Lecture 2 Hydrocarbon Habitat

Fundamentals of Petroleum Geoscience Lecture 2: Hydrocarbon Habitat

Hydrocarbon Habitat

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• The five elements of a Petroleum System– Source

• Maturation• Migration

– Reservoir– Seal – Trap– Timing

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Petroleum System Definition

Genetically-related hydrocarbons whose provenance is a source rock.

Source RockMigration RouteReservoir RockSeal RockTrap

Elements

GenerationMigrationAccumulationPreservation

Processes

Timing

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Critical Risk Elements

Trap– Geometry– Seal Presence– Seal Effectiveness

Reservoir– Presence– Effectiveness

Timing

Charge– Source Presence– Source Effectiveness

• Maturity• Migration

Accumulation

Generation

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Seal

Structure/TrapReservoir

Petroleum charge

Migration

Source Rock

Reservoir Quality In-reservoir changes

Petroleum expelled with time

Migration

ReservoirTrap

Timing

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Petroleum System Elements

24803

Source RockSource Rock

Top Seal RockTop Seal Rock

Reservoir RockReservoir Rock

Anticlinal TrapAnticlinal Trap

(Organic Rich)(Organic Rich)

(Impermeable(Impermeable))

((Porous/Permeable)Porous/Permeable)PotentialPotentialMigration RouteMigration Route

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• Kerogen converted into Hydrocarbons– Process = Maturation

• > Measure by GCMS or reflectivity

• Hydrocarbons are expelled– Driver = overpressure due to volume expansion and bouyancy

• Petroleum migrates laterally on reaching carrier bed– Bouyancy vs capillary forces

Generation and Migration

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Source Rocks• These are rocks that are capable of generating hydrocarbons.

• Oil and gas is released from organic matter within mudstones, coals or carbonates, when the rock is buried and subjected to increased heat and pressure.

• Good source rocks contain a high concentration of organic matter. The measure of a rocks organic richness is commonly called its “Total Organic Carbon” content or TOC.

• A good source rock has a TOC in excess of 1.0%

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Source Rocks• The nature of the hydrocarbons generated will depend on both the

type of organic matter a source rock contains, and the degree ofheat / pressure the rock is subjected to on burial.

• Source rocks rich in land plant material are generally “gas prone”

• Source rocks rich in marine algae are generally “oil” prone”

• With increased temperature and pressure, source rocks generate more gas

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Organic-Rich

Thin Laminae

Measured Values

3.39 378

Total Organic Carbon

2.24 12.80

In-PlacePetroleum

S1

LOMPOC Quarry SampleMonterey Formation, CA

HydrogenIndex

PyrolyticallyGeneratedPetroleum

S2

1 Inch1 Inch

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Kerogen Types

TOC 2.12 WT.% TOC .38 WT.%

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Types of Petroleum

Oil and gas are formed by the thermal cracking of organic compounds buried in fine-grained rocks.

Algae = Hydrogen rich = Oil-prone

Wood = Hydrogen poor = Gas-prone

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Source rocks – Depositional SettingsA = marine carbonateB = marine shaleC = lacustrineD/E = terrigenous (delta top)F = Refractory + woody land plant

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Age of most Global Significant Petroleum Source Systems

Percentage Global original conventional Petroleum reserves (BOE)

Oil

Gas

0 5 10 15 20 25 30 35

Proterozoic

Cambro - Ordovician

Silurian

Early Devonian

Middle - Late Devonian

Carboniferous

Permo-Triassic

Middle Jurassic

Late Jurassic

Neocomian

Aptian - Santonian

Coniacian - Eocene

Oligo-Miocene

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Reservoirs• The are any rocks that are capable of containing (reservoiring)

hydrocarbons.

• The hydrocarbons are stored in the pore space, either in between(intragranular) or within (intergranular) the grains that make up the rock.

• Hydrocarbons may also be present in fractures.

• Reservoir lithologies include:– Sandstones (approximately 60% of all discovered oil and gas is

reservoired in sandstones) – Carbonates (approximately 40% of discovered all oil and gas is

reservoired in carbonates

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• Other reservoirs include fractured and weathered rocks.

• To be effective a reservoir needs to fulfil a number of criteria:– laterally continuous– porous (effective i.e. connected porosity)– permeable

Reservoirs

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Porosity

• Porosity is the fraction (or percent) of the rock bulk volume occupied by pore space.

• This is a measure of the proportion of pore space to grains, normally quoted as a percentage

Ø % = volume of voids x 100total rock volume

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Porosity

• The porosity may be divided into macro porosity and micro porosity in rocks that have a bimodal pore size distribution.

• Some examples include: – (1) sandstones with a significant amount of clays, – (2) sandstones with microporous chert grains, i.e., interparticle and

intraparticle porosity, – (3) carbonate rocks with vuggy porosity (caverns are an extreme case)

and matrix porosity, – (4) carbonate rocks with moldic porosity and matrix porosity, – (5) carbonate rocks with interparticle porosity and intercrystalline

porosity, – (6) fracture porosity and matrix porosity

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Effective and Ineffective porosity

• The total porosity can also be divided into effective porosity and ineffective porosity.

• Ineffective pores are pores with no openings or zero coordination number.

• Effective porosity can be divided into Cul-de-sac or dead-end pores with a coordination number of one and catenary pores with coordination number of two or more.

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Porosity

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Sorting

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Porosity and Permeability versus Sorting and Grain Size

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Primary porosity

• Formed when the sediment was deposited. May be intergranular(interparticulate) or intragranular (intraparticulate) – i.e. carbonate skeletal grains.

• This is general reduced with time and burial due to:– Compaction– Diagenesis, pore filling cements

• Primary porosity in sandstones may exceed 40 to 55%. This is dependent on the grain size and packing of the sediment.

• A loose sand commonly has a porosity of around 30%. A tight sand (well cemented and compacted) may be less than 1%. Commonly sandstonereservoirs have porosities in the range 10 to 25%

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Porosity in a Sandstone

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Cements

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Secondary Porosity

• This is developed after deposition, and caused by a number of processes: Dissolution (leaching of mineral grains, common in carbonate reservoirs.

• This can be further subdivided into mouldic (fabric selective porosity) and vuggy (non-fabric selective). Very large vugs are often called cavernous porosity.

– Early cementation, fenestral porosity.– Intercrystalline porosity – dolomitiization (secondary replacement

of calcite by dolomite. Dolomitic limestone have a friable sugary (sucrosic) texture. Dolomitization is caused by the 13% shrinkage in the crystal lattice structure of dolomite compared to calcite, with resultant development of secondary porosity.

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Fracture Porosity

• Fracture porosity. Generally fractures have only a minor effect on bulk porosity of a reservoir, but they may substantially increase the permeability and therefore “effective porosity” of a reservoir.

• Fractures are rare in soft, poorly consolidated sandstones, which deform more by plastic flow. They are more common in brittle well lithified rocks, associated with folding and faulting.

• Fractures produce “dual porosity” systems.

• Care needs to be taken on interpreting fractures in the subsurface, as they may be induced by drilling in cores.

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Measuring Porosity• Porosity can be measured in the subsurface from a variety of

sources.– Wireline log analysis– Cores – Seismic

• Two main methods can be used to measure porosity of a rock sample:– Washburn-Bunting Method (gas expansion technique)

Porosity (%) = volume of gas extractedBulk volume of sample x100

– Boyles Law Method (pressure x volume = constant)

• Bulk volume of the sample can be obtained by applying Archimedesprinciple of displacement. This normally involves placing the sample into a container with a know volume of a mercury (non wetting fluid) and recording the volume displaced.

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Permeability

• This is still a fundamental equation in reservoir geology. Permeability is measured in Darcy’s (a dimensionless unit).

• A Darcy is defined as “the permeability that allows fluid of 1 centipoise viscosity to flow at a velocity of 1cm/sec for a pressure drop of 1 atm / cm”.

• As reservoirs commonly have permeabilities of around 1/1000 of a Darcy, the commonly used unit in the oil industry is the millidarcy.

• Reservoirs have common permeabilities of between 5 to 500 mD, but may exceed 1 Darcy.

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Permeability

• Permeability is the ability of a fluid to pass through a porous medium.• Early work on this was carried out by Darcy (1856) who developed

Darcy’s Law.• Q = K (P1-P2) A• µL

WhereQ = rate of flowK= permeabilityP1-P2 + pressure drop across the sampleA = cross sectional areaL = sample lengthµ = viscosity of fluid

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Hydraulic Conductivity

• K represents a measure of the ability for flow through porous media:

• K is highest for gravels - 0.1 to 1 cm/sec

• K is high for sands - 10-2 to 10-3 cm/sec

• K is moderate for silts - 10-4 to 10-5 cm/sec

• K is lowest for clays - 10-7 to 10-9 cm/sec

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Measuring Permeability• Permeability can be measured by:

1. Drill stem / Production tests2. Core analysis3. Minipermeameter4. Petrophysical analysis of well logs

• Note:

Darcy’s law is only really valid in single phase flow systems. Most reservoirs are dual phase or mulitple phase flow (oil/water, oil/gas, gas/water), the equation is still used but with some conditions / adjustments.

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Measuring Permeability

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Permeability• Many reservoirs are “dual porosity” systems, making permeability

and porosity calculations more complex.

• Permeabilities measured may be incorrect due to reservoir damage. This can result from reservoir damage due to drilling -fluid invasion and also damage to cores brought to surface and contamination again by drilling fluid (and through the later cleaning process). All of this may effect measurements obtained,

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Controls on Permeability

• Pore throat size and tortuosity

• Grain size

• In the reservoir, permeability may, and commonly does, vary depending upon the direction of fluid flow.

•• Permeabilities are often quoted as both Kh =horizontal permeability,

and Kv = vertical permeability.

• In general, in layered / bedded sedimentary rocks, Kh is greater than Kv.

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Relationship between Porosity / Permeability

• Texture

• Variation with depth in common reservoirs

• Predictability

• Good and Bad Reservoirs

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Traps• Having identified a sedimentary basin, in which a good petroleum

system exists, with adequate (and mature) source, reservoirs, and potential sealing lithologies, then comes the main task of the exploration geologist, to identify potential TRAPS.

• A trapping mechanism is “any geometric arrangement of porous and non-porous strata that interrupts the migration of oil to the surface”.

• Hydrocarbons are less dense than water, and thus once generated at depth, will rise to the earth’s surface through buoyancy.

• They flow through lithologies which are porous and permeable (reservoirs / carrier beds), but cannot flow through rocks that are tight, (have no or very low porosity and permeability).

• Tight rocks are termed seals. Typical sealing rocks are mudstones (shales) and salt, but any lithology that has no porosity / permeability can act as a seal

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Traps and Seals• A trap is a subsurface feature that prevents the migration of

hydrocarbons to the surface. There are two main types;

• Structural Traps: i.e anticlines, fault related structures

• Stratigraphic Traps: i.e reefal-buildups, sandstone pinchouts

• Combination Traps: : In many cases a trap will be a combination of both structural and stratigraphic trapping mechanisms.

• Hydrodynamic Traps: resulting from ground water flow

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Salt DomeFault

Unconformity

Pinchout

Anticline

Hydrocarbon Trap Types

American Petroleum Institute, 1986

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Structural Traps

– Anticlines• folded rock units, 4-way dip closure, mapped closures with

‘spill point’

– Fault related structures– horsts, half-hragben, tilted fault blocks, thrusted anticlines– Listric faults with roll-over anticlies

– Stacked closures

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Anticlines / Domes

GasGasOilOil

WaterWater

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Tilted Fault Block

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Stratigraphic Traps

• The drilling of a pure “stratigraphic” trap is still relatively rare, especially as a wildcat.

• Historically, the majority of stratigraphic traps were found by accident and were associated with the drilling of a structural feature.

• Commonly the stratigraphic component came to light as additionalwells were drilled that proved hydrocarbons extending outside the know “structural” closure, or a well encountered hydrocarbons in a section whist drilling for another target.

• Types of stratigraphic traps:– Pinchouts, unconformity traps, lobes, channels and levess, reefs

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Stratigraphic Traps

• Stratigraphic Traps are harder to find, often subtle, and require a more integrated geoscience approach

• To-date some 90% of all discovered stratigraphic traps have been found in the USA (95% excluding giant fields), a reflection of drilling density and maturity of exploration in the USA.

• The obvious conclusion is that with ever increasing improvements in data quality (seismic, well logs), data density and geological techniques, the proportion of stratigraphic traps drilled in most basins of the world will increase.

• A substantial part of most mature basins remaining “new” reserves are to be found in stratigraphic traps.

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Salt Domes

Drape / Folding

Truncated and folded units

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Stratigraphic Pinchout / Truncation

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Reefal Buildup

3D Seismic over reefal buildup

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3D Seismic Image - Submarine Fan

Armentrout et al., 1996

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22

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1

2

3

Less Confined

Flow

Confined Flow

New Tools Better Data Improved Understanding

Hummocky Channel Levee

Lobate Mound

Sheet-Form Fan

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Prospect Mapping using 3D Seismic

Sequence Boundary

RMS-ampinterval

Confined Flow

TWTHorizon

N

LessConfinedFlow

5 km0

Sequence Boundary

TWT Horizon

N 5 km0

Overlay of Reservoir on Structure Stratigraphic Interval for Reservoir

ProspectsProspects

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1400

1500

1600

1700

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19003500

4000

4500

5000

5500

6000

6500

5100

5000

4900

4800

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46001900

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1700

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1500

1400

1300

12006500

6000

5500

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4000

35004600

4700

4800

4900

Dep

th (f

t)

3D Seismic Image of Channel Sand

Monson, Mobil, 1998

VoxelGeo Display

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Stratigraphic Traps - Channels

3D seismic, timesliceamplitude map, reds are oil filled sands

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Timing

• The final element is the geological history of the basin and the timing of hydrocarbon generation versus reservoir / seal deposition andmost importantly trap formation.

• Post trap formation effects (tectonic movements / tilting etc) may also “breach” an earlier effective trap.

• This part of the exploration programme is the pulling together of all the available information to develop understand the “Petroleum System” and identify effective “Play Types” in a basin.

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Generation and Migration

Reservoir Extent

Source ExtentSeal

Extent

Critical Reconstruction

Present Past

Components

HC Charge

Preservation

Time Present

Timing Sheets

Play Maps

Petroleum System, Play Definition, and Risk

Jeff Brown, Mobil, 1999

Trap

TIMING

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Petroleum System DefinitionGenetically-related hydrocarbons whose provenance is a source rock.

Source RockMigration RouteReservoir RockSeal RockTrap

Elements

GenerationMigrationAccumulationPreservation

Processes

55


TeapotOwens

Pod of ActiveSource Rock

Just

Big Oil

Hardy Lucky

Zero Edge ofReservoir Rock

Immature Source Rock

Raven

Marginal

250 Ma

A A’

Deer-Boar Petroleum System at Critical Moment

MagoonMagoon and Dow, 1994and Dow, 1994Re

serv

oir R

ock

David

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OverburdenSealReservoirSource

STRATIGRAPHICSTRATIGRAPHICEXTENT OFEXTENT OF

PETROLEUM SYSTEMPETROLEUM SYSTEM

Trap Trap

Essential elements of petroleum

systemPOD OF ACTIVE SOURCE ROCK

Basement

Older rocks Sedi

men

tary

basi

n-fil

l

GEOGRAPHIC EXTENT OF PETROLEUM SYSTEM250 MaTrap

Petroleum accumulationTop of oil windowBottom of oil windowLocation for burial history chart

A A’

Petroleum System at Critical MomentCritical Moment = Time of Expulsion/Migration

Modified from Magoon and Dow, 1994

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Basement

GEOGRAPHIC EXTENT OF PETROLEUM SYSTEM Present-Day

STRATIGRAPHICEXTENT OF

PETROLEUM SYSTEM

Petroleum accumulationTop of oil window

Bottom of oil window

Trap TrapTrap

SealReservoirSourceOlder Rock

Overburden

A A’

Magoon and Dow, 1994

Present-Day Petroleum System

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Modelling Generation and Migration

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400 300 200 100

Paleozoic Mesozoic Cen.

P NKJTPMD P Lith

olog

y

RockUnit

Dep

th (K

m)

Sou

rce

Res

ervo

irS

eal

Ove

rbur

den

1

2

3

Placer FmGeorge Sh

Boar Ss

Deer ShElk Fm

Top gas window

Critical Moment

ThickFm

R

Burial History Chart

MagoonMagoon and Dow, 1994and Dow, 1994

Generation

Time of Expulsion and Migration. (Trap must already exist)

Top oil window

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400 300 200 100 Geologic TimeScale

PetroleumSystem Events

Rock Units

Source Rock

Reservoir Rock

Seal Rock

Trap Formation

Overburden Rock

Gen/Migration/Accum

Preservation

Critical Moment

Paleozoic Mesozoic Cenozoic

D M P P TR J K P N

Elem

ents

Proc

esse

s

Magoon and Dow, 1994

Petroleum System Events ChartTiming of Elements and Processes

Critical MomentTime of Expulsion and Migration. (Trap must already exist)

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Petroleum System - A Dynamic Entity

Spill PointSpill Point

Seal Rock(Mudstone)Reservoir Rock

(Sandstone)Migration from‘Kitchen’

1) Early Generation

2) Late Generation

Gas displaces all oil

Gas beginning to displace oil

Displaced oil accumulates

Lecture 2 Hydrocarbon Habitat

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