Lecture 2 Hydrocarbon Habitat

Fundamentals Fundamentals of Petroleum Geoscience

Lecture 2: Hydrocarbon Habitat

Hydrocarbon Habitat



• The The fiv five e ele eleme ment nts s of of a Petr Petrol oleu eum m Sys Syste tem m – Source • Maturation • Migration

– Res eser erv voir – Seal – Trap – Timing

2



• The The fiv five e ele eleme ment nts s of of a Petr Petrol oleu eum m Sys Syste tem m – Source • Maturation • Migration

– Res eser erv voir – Seal – Trap – Timing

2



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

Elements Source Rock Migration Route Reservoir Rock Seal Rock Trap

Processes

Timing

Generation Migration Accumulation Preservation 3

Fundamentals of Petroleum Geoscience


Critical Risk Elements Trap

Accumulation

– Geometry – Seal Presence – Seal Effectiveness

Reservoir – Presence – Effectiveness

Timing

Charge Generation

– Source Presence – Source Effectiveness • Maturity • Migration 4



Seal

Structure/Trap Reservoir Reservoir

Trap

In-reservoir changes

Reservoir Quality Petroleum charge Migration Migration Timing

Petroleum expelled with time

Source Rock

5



Petroleum System Elements

Anticlinal Trap

Top Top Seal Seal Rock (Impermeable ) Reservoir

Rock

(Porous/Permeable) Potential Migration Route

Source Rock

(Organic Rich)

24803

6



Generation and Migration • 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

7



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%

8



Source Rocks •

The nature of the hydrocarbons generated will depend on both the type of organic matter a source rock contains, and the degree of heat / 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

9


OrganicRich

Thin Laminae


Measured Values Total Organic Carbon

1 Inch

Hydrogen Index

3.39

378

In-Place Petroleum S1

Pyrolytically Generated Petroleum S2

2.24

12.80

LOMPOC Quarry Sample Monterey Formation, CA 10



Kerogen Types

TOC 2.12 WT.%

TOC .38 WT.% 11



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

12



Source rocks – Depositional Settings A = marine carbonate B = marine shale C = lacustrine D/E = terrigenous (delta top) F = Refractory + woody land plant

13



Age of most Global Significant Petroleum Source Systems Oligo-Miocene Coniacian - Eocene Aptian - Santonian Neocomian Oil

Late Jurassic Gas

Middle Jurassic Permo-Triassic Carboniferous Middle - Late Devonian Early Devonian Silurian Cambro - Ordovician Proterozoic

0

5

10

15

20

25

30

35

Percentage Global original conventional Petroleum reserves (BOE)

14



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 15



Reservoirs •

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

16



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 100 total rock volume

17



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

18



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.

19



Porosity

20



Sorting

21



Porosity and Permeability versus Sorting and Grain Size

22



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 sandstone reservoirs have porosities in the range 10 to 25% 23



Porosity in a Sandstone

24



Cements

25



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. 26



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.

27



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 extracted Bulk volume of sample x100 – Boyles Law Method (pressure x volume = constant) Bulk volume of the sample can be obtained by applying Archimedes principle 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. 28



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.

29



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 Where Q = rate of flow K= permeability P1-P2 + pressure drop across the sample A = cross sectional area L = sample length µ = viscosity of fluid 30



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

31



Measuring Permeability •

Permeability can be measured by: 1. 2. 3. 4.

•

Drill stem / Production tests Core analysis Minipermeameter 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.

32



Measuring Permeability

33



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,

34



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.

35



Relationship between Porosity / Permeability •

Texture

•

Variation with depth in common reservoirs

•

Predictability

•

Good and Bad Reservoirs

36



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

37



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

38



Hydrocarbon Trap Types

Anticline

Fault

Salt Dome

Pinchout

Unconformity

39 American Petroleum Institute, 1986



Structural Traps – Anticlines • folded folded rock rock unit units, s, 4-way 4-way dip closur closure, e, mappe mapped d clos closure ures s with with ‘spill point’

– Fault Fault relate related d struct structure ures s – horsts horsts,, half-hr half-hragb agben, en, tilte tilted d fault fault blocks, blocks, thrust thrusted ed anticl anticline ines s – List Listric ric faul faults ts with with rollroll-ove overr antic anticlilies es

– Stac Stacke ked d clo closu sure res s

40



Anticlines / Domes

Gas Oil Water

41



Tilted Fault Block

42



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 additional wells 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

43



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.

44



Salt Domes Drape / Folding

Truncated and folded units 45



Stratigraphic Pinchout / Truncation

46



Reefal Buildup

3D Seismic over reefal buildup 47



3D Seismic Image - Submarine Fan New Tools

Better Data

Confined Flow

Improved Understanding

1

Hummocky Channel Levee

1

2

2 Less Confined Flow

Lobate Mound

3

3 Sheet-Form Fan

48

Armentrout et al., 1996



Prospect Mapping using 3D Seismic RMS-amp interval

TWT Horizon

Sequence Boundary

TWTHorizon

Sequence Boundary

Stratigraphic Interval for Reservoir

Overlay of Reservoir on Structure

Confined Flow

Less Confined Flow

Prospects 0

5 km

N

0

5 km

N

49



3D Seismic Image of Channel Sand 4900

1400

4800 1500

4700

1600

4600 3500

1700

4000

1800

) t 4500 f ( h t p 5000 e D

1900 3500 4000

5500

4500

6000 5000 6500 1200

5500 1300

6000 1400

6500 5100

1500 5000

1600

VoxelGeo Display

4900 1700

4800 1800

4700 1900 4600

50 Monson, Mobil, 1998



Stratigraphic Traps - Channels

3D seismic, timeslice amplitude map, reds are 51 oil filled sands



Timing •

The final element is the geological history of the basin and the timing of hydrocarbon generation versus reservoir / seal deposition and most 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.

52



Petroleum System, Play Definition, and Risk Trap

Play Maps Seal Extent

Source Extent

Timing Sheets Generation and Migration

Time

Present

Components HC Charge Reservoir Extent

TIMING

Preservation

Critical Reconstruction

Present

Past

53 Jeff Brown, Mobil, 1999



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

Elements Source Rock Migration Route Reservoir Rock Seal Rock Trap

Processes Generation Migration Accumulation Preservation

54



Deer-Boar Petroleum System at Critical Moment 250 Ma Raven

A

Owens

Teapot

Pod of Active Source Rock

Just

Marginal

k c o R r i Big Oil r v o s e e Hardy R

A’

Lucky

David

Immature Source Rock

Zero Edge of Reservoir Rock Magoon and Dow, 1994

55



Petroleum System at Critical Moment Critical Moment = Time of Expulsion/Migration GEOGRAPHIC EXTENT OF PETROLEUM SYSTEM

A

250 Ma

Trap

STRATIGRAPHIC EXTENT OF PETROLEUM SYSTEM

t e n m s e a B

Essential elements of POD OF ACTIVE petroleum system SOURCE ROCK

Trap

Trap

Overburden Seal Reservoir Source

Petroleum accumulation Top of oil window

Older rocks

A’

y r l a l i t f n e n i m s i d a e b S

Bottom of oil window Location for burial history chart

56 Modified from Magoon and Dow, 1994



Present-Day Petroleum System GEOGRAPHIC EXTENT OF PETROLEUM SYSTEM

A

Present-Day

Trap

Trap

Trap

A’

STRATIGRAPHIC EXTENT OF PETROLEUM SYSTEM

t e n m s e a B

Petroleum accumulation Top of oil window Bottom of oil window

Overburden Seal Reservoir Source Older Rock 57 Magoon and Dow, 1994



Modelling Generation and Migration

58



Burial History Chart 400

300

200

Paleozoic D

M

P

100

Mesozoic P

TR

J

Cen. K

P

N

y g o l o h t i L

) n r e m e i d o l r K v ( c r r a u e e b h t u o s S r p S e e v e R D O

Rock Unit

Thick Fm

Generation

1

2 Placer Fm George Sh Top oil window

Boar Ss

Top gas window

Deer Sh Elk Fm

Critical Moment

3

Magoon and Dow, 1994

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

59



Petroleum System Events Chart Timing of Elements and Processes 400

300

200

Paleozoic

D

M

P

100

Mesozoic

P

TR

J

Geologic Time Scale Cenozoic

K

P

N

Petroleum System Events Rock Units Source Rock Reservoir Rock Seal Rock Overburden Rock Trap Formation Gen/Migration/Accum Preservation

Magoon and Dow, 1994

Critical Moment

Critical Moment

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

s t n e m e l E s e s s e c o r P 60

Lecture 2 Hydrocarbon Habitat

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