Week 1 - 3 Course contents Introduction To Petroleum Geology Philosophy of the science of petroleum geology and relationship of petroleum geology with

Week 1 - 3

Course contents Introduction To Petroleum Geology

• Philosophy of the science of petroleum geology and relationship of petroleum geology with other branches of

science.Origin, Migration and Entrapment Of Hydrocarbons

•Generation, migration and trapping mechanism of hydrocarbons.

• Distribution of source rock over geologic time

Introduction to Petroleum Geology

LO: At the end of this lecture you will be able to understand the philosophy & Technology of petroleum geology.

Subject covers geologic history of petroleum describing how petroleum is generated, migrated and accumulated in the sub-surface, characteristics of reservoir rocks and reservoir dynamics. Course also covers volumetric estimation of in-place hydrocarbon reserves.

Course contents Introduction To Petroleum Geology

• Philosophy of the science of petroleum geology and relationship of petroleum geology with other branches of

science.Origin, Migration and Entrapment Of Hydrocarbons

•Generation, migration and trapping mechanism of hydrocarbons.

• Distribution of source rock over geologic time


Lesson Outcomes

At the end of three weeks students• should be able understand the relation of petroleum geology with other branches of science and also the philosophy of petroleum geology.

•should be able to understand how oil generates, migrates and are trapped in the subsurface reservoirs.



Petroleum Accumulates

Petroleum Migrates

Petroleum Generates24803

Petroleum System ElementsPetroleum System Elements

120° F120° F

350° F350° FGenerationGeneration

MigrationMigration

Seal RockSeal Rock

Reservoir RockReservoir Rock

OilOil

WaterWater

Gas CapGas Cap

EntrapmentEntrapment

Fundamentals of Petroleum Geology

Generation of Petroleum

1. Abiogenic Theory

The abiotic hypothesis is that the full suite of hydrocarbons found in petroleum can be generated in the mantle by abiogenic processes and these hydrocarbons can migrate out of the mantle into the crust until they escape to the surface or are trapped by impermeable strata, forming petroleum reservoirs.

2. Biogenic Theory: Petroleum is generated by effects of heatand pressure on the organic remains of living organic matter.

1. Meteorites and other extra-terrestrial bodies have evidence of presence of methane

2. All the compounds of petroleum can be prepared by inorganic methods.

3. Thermodynamically methane can polymerize tohigher homologue at high temperature & pressure (mantle pressure & temperature).

4. Oil is found in granitic rocks of Vietnam

Evidences in favour of Abiogenic Theory of Petroleum Generation

1. Unpredictable.

2. Does not accept biomarkers.

3. Cannot explain optical rotation

4. Can not explain presence of porphyrins

4. Average carbon isotope composition of petroleum do not support

Problems of Abiogenic Theory of Petroleum Generation

1. Both organic matter and petroleum are made of dominantly Carbon and Hydrogen. Further, both of them also have nitrogen.

2. Optical rotation of plane polarized light is seen in some organic substances and petroleum.

3. Presence of porphyrins in petroleum.

4. Presence of bio-markers in petroleum and

5. Odd-Even periodicity are well documented in petroleum paraffins and living organisms suggesting organic origin of petroleum.

6. Palynological studies have also suggested the organic origin of petroleum.

Evidences in favour of Organic Source

• It is not possible to generate long chain hydrocarbons by polymerization at low temperature–low pressure natural conditions because there is a constant Gf (Gibbs free energy change) increase for every increase in CH2 unit (2.2 kcal/mol).

• The average concentration of hydrogen in proportion to carbon is less in organic matter than in petroleum

•Presence of helium in some gases cannot be explained by organic theory

Limitations of Biogenic Theory of Petroleum Generation

3. Source Rock in Petroleum Geology

Week 4 -9

LO: At the end of you will appreciate what are the geochemical parameters necessary to resolve the petroleum geology problems. How these parameters are generated and what are the limitations of each these parameters

Shale: Shale breaks into thin pieces with sharp edges. It occurs in a wide range of colors that include: red, brown, green, gray, and black. It is the most common sedimentary rock and is found in sedimentary basins worldwide.

Marcellus Shale is a petroliferous black shale deposited during the Devonian in eastern USA. It is also explored as a shale gas resource. This photo shows an outcropof the Marcellus in New York State

Distilling Oil & water from KimmeridgeOil Shale using gas lighter.

Kimmeridge oil shale burns with Curling gasy flame well above the Burning rock & noxious fumes

Source Rock

Definition: A rock that is capable of generating and expelling hydrocarbons to form commercial oil/gas pools.

Characteristics: 1. Fine grained sedimentary rock with high TOC and H/C deposited in an anoxic environment.

Model of Petroleum(Wiehe, I.A., 1999)

A – Asphaltene (solute), R – Resins (dispersant)a – Aromatics (solvent), s – Saturates (non-solvent)

s s s s a a a s s a R R R a s s a R A A R a s s a R A A R a s s a R R R a s s a a a s s s s

Petroleum is a colloidalsolution of resins &asphaltenes in aromaticsand saturates

Source Rock Parameters• TOC : Total Organic Carbon – Quantity• H/C : Hydrogen to Carbon (molar) – Quantity & Quality• Pyrolysis Data: S1, S2, Tmax, HI, OI, PI – Quantity, Maturity & Quality

• Vitrinite reflectance (Vro) - Quality • Visual kerogen analysis - Quality• GC: Whole oil, C15+, Saturates - Quality• Py - GC - Quality• Biomarkers – Quality, Maturity• Isotopes – Quality, Maturity

TOC

H/C

Pyrolysis

VroPyrolysisBiomarkerIsotopes

PyrolysisPy-GCGC, VKABiomarker

Sedimentary Organic Matter

(Break down of cell material by intracellular hydrolytic enzymes)

Lost organic Matter

Refractory OM.(Lignins & Lipids)

Living Organism Death Organic Matter (OM)

LeachingDecomposition

Slow Decomposition

Labile(Carbohydrates & Proteins)

(Bacterial colonization)

Deposit with

sediments

∞ (py. productivity, bathymetry)

Sedimentary Organic Matter

Primary Production of Organic Matter

Carbon Flux Recycled/Dissolved

Preserved Degraded

Kerogen/Coal

Kerogen formation

Algaenan: Highly aliphatic, non-hydrolysable, insoluble macro-molecular constituent have been identified in a number of microalga cells and their selective preservation shown to playmajor role in the formation of kerogen. All the algaenans so farexamined comprise a network of long polymethylenic chains, except L race of Botryococcus braunii that contains C40 isoprenoid chains with lycophane type skeleton.

Melanoidin: Millard (1912) described the condensation reaction of an amino group and a reducing compound. It is believed that proteins and lipids react to form kerogen.

Mechanism of kerogen formation from organic matter is still not known. Earlier, it was believed that the kerogen is formed by condensation reactions, the same process of coalification.

At present the theories of ‘Algaenan’ and ‘Millard condensation to form melanoidin’ are in vogue.

Kerogen

Crum-Brown (1912) first used the term to describe OM of a Scottish Shale that produced waxy oil upon distillation.

White (1915) & Trager (1924) extended this definition to all OM in rocks capable of oil generation.

Forsman & Hunt (1958) defined kerogen as dispersed OM of ancient sediments insoluble in usual organic solvents.

This was later extended by Durand (1980) to all insoluble sedimentary OM including pure organic deposits such as humic and algal coals and various asphaltic substances as

well as insoluble OM in recent sediments & in soils.

Kerogen

Kerogen formation is the first step of organic transformation after the sedimentation of dead organic matter. After deposition of the organic rich sediments, it gradually changes to source rock during which sediments are converted to rock and organic matter is converted to kerogen. This process is known as the diagenesis of the source rock and during this process small amounts of biogenic methane is also evolved.

Engler (1913) heated oleic acid and other organic material at temperatures below 2500C and obtained paraffin, napthene and aromatic hydrocarbons in entire petroleum range. Later, he described the generation of petroleum from organic matter as a two step process involving bitumen as an intermediate.

Kerogen Bitumen Oil + Gas + Residue

Kerogen

Larskaya and Zhabrev (1964) were the first geochemists to demonstrate that the generation of hydrocarbons from the kerogen of shale increases exponentially with depth.

They found extractable organic matter (bitumen) of shale in western ciscaspian region change very little in the temperature range of 200C to 500C after which it increased markedly.

Kerogen Kerogen isolation (Durand & Nicaise,1980):

After the rock is ground to fine powder, carbonate minerals are removed by treatment with HCl in inert N2 atmosphere at 800C. Silicates are then dissolved in HF, also in inert N2 atmosphere at 800C, leaving a kerogen concentrate consisting of kerogen and small amounts of acid resistant minerals like pyrite. Each acid treatment is followed by thorough water washing with special care to ensure dissolution of newly formed Fluorosilicates. Recovered kerogen is dried at 1000C underinert N2 atmosphere.

For most of the practical Purpose this kerogen concentrate is sufficiently pure, however for some special studies (like sulfur content) inorganic component is removed by floatation in a heavy liquid like, aquous zinc bromide.

Kerogen Type-I: • Contain alginite, amorphous organic matter, cyanobacteria, fresh water algae and land plant resins.• H/C > 1.25 & O/C < 0.15• Show great tendency to readily produce liquid hydrocarbons• It derives principally from lacustrine algae and forms only in anoxic lakes and several other unusual environments• Has few cyclic and aromatic structures.•Formed mainly from proteins and lipids.

Type-II: • Contain alginite, amorphous organic matter, cyanobacteria, fresh water algae and land plant resins.• H/C < 1.25 & O/C 0.03 to 0.18• Show great tendency to produce a mix of oil & gas.• It derives principally from exinite, cutinite, resinite and liptinite

Kerogen

Type-III:• H/C < 1.0 & O/C 0.03 to 0.3• Material is wood or coal• Tends to produce gas• Derived mainly from plant matter of cellulose & lignin

•Type-II-S• Same as Type-II but rich in sulfur

•Type-IV• H/C <0.5• It is mostly decomposed organic matter of polycyclic aromatic hydrocarbons.

Algaenan: Highly aliphatic, non-hydrolysable, insoluble macro-molecular constituent have been identified in a number of microalga cells and their selective preservation shown to playmajor role in the formation of kerogen. All the algaenans so farexamined comprise a network of long polymethylenic chains, except L race of Botryococcus braunii that contains C40 isoprenoid chains with lycophane type skeleton.

Melanoidin: Millard (1912) described the condensation reaction of an amino group and a reducing compound. It is believed that proteins and lipids react to form kerogen.

Mechanism of kerogen formation from organic matter is still not known. Earlier, it was believed that the kerogen is formed by

condensation reactions, the same process of coalification.At present the theories of ‘Algaenan’ and ‘Millard condensation

to form melanoidin’ are in vogue.

Kerogen

Bitumen

P e t r o l e u m

Lipids, Proteins & Carbohydrates

Hydrocarbons &Protohydrocarbons

In organismsBacterial activity & Low temperature chemical reactions

Living Organisms

500C

High Temperature Reactions

Methane

Heavy Oil Light Oil & Gas

Gas

Pyrobitumen

2000C

2500C

Diagenesis

Catagenesis

Metagenesis

Organic Diagenesis is the biological, physical and chemical alteration of the organic debris before pronounced effect of temperature. It covers temperature range upto 500C.

Catagenesis is the stage in which increasing temperature cause kerogen to thermally decompose to bitumen to oil, condensate and gas. It coverstemperature range of 500C to 2000C.

Zone of higher temperature from 2000C to 2500C in which small amounts of methane is formed and remaining organic matter is converted to pyrobitumenIs called Metagenesis.

Diagenesis

The quantity of petroleum generated is determinedmainly by the amount of hydrogen in the organic matter in the sediment. It follows that strongly reducing environments such as stagnant lakes orsilled basins preserve and enhance the quantity and hydrogen content of OM; whereas oxidising environments lower it.

OM deposited in sediments consists of:Carbohydrates, Proteins, Lipids, Lignins andChitin, Waxes, Resins, Pigments, Fats & Oils.

Diagenesis

Some of them are consumed by burrowing organisms;Some may be complexed with mineral matter and some is attacked by microbes that use enzymes to degradebiopolymer into simple monomers from which they were originally formed.

Some degraded bio-monomers undergo no further reaction but others condense to form complex high molecular weight geopolymer which along with undegraded biopolymer become the precursor of KEROGEN.

DiagenesisDuring Diagenesis under reducing conditions, thiscomplex mixture of geo- and biopolymers and monomers undergo a whole series of low temperature biological & chemical reaction that result in the formation of more hydrocarbon like material through the loss of oxygen, nitrogen and sulfur.

OilWindow

Immature

Post mature for oil

Mg of HC / 100 mg of TOC

Dep

th

10 20 30

Anoxic< 0.1 ml/L

Suboxic1.0 to 0.1 ml/L

Oxic> 1.0 ml/L

SedimentLaminated

Micro – MacroBurrowed

Coarse Bioturbation

% TOC

H/C of OM

OM Type

3 to 20 1 to 3 0.05 to 1.0

1.6 1.2 0.8

I and II II and III III and IV

Catagenesis

It is the stage of reaction of OM at higher temperature at a greater depth. Over time, these high temperatures

cause thermal degradation of kerogen to produce petroleum.

Engler (1913) heated mixture of oleic acid and other OMat a temperature below 2500C and obtained petroleum. Later, he described kerogen is first converted to bitumenas an intermediate. Miknis et al. (1987) confirmed this

proposition.

Kerogen Bitumen Petroleum

Oil Window : The depth interval in which a petroleum source rock generates and expels most of its oil is called the oil Window. Most oil windows are in the temperature range of 600C (1400F) to 1600C (3200F).

The stratigraphic intervals above, within and below the oil window are known as immature, mature and post mature for oil generation. Post mature for oil is mature for gas window.

Gas Window : The depth interval in which a petroleum source rock generates and expels most of its gas is called the gas window. Most gas windows are in the temperature range of 1000C (2120F) to 2000C (3920F).

Metagenesis

This is the last stage of significant thermal alteration of OM. Here methane generation diminishes and graphitic structure begins to form. Metagenesis occurs at the temperature rangeOf 2000C (3920F) 2500C (4820F).At such temperatures, atomic H/C of kerogen falls to less than 0.4, typical of the kerogen in phyllite.

High temperature end of organic thermal alteration overlapswith low temperature beginning of inorganic thermal alteration.

Migration of Petroleum

1. Geological Framework2. Short or Long Migration3.Primary Migration4.Secondary Migration

• Capillary Pressure• Buoyancy• Dissolved Gas effects

5. Tilted Oil-Water contact

Geological Framework:

1. Nearly every petroleum pool exists within an environment of water.

2. The gas and oil are chiefly immiscible in water and both have lower density than surrounding water.

3. The microscopic shape and size of pores, the tortuous path of permeability and chemical character of the rock varies.

Geological Framework4. The minimum time for oil & Gas to generate,

migrate and accumulate is probably less than one MY.

5. The fluid pressure within the reservoir rock may also vary within the life of the reservoir, depending on the geological history of the region. They have been observed to range from one atmosphere to 1000 atmosphere or greater and they may have fluctuated up or down many times

Geological framework:

6. Temperature of the reservoir rock may vary 50 – 1000C although extremes up to 1630C is known.

7. The geologic history of the trap may vary widely – from a single geologic episode to a combination of many phenomena extending over a long period of geologic time.

8. Reservoir rocks that contain petroleum differ from one another in various ways: They range in geologic age from Precambrian to Pliocene; in composition from siliceous to carbonate; in origin from sedimentary to igneous; in porosity from 1 to 40% and in permeability from one millidarcy to many darcies.

Short or Long Migration:1.The best evidence of short migration are the pools in isolated sand

lenses.2. There are several good reasons for believing long distance migration:

a) Common occurrence of oil and gas seepage.b) Production of oil and gas from pools.c) A structural trap may not form until long after the reservoir rock

is in place.

Migration of Petroleum

Primary Migration:Defined as the migration of petroleum from the source rock to the carrier bed.Earlier theories believed oil and gas came out of source rock with squeezing Water (Hydrodynamic theory by Munn(1909); Hydraulic buoyancy theory by Rich(1921); Mrazec(1910);Daly(1917); Sedimentary compaction theory by King(1899); Monnet(1922); Lewis(1924) and the Compaction – hydraulic Theory by Cheney(1940)

Secondary MigrationIn order to evaluate the conditions under which oil migration can occur it is necessary to estimate the difference of capillary pressure at the leading and rear end of the migrating oil body. The geometry of an isolated oil globule in a pore when there is no effective force tending it to cause migrate is shown in Fig.AThe capillary pressure is

Which is approximately same at every corners. Fig.B shows the distorted Shape of this globule as it is just before breaking through the constrictionAt point A and is migrating to the next pore. In this condition the capillary pressure at the foremost and rear end are -

2γ cosØr

Pc = ------------

2γ cosØrc

Pc = ------------2γ cosØ

rpPc = ------------

Fig.A

Fig.B

rp

Therefore, ΔPc = 2γ CosØ (1/rc – 1/rp)

Assume, for an average porous sand rc of the pore connecting capillaries is between ½ and ¼ of rp of the pores, then

ΔPc = 2γ CosØ /rp or ΔPc = 6γ CosØ /rp)

If we take an average, rc = rp/3, then

ΔPc = 4γ CosØ/rp

If contact angle is 600, then cosØ = ½ and ΔPc = 2γ/rp

Secondary Migration

Secondary Migration

Consider now the possibility of creating this ΔPc from flowing formation water.A normal hydrodynamic gradient of 10ft/mile can produce 0.1dyne/cm2 for coarse sand 0.02 dynes/cm2 for fine sand. An extreme gradient of 100ft/mile will produce 1.0dyne/cm2 for coarse and 0.2 dynes/cm2 for fine sand.

Capillary Pressure Difference Required for Migration (dynes/cm2)

γ

(Dyne/cm2)

V. coarse sand (rp=0.02cm)

Coarse Sand

(rp =0.01cm)

Av. Sand

(rp=0.005cm)

Fine Sand

(rp=0.002cm)

V.Fine Sand

(rp=0.001cm)

30 3,000 6,000 12,000 30,000 60,000

25 2,500 5,000 10,000 25,000 50,000

20 2,000 4,000 8,000 20,000 40,000

10 1,000 2,000 4,000 10,000 20,000

5 500 1,000 2,000 5,000 10,000

1 100 200 400 1,000 2,000

BuoyancyUpward movement due to buoyancy begin

where sufficient local concentration of oil & gas has developed.

Buoyancy

The force of buoyancy acts to trap hydrocarbon in two ways :

1. when HC entrained in flowing water reach an anticlinal area their buoyancy causes them to resist further movement.

2. The oil & gas migrate up dip by buoyancy till buoyancy or capillary pressure no longer exceeds Pd.

Buoyancy

Buoyancy spplimented by hydro-dynamic force

Buoyancy opposed by hydrody-namic force

Buoyancy spplimented by hydro-dynamic force

Effect of water flowdirected down dip &through a barrier zone of higher displacement pressure

Effect of water flowdirected up dip &through a barrier zone of higher displacement pressure

DolomiteDolomite

QZ + HAc+ Crude

Oil

Gas

Effect of gas in migrationThe importance of gas in the movement of oil through

reservoir was long ago pointed by Johnson (1912).

Thiel’s Experiment (1920): Crude oil diluted with 1/3rd kerosene + sea water acidized with 0.5% HAc + 20mesh quartzite was placed in a 1” dia 4’ length tube bent in the form of anticline. 4” of both ends of tube were filled with crushed dolomite and finally ends were sealed. CO2 generated by reaction of dolomite with HAc moved up the tube carrying oil with it. Similar experiment with no gas resulted no concentration of oil.

This was supported by Emmons & Mills Experiment

All the theories involve movement of large quantity of water with dispersed oil and gas.Oil and Gas in solution of reservoir waters is cited as evidence. The solubility of natural gas in water ranges from 4 cft/barrel at 400 psi that goes to 22 cft/barrel between 2000 to 6000psi. Lighter components of oil are soluble in water, the amount increase with pressure. At atmospheric pressure the solubility is 0.014% volume.

Water squeezed out of clays & Shales:If water containing oil & gas is squeezed out of clay & shale into permeablereservoir rock, then it must displace water that is already present in the permeable rock. This would cause a flow of water through the reservoir rock to the surface. As the water moves through the reservoir, the entrained petroleum may be carried along in a miscible phase until enough petroleum Accumulates in phase continuity to develop buoyancy.

Normal Water Circulation:

At the time of diagenesis the pore spaces of the reservoir are filled with water. Regional circulation pattern develop and continually change as fluid pressure gradient change. Fluid pressure may change due to:

i. Diastrophism, ii. Mountain building, iii. Erosion, iv. Deposition, v. Recharge from meteoric waters, vi. Osmosis, vii. Faulting,viii. Folding, ix. Cementation affecting permeability may change direction x. Chemical deposit and xi. Igneous intrusion

The present concept of mechanism of primary migration suggests petroleumglobules develop excess pressure in the pores of shale and generate micro-cracks in the rock through which globules come out. Once the globules comeout, pore pressure also get reduced and micro-cracks are annealed.

Secondary Migration:Defined as the migration of petroleum from the carrier bed to the trap.1.Entrained Particles –

a) Microscopic and sub-microscopic particles of petroleum hydrocarbonentrained in the moving water would be carried along until obstructed by the structure or separate and aggregate till buoyancy get effective.

2.Capillary Pressure:

The basic requirement for the large patches of oil in water wet reservoir is that the capillary pressure of the oil-water interface exceeds the displacement pressure. For any specific combination of oil, water and porethe displacement pressure Pd is a constant value. On the other hand, capillary pressure is dependent on buoyancy, pressure gradient, and the length of the oil phase. Whenever these forces are sufficient to cause capillary pressure to exceed Pd the oil will enter and pass through the pores.

Reservoir Rock Reservoir Rock is defined as the rock having appreciable

porosity to trap the hydrocarbons and sufficient permeability for the oil to flow during migration and production.

Characteristically, these are coarse grained clastic sedimentary rocks or porous limestones or fractured igneous or metamorphic rocks.

Seal Rock Seal Rock is defined as the rock that prevent further vertical

and lateral migration of the hydrocarbons from the trap.

Characteristically, these are fine grained impermeable sedimentary rocks.

Hydrocarbon Traps Hydrocarbon Traps are:

(1)Structural: If trapping is exclusively due to structural style, or(2)Stratigraphic: If trapping is exclusively due to stratigraphic

disposition, or(3)Combination: If trapping is neither due to structure & nor

due stratigraphic disposition alone.

Structural Traps: Folds, FaultsStratigraphic Traps: Lenticular Sands, Channel fillsCombination Traps: Up-dip pinchouts

PAB 1023 Petroleum Geoscience 56

Structural Traps

• Structural traps are formed usually by tectonic forces.

– An anticline is where rocks are folded or bent upwards. Hydrocarbons migrate up the flanks of the anticline and are trapped in the crest.

– Faults occur where there is movement along a joint or fracture. Offset of the beds could result in an impermeable layer being on top of a permeable layer.


Structural Traps

:


Structural Traps Terminology


Six trapping and two non-trapping configurations against a fault, depending on whether the fault is normal or reverse, on the direction of dip of the beds relative to the fault plane, and on the amount of displacement of the reservoir.


Sealing Faults• An investigation into the sealing qualities of faults affecting

roll-over anticlines in the Niger Delta, where the reservoirs overlie overpressured shales (bulging).

• Where a reservoir is full to spill-point against a fault, and where an oil-water contact is continuous across a fault, it is presumed that the fault is non-sealing; elsewhere it appears to form a trap. The difference is believed to be due to clay being smeared into the fault plane, where there is enough of it in the section, as the fault moved


Sealing Faults


STRUCTURAL TRAP

• A review of 200 giant oilfields (those containing 500 million barrels or more) emphasize the importance of structural, essentially anticlinal, traps in both number and size.

• The number of structural field of this size may partly reflect the fact that structural traps are easier to find than the others, but the oil reserves they contain show clearly that generally they are also bigger.


STRUCTURAL TRAP

• The trouble, from our present-day point of view, is that in most parts of the world the larger anticlines have now been drilled. What our efforts are increasingly directed towards, therefore, are the more obscure and generally smaller prospects.


Stratigraphic Traps

• Rock layers changing from a good reservoir to non-reservoir due to change in rock type (pinch-out), reservoir quality (diagenesis), or removal (erosional unconformity)


Pinch Out• Sometimes due to a lateral

change in the environment of deposition a lens of permeable sand is surrounded by less permeable siltstones and shales, forming a pinch out trap.

• This commonly happens in stream environments where sand is deposited along the stream channel which is surrounded by a flood plain characterized by finer grained sediments.


Channel Trap


Unconformity• Unconformities are another

type of stratigraphic trap. – They represent a gap in the

geologic record, in other words a period of erosion and/or nondeposition.

• They can result in a permeable reservoir rock being truncated and overlain by an impermeable unit.


hydrodynamic trap

• Oil, attempting to escape to surface up a reservoir, is held against an unevenness of its upper surface by water flowing in the opposite direction. There is no structural or stratigraphic closure. – Note that the oil-water

contact is tilted down in the direction of water flow.


Diapiric Traps

• Caused by upward movement of sediments that are less dense than those overlying them– Salt– Clay


Salt Dome Associated Traps


Seal

• A seal is a fine-grained rock that prevents the oil migrating to the surface @ vertical migration (which happens in many parts of the world - leading to natural oil seeps).

• The seal is an important component in a prospect.


SEAL

• Common seals include – salt evaporites, chalks provides an effective seal,

but – Muddy @ clay-rich rocks @ mudrock, shale

represent most seals. – Siltstones (very fine-grained)


HOW WE QUANTIFY SEAL?

• Analysis of seals involves assessment of their thickness and extent, such that their effectiveness can be quantified.

• Knowledge of sequence stratigraphy is crucial for better understanding of seal.

Pyrolysis is the technique of heating in absence of oxygen. It is usually carried in an atmosphere of helium or nitrogen. In this process the available organic matter in the rock (in the form of kerogen) is cracked down to smaller fragments. Since formation of petroleum takes place deep inside the earth, it is expected to be formed by pyrolysis cracking. But the actual environment of kerogen cracking for generation of petroleum is not yet known. Therefore, following varieties of pyrolysis techniques have been attempted.

1. Dry pyrolysis 2. Hydrous pyrolysis3. Closed system pyrolysis 4.Closed anvil pyrolysis

Pyrolysis

Dry pyrolysis is carried out using dry samples of carbonate free rock or kerogen. Samples are made carbonate free by prolonged treatment of samples with dilute hydrochloric acid and then thoroughly water washed and dried. The most commonly used dry pyrolysis instrument is Rock-Eval pyrolyser, introduced by IFP sponsored Vincii Laboratory, France. This instrument pyrolyses the sample in nitrogen or helium environment using desired programmed heating conditions. This programmed heating condition is a facility to simulate heating as per thermal history of the basin. This instrument is fitted with a flame ionization detector (FID) to detect the released hydrocarbons.

Pyrolysis

Programmed Heating

He Supply

Rock Sample

CO2

Trap

CO2

Detector

Hydrocarbon Detector (FID)

RecorderS1 & S2

S3

Schematic diagram of a Rock-Eval Pyrolyzer

(Figure adapted from ‘GeochemistryIn Petroleum Exploration’ by D. W. Waples (1985))

The instrument is heated at 3000C before any sample is introduced. When the sample is introduced at 3000C, it is believed all the available bitumen in the rock is released as S1 (P1) peak. The instrument is then heated as per program till 6500C (RK-Eval 2). The hydrocarbons generated by cracking in this process is recorded as S2 (P2) peak. The instrument also holds a CO2 trap. This trap

accumulates all the CO2 generated from beginning to end and releases as S3.

Time / Temperature

Intensity

3000C 6500C

S1S2 S

3

S2 is representative of cracked hydrocarbons available by programmed cracking of kerogen. High S2 therefore represents high amount of kerogen in the rock suggesting a possible good source rock. Tmax is the temperature corresponding to maximum of S2 generation. This also indicates the maturity of the corresponding source rock. This is because a highly matured source rock is supposed to start cracking at elevated temperature compared to low matured kerogen.

source rock and its peak generation, Tmax, is also supposed to be high. S3 is the amount of CO2 generated from start to the end of the pyrolysis program. This CO2 is generated using oxygen from the oxygen bearing radicals (-COOH, -CHO, -CO-, -O- etc) in the compounds of kerogen. High amount of S3 thus represents high amount of such oxidised

radicals and in turn suggest more oxygenated environment of kerogen deposition.

Pyrolysis results also generate three more derived parameters – hydrogen index (HI), oxygen index (OI).and production index (PI).

HI = 100(S2 / TOC) --- (1)OI = 100(S3 / TOC) --- (2)PI = S1/ (S1+S2) --- (3)

S2 is believed to be the cracked hydrocarbon produced from kerogen and therefore, higher S2 represent higher produced hydrocarbon.

Produced hydrocarbon is essentially a function of carbon & hydrogen. Therefore, division of S2

with TOC is expected to be function of hydrogen. Similarly, S3 / TOC is a function of oxygen and is called oxygen index.

Production index is the ratio of S1 and (S1+S2). Assuming S1 be the generated bitumen and S2 be the potential, PI suggests fraction of the total potential generated.

Limitations: Although Rock-Eval is used very frequently for source rock analysis but it has got serious limitations for which it is not considered as confident source rock evaluator but it is used as source rock screening aparatus. The following limitations should be borne in mind during interpretation of data:.•The reliability of S2 peak depends on the assumption that at 3000C all

the free hydrocarbons will expel as S1 from kerogen, leaving only kerogen decomposition to form S2 at higher temperatures. Expulsion

however depends on the vapour pressure. A C24H50 free hydrocarbon

has a boiling point of 3900C STP (Ferris, 1955). Consequently it comes off as S2 in stead of S1.Three straight chain paraffins, C16H34, C22H46 and C24H50 were mixed

with calcite and pyrolysed (Tarafa, 1983). C16 peak came off as S1 but

C22 was split between S1 and S2 and most of C24 came off as S2 peak. It

was concluded that free hydrocarbons containing 24 or more atoms of carbon come off as S2 and would result higher HI.

•Because of the same earlier reason S1 is also to be considered with

suspision. Further, S1 may be high in reservoir rocks and carrier beds

due to migratory hydrocarbons. It becomes difficult to distinguish autochthonous generated hydrocarbons from allochthonous migratory hydrocarbons.•Hydrogen index (HI) does not necessarily represent the amount of hydrogen concentration in the source rock. This is because FID measures the number of C-C bonds in hydrocarbon.Therefore, hexane and benzene both should have equal HI but actually, hexane has got H/C=2.33 and benzene has H/C=1.0. This suggests all the high HI values do not correspond to high concentration of hydrogen in the sample, rather it may be due to high aromaticity of the organic matter in the sample.

•Whereas both S1 and S2 are not confident, then the derived parameters like, HI, Tmax and PI are also not dependable.

Lithology TOC S1 S2 Tmax HI PIRemark as

possible Source Rock

1. Shale 0.68 0.15 0.68 429

2. Shale 1.19 0.17 1.36 438

3. Shale 0.98 0.06 1.47 433

4. Shale 0.9 0.08 1.2 439

5. Shale 0.43 0.01 0.21 440

6. Shale 1.01 0.06 1.19 438

7. 80% Sh, 20% silt 0.86 0.38 2.86 438

8. 90% Sh, 10% silt 0.62 0.32 2.53 438

9. Shale 0.36 0.18 1.1 440

10. Shale 1.4 1.4 2.8 450

Complete the table given below with remarks of possible source rock.

Documents

Week 1 - 3 Course contents Introduction To Petroleum Geology Philosophy of the science of petroleum geology and relationship of petroleum geology with