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Petroleum Geology

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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

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

These Include:

Mature source rockMigrationReservoir rockSeal or cap rock

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Cross Section Of A Petroleum System

Overburden Rock

Seal or Cap[Rock

Reservoir Rock

Source Rock

Underburden Rock

Basement Rock

Top Oil WindowTop 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

Sedi

men

tary

Bas

in F

ill

R

Stratigraphic Extent of

PetroleumSystem

ActiveSource Rock

Extent of Play

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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.”

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Cross-Cutting Relationships

Angular Unconformity

Igneous Sill

A

B

C

D

EF

GHIJK

IgneousDike

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Fall 2003 EASA-123 Intro to Earthquakes Lecture-3 17

Mechanical Layers:

1. Lithosphere

2. Asthenosphere

3. Mesosphere

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0

50

100

150

200

250

300

350

400

450

500

550

600

0

10

20

30

40

50

60

Cry

ptoz

oic

(Pre

cam

bria

n)

Phanerozoic

Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

PermianPennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

Mill

ions

of y

ears

ago

Mill

ions

of y

ears

ago

Bill

ions

of y

ears

ago

0

1

2

3

4

4.6

Paleocene

Eocene

Oligocene

Miocene

PliocenePleistoceneRecent

Qua

tern

ary

perio

dTe

rtia

rype

riod

Eon Era Period Epoch

Geologic Time Chart

Pal

eozo

icM

esoz

oic

Cen

ozoi

c Er

a

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Rocks

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Classification of RocksSEDIMENTARY

Roc

k-fo

rmin

gpr

oces

sSo

urce

of

mat

eria

l

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

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The Rock Cycle

Magma

MetamorphicRock

SedimentaryRock

IgneousRock

Sediment

Heat and Pressure

Weathering,Transportationand Deposition

Weathering, Transportation,

and Deposition

Cooling and

SolidificationMelt

ing (Crystalization)

Hea

tAnd

Pres

sure

(Met

amor

phis

m)

Weathering,

TransportationAnd

Deposition

Cementation andCompaction(Lithification)

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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.

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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

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Metamorphic Rocks 2) Metamorphic rocks formed by the action of temperature and/or pressure on

sedimentary or igneous rocks.

Examples are• Marble - formed from limestone• Hornfels - from shale or tuff• Gneiss - similar to granite but formed by metamorphosis

Field Example:

1. Point Arguello - Monterey Formation is actually layers of fractured Chert and Shale. Oil is in the fractures

2. Long Beach, Calif. - Many SS producers on an Anticline above fractured Metamorphic basement rock

3. Austin, TX eastward - Lava flows of Basalt (Serpentine) from Volcanoes in ancient Gulf of Mexico

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Sedimentary Rocks These are the most important for the oil industry as it contains

most of the source rocks and cap rocks and a majority of the reservoirs.

Sedimentary rocks come from the debris of older rocks and are split into two categories

Clastic and Non-clastic.• Clastic rocks - formed from the materials of older rocks by the

actions of erosion, transportation and deposition.• Non-clastic rocks - from chemical or biological origin and then

deposition.

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Rock ClassificationClasticsRock type Particle diameter Conglomerate Pebbles 2 - 64mm Sandstone Sand .06 - 2mm Siltstone Silt .004 - .06mm or 4 to 65 microns Shale Clay < .004mm or 4 microns

Non-ClasticsRock type Composition Limestone CaCO3 Dolomite CaMg(CO3)2 Salt NaCl Anhydrite CaSO4 Gypsum CaSO4.2H2O Coal Carbon

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Siltstone, mudand shale

~75%

Sedimentary Rock Types• Relative abundance

Sandstoneand conglomerate

~11%

Limestone anddolomite

~13%

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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

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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.

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Depositional Environments The depositional characteristics of the rocks lead to some

of their properties and the reservoir property.• The reservoir rock type clastic or non-clastic.• The type of porosity (especially in carbonates) is determined

by the environment plus subsequent events. The structure of a reservoir can also be determined by

deposition; a river, a delta, a reef etc. This can also lead to permeability and producibility of

these properties are often changed by further events.

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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.

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Clastic Sedimentary Rocks

BrecciaBreccia

SandstoneSandstone

ConglomerateConglomerate

ShaleShale

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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)

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Clastic Rocks

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

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Sedimentation

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Sedimentation

Sedimentary muds become sedimentary rocks.• Calcareous muds become limestone.• Sands become sandstone.

Grains in the matrix and the fluids reacting to create new minerals changing the matrix and porosity. Fluids can also change creating a new set of minerals.

This whole process is called Diagenesis.

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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

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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)

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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.

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Fan Deposition

Example Alluvial sedimentation

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Sandstone Composition Framework Grains

Qtz

Qtz

Qtz

Qtz

Ankerite

Quartz

Quartz

Quartz

Qtz

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Scanning Electron MicrographNorphlet Formation, Offshore Alabama, USA

Pores Provide theVolume to ContainHydrocarbon Fluids

Pore Throats RestrictFluid Flow

PoreThroat

Porosity in Sandstone

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100

10

1

0.1

0.01 0.01

0.1

1

10

100

1000

2 6 10 14 2 6 10 14 18

Perm

eabi

lity

(md)

Porosity (%)

Authigenic Illite Authigenic Chlorite

(modified from Kugler and McHugh, 1990)

Effects of Clays on Reservoir Quality

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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.

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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 by magnesium. 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.

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Depositional Environment Carbonates

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

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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.

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Hydrocarbon Generation, Migration, and Accumulation

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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.

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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.

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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.

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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

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Plate Tectonics and Structural Geology

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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

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Structural Features

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Folded Structures

Anticline Syncline

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Fold Terminology

Anticline

SynclineOldest rock

Youngest rock

Modified from xxx)

Limb

Limb

N

Limb

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H.W.

FaultScarp

Downthrown

Upthrown

Strike direction

F.W.

Fault planeDip angle

Normal Fault

Key bed

Fault scarpD

ownthrow

n

Upthrow

n

F.W.

H.W.

Fault plane

Dipangle

Strike direction

Reverse Fault

Faults

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Faulting (normal faults)

Example Kabab Canyon, Utah

Photograph by XXX

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Strike Slip Fault(Left Lateral)

N

Fault Plane

Strik

e Dip Angle

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Heterogeneity

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Geologic Reservoir Heterogeneity

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Scales of Geological Reservoir Heterogeneity

Fiel

d W

ide

Inte

rwel

lW

ell-B

ore

(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

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Hydrocarbon Traps

Structural traps

Stratigraphic traps

Combination traps

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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

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Structural Hydrocarbon Traps

SaltDiapir

Oil/WaterContact

GasOil/GasContact

Oil

ClosureOilShale Trap

Fracture Basement

(modified from Bjorlykke, 1989)

Fold Trap

Seal

OilSalt

Dome

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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

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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

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Petroleum Exploration: Geophysical Application to Petroleum Geology

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Petroleum Exploration-Geophysical Methods

Gravity methods

Magnetic surveys

Seismic surveys

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-3-2-1+1

Salt2.1 gm/cm3

Corrected Gravity(Bouguer Anomaly)

UncorrectedGravity

Clastics2.4 gm/cm3

Meter

GravityValue (mgal)

Principle of Gravity Surveys

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Principle of Magnetic Surveys

+

-

Basement

Sedimentary Basin

Magnetization

Measured

(from xxx, 19xx)

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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

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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.

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Sea bed

Boat

Cable with hydrophones

Sea Surface

Source(Airguns)

Sedimentary Layers

Incidentwaves Reflected

waves

Marine Acquisition System

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Crossline 470 (East)

N S

Source

ReservoirsSeal (unconformity)

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Applications of Seismic Data

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

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SeismicAmplitude

Mapof a

Horizon

Channels

Modified from Brown, 1996

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

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4-D Seismic Surveys

The “4” in 4-D seismic is time A 4-D survey means that at least two 3-D seismic

surveys have been made at different times over the same field

Reflection character (attributes) change through time

These changes result from migration of the water contact in the reservoir

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Exercises:Petroleum Geology

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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_______

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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).

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N

a b

c d

23

1

4

34 4

Well

Exercise 3

Well

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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?