps



Structural traps



Stratigraphic traps



Combination traps

58

Traps General

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

59

Structural Hydrocarbon Traps Shale

Oil

Trap

Sea l

Oil/Gas Contact

Gas

Closure

Oil/Water Contact Oil

Fracture Basement

Salt Dome

Fold Trap

Salt Diapir

Oil

(modified from Bjorlykke, 1989) 60

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 an impermeable bed. Faults are found in conjunction with other structures such as Normal Reverse Fault??? anticlines, domes and salt or domes.

Drag Faults - Wyoming, most Rocky Mountains Normal Faults - Nigeria, Hibenia (E. Canada), Vicksburg Trends (Victoria, TX)

61

Stratigraphic Traps Michigan - Belle River Mills Devonian reefs (Barriers and Atolls) Alberta CA. (Leduc & Redwater) Midland Basin &Delaware Basin of West TX - Barrier Reefs

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

62

Petroleum ExplorationGeophysical Methods 

Gravity methods



Magnetic surveys



Seismic surveys

64

Principle of Gravity Surveys Uncorrected Gravity +1 Gravity -1 Value (mgal) -2 -3

Corrected Gravity (Bouguer Anomaly) Meter

Clastics 2.4 gm/cm3

Salt 2.1 gm/cm3

65

Principle of Magnetic Surveys

Sedimentary Basin Basement

+

-

Magnetization Measured (from xxx, 19xx)

66

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

67

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.

68

Marine Acquisition System Boat Sea Surface Source (Airguns) Incident waves

Cable with hydrophones

Reflected waves

Sea bed Sedimentary Layers

69

Crossline 470 (East) N

S Seal (unconformity) Reservoirs

Source

70

Applications of Seismic Data   

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

71

- 12 6 0

0

Structural Map, VLE 196 Field 2

6

0

0

-1

-1

28 0 0

30 00

-1

- 12600

W

-1

22 0

0

-1 2 4 0 0

O

-1 2 4 0 0

Structural interpretation based on 3-D seismic and well log data

-1 1 6 0 0

-1

00

2

4

0

0

26 -1

24

1400

-1

00

-1

0 -12

Top Misoa C-4 Sand Elevation (ft) N

00

00

- 12400

-1

30

00

-12,800 -13,200

00

N

W

-12,400 -12,800

O

-12,000 -12,400

t F a u l 4 0 0 V L E

-11,600 -12,000

-1 26

- 11 6 0 0

- 11800

11,400 -11,600

40

0

60

0

W

O

12

- 1 2

0 40

3000 ft 1000 m

-

-1 2

0

0

- 11 60 0

Sea-level datum

0

80

- 1

28

12

72 00 - 128

Channels

Seismic Amplitude Map of a Horizon 3-D Seismic data define reservoirquality,channel-fill sand deposits

Modified from Brown, 1996

73

Fluid Level Boundaries on 3-D Data Not Interpreted

Flat spot on seismic line indicates petroleum / water contact

Interpreted

Fault Modified from Brown, 1996

74

Exercise 1 1. Oil forms at lower temperatures than gas. T_____ F ______ 2.The law of (original horizontality, uniformitarianism, superpos ition) states that, in a normal sedimentary sequence, younger layers occur on top of olders.layer 3.The largest division of geologic time is the (era, eon, period, epoch). 4.Hydrocarbons are most abundant in (metamorphic, igneous, sedimen tary) rocks. 5. The most abundant sedimentary rock type is shale. T____ F__ ____ 6. Name 3 clay minerals common in sandstone reservoirs A. _____________________ B.____________________ 7.

C. _________ ___________

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

8.

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

9.

Alluvial, desert, delta, beach and shallow shelf sediment make he best t reservoirs

10.

T_______ F_______

Exercise 2 1.

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).

81

Exercise 3 N 4

Well 4

3

3

4

2 1

a

b Well

c

d 82

• 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).

Exercise 4 1.

2.

3. 3.

4.

5.

Hydrocarbons reservoirs are normally in (igneous, metamorphic, sedimentary) rocks. Fluorescence of drill cuttings or core indicates (oil, gas, water) is present. Reservoir traps are (very impermeable, highly permeable). What are 2 uses of seismic data in petroleum exploration and development? 1.

________________________________________________

2.

_________________________________________________

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

83

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.”

5

•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) > 64 mm

PIROKLAS Bomb Block

2 - 64 mm

Lapillus / Lapili

< 2 – 1/16 mm

Butiran debu kasar Butiran debu halus

< 1/16 mm

ENDAPAN PIROKLASTIK

NAMA BATUAN

Lapisan bom/blok atau tefra bom/blok Lapisan lapili atau tefra lapili Debu kasar

Aglomerat, breksi piroklastik

Debu halus

Tuf debu halus

Lapillitone / tuf lapili Tuf debu kasar

• 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-transportasisedimentasi-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 Rudit, arenit, lutit

Endapan biokimia – Pengendapan biogenik – organik kimia Batugamping, Ironstones, dolomit, rijang, evaporit fosfat, batubara

Volkaniklastik 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). Kadangkadang 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 framework 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 ALLUVIAL FAN

 1998 Pamela J. W. Gore

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, crossbedding, 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 REEF

 1998 Pamela J. W. Gore

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, crossbedding

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 DELTA

 1998 Pamela J. W. Gore

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, crossbedding

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