assignment 200

151PTE321 Engineering Geology

Lecture 8

Learning Outcomes

At the end of this chapter the student will be able to:

• Learn about the concept of Petroleum System;

• Understand the process of Oil and Gas generation;

• Recognize the different elements of a Petroleum System;

• Recognize the different kind of hydrocarbons generated by the source rock;

• Recognize the different kind of environments needed for the generation of

hydrocarbons;

Disclaimer:

These notes are not intended to cover all the necessary knowledge for a Petroleum Geology course for engineering students. The notes were

created with the intention of serving as a guide and summary on the subject. For more detail it is recommended that the student consults any of

the textbooks recommended by the instructor.

Petroleum Systems

Elements

Source Rock

Migration Route

Reservoir Rock

Seal Rock

Trap

Processes

Generation

Migration

Accumulation

Preservation

A Petroleum System requires that certain geologic factors and geologic events

happen at coordinated times.

Geological conditions and geochemical

processes

1. Occurrence of source rocks which generate petroleums under proper

subsurface temperature conditions.

2. Sediment compaction leading to expulsion of petroleum from the source

and into the reservoir rocks (primary migration).

3. Occurrence of reservoir rocks of sufficient porosity and permeability

allowing flow of petroleum through the pore system (secondary migration).

4. Structural configurations of sedimentary strata whereby the reservoir

rocks form traps, i.e. closed containers in the subsurface for the

accumulation of petroleum.

5. Traps are sealed above by impermeable sediment layers (cap rocks) in

order to keep petroleum accumulations in place.

6. Correct timing with respect to the sequence by which the processes of

petroleum generation/migration and trap formation have occurred during

the history of a sedimentary basin.

7. Favorable conditions for the preservation of petroleum accumulation

during extended periods of geologic time, i.e. absence of destructive, such

as the fracturing of cap rocks leading to dissipation of petroleum

accumulations, or severe heating resulting in the cracking of oil into gas.

modified from Magoon and Dow, 1994

Source Rocks

• Most major source rocks are fine grained, clay-rich siliciclastic rocks

(mudstones and shales) and biogenic limestone’s. Coal may also be

considered as a source rock, even though it is present in relatively small

quantities within the earth when compared to shale’s and biogenic

limestone’s.

• Hydrocarbon originates from micro organisms in seas and lakes. When

they die, they sink to the bottom where they form organic-rich "muds" in

fine sediments.

• In subaerial environments organic matter is readily destroyed by

chemical and microbial oxidation shortly after deposition.

• Good quality petroleum source rocks can be deposited in marine or lake

environments as organic-matter-rich muds providing that bottom waters are

oxygen-deficient, i.e. that reducing conditions prevail.

Plankton (singular plankter) are the diverse collection of organisms that

live in large bodies of water and are unable to swim against a current.

They provide a crucial source of food to many large aquatic organisms,

such as fish and whales (wikipedia).

PLANKTON

These organisms include bacteria, archaea, algae, protozoa and

drifting or floating animals that inhabit—for example—the pelagic zone of

oceans, seas, or bodies of fresh water. Essentially, plankton are defined

by their ecological niche rather than any phylogenetic or taxonomic

classification.

Though many planktonic species are microscopic in size, plankton

includes organisms over a wide range of sizes, including large

organisms such as jellyfish. Technically the term does not include

organisms on the surface of the water, which are called pleuston - or

those that swim actively in the water, which are called nekton.

PLANKTON

PLANKTON Distribution A schematic showing the abundance of plankton in the oceans. The schematic was based on

an image in the book "Waardeer de oceanen" by Lawrence Williams. Plankton is a fish feed

giving an indication of the richness of fish species and their numbers in that part of the ocean.

It is an ocean fertiliser natural in origin (eg unlike others as ammonium-nitrate). Its variation

in abundance shows what parts of the ocean are in more need of protection. Plankton is

richer near land surfaces (trough soil runoff carrying nutrients) It is especially rich near

river mouths.

Source Rocks

• If the concentration of oxygen dissolved in these waters is less than 0.1

ml/l the environment is referred to as anaerobic,

• If it is in the range of 0.1-1.0 ml/l the environment is referred to as

dysaerobic

• If higher oxygen concentrations prevail, the environment is known as

oxic.

• Anaerobic or dysaerobic environments require stagnant water

conditions, because turbulent water circulation results in the replenishment

of oxygen contents.

• A petroleum source is characterized by three essential conditions: it must

have a sufficient content of finely dispersed organic matter of biological

origin; this organic matter must be of a specific composition, i.e.

hydrogen-rich (reducing environment, strips oxygen from sediments);

and the source rock must be buried at certain depths and subjected to

proper subsurface temperatures in order to initiate the process of

petroleum generation by the thermal degradation of kerogen.

Source Rocks

• The sediments are compacted to form organic-rich rocks with very low

permeability.

• With increasing depth, temperature increases, geologic time passes,

and chemical action occurs, converting the organic debris into

hydrocarbons.

• In most geologic situations, the resultant hydrocarbons formed have been

forced out of the sediments during lithification.

• The hydrocarbon can migrate very slowly to nearby porous rocks,

displacing the original formation water.

Models of Deposition of Organic Matter

• There are three basic depositional scenarios which ensure favorable

conditions for the preservation of organic matter.

• The depositional system of the so called stagnation model requires a

restricted basin, i.e. a marine basin which has highly restricted water

circulation with the open ocean. This is the case today, e.g. of the Black Sea

which is up to 2,500 m deep but only has a narrow 25 m deep connection to the

Mediterranean Sea.

from D Leythaeuser, “Encyclopaedia of Hydrocarbons”

Models of Deposition of Organic Matter

• The second principal depositional system in this context is the so-called

productivity model. In certain areas of today’s world oceans, nutrient-rich

bottom water currents upwell across the shelf edges from deep parts of the

continental slopes. When they reach the near-surface interval penetrated by

sun light (photic zone), a massive growth of marine algae occurs

(phytoplanctonic blooms). Enormous quantities of algal biomass are produced

by this photosynthetic activity. This is the basis of the marine food chain, i. e.

algae are eaten by zooplankton which in turn are eaten by fish, etc.

from D Leythaeuser, “Encyclopaedia of Hydrocarbons”

• Currents of water masses of higher density originate in Arctic and Antarctic

oceanic realms and flow along the deep ocean topography towards lower

latitudes.

• Wherever they encounter major topographic elevations they displace

nutrient-rich bottom water masses towards the surface of the ocean. In this

way, a series of processes and effects are initiated which are similar to those

in the upwelling regime leading to the establishment of an open-ocean

oxygen-minimum zone. Wherever this oxygen minimum zone impinges on a

continental shelf, organic matter-rich sediments are deposited.

Models of Deposition of Organic Matter

from D Leythaeuser, “Encyclopaedia of Hydrocarbons”

Models of Deposition of Organic Matter

• All the depositional environments of marine and freshwater systems can also

receive an input of organic matter derived from higher land plants

transported by rivers or glaciers, or wind-blown.

• In contrast to algal or bacterial biomass which is rich in hydrogen, land

plant-derived organic matter tends, due to high contributions by cellulose

and lignin-derived precursor materials, to be rich in oxygen.

from D Leythaeuser, “Encyclopaedia of Hydrocarbons”

• The great variety of kerogens occurring

in nature can be classified into three

broad categories referred to as type I-,

type II- and type III-kerogens (Tissot

and Welte, 1984).

• The high H/C-ratio of type I-kerogens,

goes back to a high input of algae and

bacterial biomass.

• The elevated H/C-ratio of type II-

kerogens is mostly derived from a high

contribution of algal biomass.

Models of Deposition of Organic Matter

from D Leythaeuser, “Encyclopaedia of Hydrocarbons”

• It is the relative abundance of each of

these organic materials which

determines whether the resulting source

rock will generate predominantly oil or

gas upon burial.

• The solid organic matter in source rocks

which is insoluble in low-boiling organic

solvents is called kerogen.

Models of Deposition of Organic Matter

• Kerogens of type III, in contrast, have high O/C- and low H/C-ratios.

• The elevated oxygen content, is either due to a high input of higher land

plants, which are always rich in cellulose and lignin-derived structures, or to

the deposition of any kind of organic matter derived from marine organisms

under dysaerobic to oxic environments.

• Most prolific source rocks for oil have type II-kerogens.

• Type I-kerogens are rare in terms of worldwide occurrence and mostly

restricted to oil shales (rocks which do not contain oil, but high concentrations

of kerogen, which yield oil artificially when the rock is heated to 500°C in an

inert atmosphere).

• Source rocks bearing type III-kerogen generate little oil but more gas and

condensate upon exposure to proper subsurface temperatures.

Organic Matter in Sedimentary Rocks

• Based on empirical evidence, minimum concentration levels of 1.5% and

0.5% total organic carbon (TOC) in source rocks of siliclastic and carbonate

lithologies respectively have been established (Hunt, 1996).

• The organic carbon concentration is an approximate measure of the organic

matter content of a rock.

• Organic matter is predominantly composed of organic carbon, but also

contains minor amounts of N, S, and O.

• This minimum concentration of organic carbon in source rocks is controlled by

the relationship between the quantity of petroleum generated and the

internal storage capacity of the rocks in terms of their porosity.

• If too little organic matter is present, the small quantities of petroleum

generated will not exceed the storage capacity of the rock, i.e. no petroleum

expulsion will take place.

Interpretation of Total Organic Carbon (TOC)

Hydrocarbon

Generation

Potential

TOC in Shale

(wt. %)

TOC in Carbonates

(wt. %)

Poor 0.0-0.5 0.0-0.2

Fair 0.5-1.0 0.2-0.5

Good 1.0-2.0 0.5-1.0

Very Good 2.0-5.0 1.0-2.0

Excellent >5.0 >2.0

The reason why petroleum source rocks of carbonate lithologies tend

to have significantly lower TOC concentrations has to do with the quality

and composition of the organic matter present. In carbonate source

rocks, the organic matter tends to be richer in hydrogen.

Temperature window for Hydrocarbon Generation

Sea level

Depth in feet

-5,000

-10,000

-15,000

Oil window

Gas

formed

only

50

Temperature oF

662

104

• In order for hydrocarbons

to be generated from

organic deposition,

temperatures must rise

above 104ºF (40ºC) but not

exceed 662ºF (350ºC).

• Higher temperatures will

destroy any remaining

organic materials or

hydrocarbons already

generated.

• In order for hydrocarbons

to be generated, a proper

sequence of geologic

events and conditions must

occur.

Hydrocarbon Migration

from “Petroleum Geology”, Schlumberger.

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.

Reservoir Rock

The term “reservoir” implies storage.

Reservoir rock is rock where hydrocarbons are stored and from which they can

be produced.

This reservoir rock may or may not be source rock.

The fluids of the subsurface migrate according to density.

The dominant fluids in hydrocarbon regions are hydrocarbon gas,

hydrocarbon liquids, and salt water.

Since hydrocarbons are the less dense of these fluids, they will tend to migrate

upward, displacing the heavier salt water down elevation.

Hydrocarbons may therefore be forced from their source rock during

lithification, and migrate into the reservoir rock in which they are stored.

The fluids present will separate according to density as migration occurs.

In order for a rock to be a potential reservoir rock, two properties, porosity and

permeability, must exist of sufficient magnitudes to justify economic

development of the hydrocarbon reservoir.

Basic Geological Conditions that Create

Petroleum Traps

• Hydrocarbon traps are any combination of physical factors that

promote the accumulation and retention of petroleum in one location.

• Traps can be structural, stratigraphic, or a combination of the two.

• Geologic processes such as faulting, folding, and deposition and

erosion create irregularities in the subsurface strata which may cause

oil and gas to be retained in a porous formation, thereby creating a

petroleum reservoir.

• The rocks that form the barrier, or trap, are referred to as caprocks.

Down figure illustrates the basic morphology and terminology of an

anticlinal trap.

Note also that the gross pay is the vertical thickness from the top

of the reservoir to the oil-water contact.

Basic Parameters of a Trap

from “Petroleum Geology”, Prof. Farooq Shareef

• Structural traps are due to post-depositional tectonic processes (e.g., folds

and faults).

• Structural traps are created by the deformation of rock strata within the earth’s

crust. This deformation can be caused by horizontal compression or tension,

vertical movement and differential compaction, which results in the folding,

tilting and faulting within sedimentary rock formations.

• Stratigraphic traps are due to syn-depositional sedimentary processes (reefs)

or post-depositional diagenetic ones (dolomitization).

• Hydrodynamic traps are caused by flowing water.

Classification of Hydrocarbon Traps

Structural Traps

There are three main types of structural traps. One type is the anticline, or

upfold of rock. These are illustrated in Figure.

Diagrams depict compressional and compaction anticlines.

from “Petroleum Geology”, Prof. Farooq Shareef

Structural Traps

Anticlinal and Dome Trap

The rock layers in an anticlinal trap were originally laid down horizontally then

folded upward into an arch or dome. Later, hydrocarbons migrate into the porous

and permeable reservoir rock. A cap or seal (impermeable layer of rock) is required

to permit the accumulation of the hydrocarbons.

from “Basic Petroleum Geology and Log Analysis”, Halliburton, 2001

Structural Traps

A second type of structural trap is the fault trap, where porous, permeable

reservoir beds are faulted against impermeable beds. Some fault planes seal,

others act as permeable conduits. These are illustrated in Figure.

from “Petroleum Geology”, Prof. Farooq Shareef from “Basic Petroleum Geology and Log Analysis”, Halliburton, 2001

Structural Traps

The third type is the growth fault, which shows sediment thickening on the

downthrown side. These are illustrated in Figure.

Diagrams depicts a growth fault with associated rollover anticline.

from “Petroleum Geology”, Prof. Farooq Shareef

Hydrodynamic Traps

The downward flow of water in a permeable bed may cause oil to be trapped in

flexures which lack vertical closure. This is a type of pure hydrodynamic trap,

illustrated in Figure.

Cross section to illustrate oil trapped hydrodynamically by flowing water.

from “Petroleum Geology”, Prof. Farooq Shareef

A trap created by piercement or

intrusion of stratified rock layers from

below by ductile nonporous salt.

The intrusion causes the lower

formations nearest the intrusion to

be uplifted and truncated along the

sides of the intrusion, while layers

above are uplifted creating a dome

or anticlinal folding.

Hydrocarbons migrate into the

porous and permeable beds on the

sides of the column of salt.

Hydrocarbons accumulate in the

traps around the outside of the salt

plug if a seal or cap rock is present.

from “Basic Petroleum Geology and Log Analysis”, Halliburton, 2001

Salt Dome or Salt Plug Trap

Stratigraphic Traps

Stratigraphic traps are formed as a

result of differences or variations

between or within stratified rock

layers, creating a change or loss of

permeability from one area to

another. These traps do not occur

as a result of movement of the

strata.

from “Basic Petroleum Geology and Log Analysis”, Halliburton, 2001

• An angular unconformity is one in which

older strata dips at an angle different

from that of younger strata.

• An angular unconformity trap occurs

when inclined, older petroleum bearing

rocks are subjected to the forces of

younger non-porous formations.

• This condition may occur whenever an

anticline, dome or monocline are eroded

and then overlain with younger, less

permeable strata.

Figure 40 Eroded anticline

Figure 41 Eroded monocline

from “Basic Petroleum Geology and Log Analysis”, Halliburton, 2001

Angular Unconformity Trap

from “Basic Petroleum Geology and Log Analysis”, Halliburton, 2001

Angular Unconformity Trap

Fractured Basement

from “Petroleum Geology”, Schlumberger.

Re-distribution of petroleum

• Of the total amount of petroleum generated in the source rocks 75% is

expelled in the course of primary migration into nearby high porosity/

permeability carrier beds.

• During secondary migration, about 50% of the petroleum which has

entered the carrier beds remains in the form of impregnations adsorbed

on mineral surfaces.

• About 40% has, at an earlier stage in the history of this sedimentary basin,

accumulated in reservoir traps.,

• The remaining 10% is on its secondary migration route bypassing all

traps and eventually leaking out at the Earth’s surface. This process is

called petroleum seepage.

• About 25% of the original petroleum accumulated gets lost by cap rock

leakage occurring at a slow rate over long periods of geologic time.

• Of the remaining petroleum, another 25% gets lost in the course of

chemical, physico-chemical and bacterial processes (discussed above).

• In summary, only about

10% of the petroleum

generated in the source

rocks can be discovered

by exploration and

produced for economic

usage.

• There are only a few

reported cases that fall

into this category. These

include the La Luna-

Misoa petroleum

system of Venezuela

and the Arabian/Iranian

Basin in the Middle

East.

from D Leythaeuser, “Encyclopaedia of Hydrocarbons”

Distribution of oil and gas fields based on

geologic age It is important to know the geologic age of reservoir rocks because rocks of

different ages frequently have different petroleum characteristics and

productivity. It is also important to note that the age of the rock does not

necessarily coincide with the time of oil accumulation. You can only know

that it accumulated sometime after the formations deposition.

Geologic Age

Era Period % of Fields

Cenozoic Neogene 18

Palaeogene 21

Mesozoic

Cretaceous 27

Jurassic 21

Permo-Triassic 6

Paleozoic

Carboniferous 5

Devonian 1

Cambrian-

Silurian

1

TOTAL 100

What’s so special about the Mesozoic?

• The worldwide climate was tropical.

• Plankton were abundant in the ocean.

• Ocean bottoms were stagnant and anoxic, unlike today’s ocean.

• Black, organic-rich muds accumulated to form later source rocks.

Distribution of oil and gas fields based on

geologic age

Warm

water

Low-oxygen

layer

Cold

water Modern Ocean

Mesozoic Ocean

Distribution of oil and gas fields based on

geologic age