3. Petroleum Accumulation
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Introduction
For petroleum to accumulate; there must be:
1. Source Rocks: These contain organic material from plants or algae. They are buried
and cooked below the earth’s surface for millions of years.
2. Migration: Once the oil is formed, it is forced by gravity to move out of the source
rock and upwards towards the surface. This is a very slow process travelling only a
few kilometers over millions of years.
3. Reservoir rock: The oil or gas will flow until it collects in a reservoir rock, which has
pores (holes) that act like a sponge. Trap (a barrier to fluid flow so that
accumulation can occur against it).
4. Trap: To trap the oil and gas the reservoir rock needs to have an impermeable rock,
called a ‘cap-rock’ above it and around it. This cap rock will not allow oil or gas to
travel through it. If there is no cap-rock the oil and gas will reach the surface as oil
or gas seep.
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Most knowledge has been obtained from experience and observation, but certain generalizations can be made:
• Petroleum originates from organic matter
• To become commercial, the hydrocarbons must be concentrated
• Petroleum reservoirs primarily occur in sedimentary rocks
1- Origin of oil and gas and Source rocks
The story of oil and gas begins hundreds of millions of years ago when the Earth was covered in swamplands filled with
huge trees and the seas were teaming with microscopic plants and animals. The oil and gas deposits started forming
about 290 to 350 million years ago during the Carboniferous Period, which get its name from the basic element in oil and
gas; carbon.
A popular belief is that oil comes from dead dinosaurs. It doesn’t. The giant reptiles lived mostly from 65 to 250 million
years ago, and most scientists believe oil actually comes from the tiniest plants and animals that preceded them.
As they died and sank to the ocean floor, the decomposing organisms, along with mud and silt, created hundreds of feet
of sediment, Sand, clay and minerals settled over this organic-rich mud and solidified into rocks. The weight of the rocks
above pressed the mud into a fraction of its original thickness. Heat from within the Earth cooked the mud’s organic
remains into a soup of hydrocarbons, the main element of petroleum and natural gas.
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Diatoms are an important group of phytoplankton. They contain a silica skeleton and may
reach 1 mm in diameter (right). Other phytoplankton organisms have a carbonate
skeleton.
Zooplankton includes planktonic foraminifera, radiolaria, and planktonic crustacea.
One theory for the origin of oil that Oil and gas originate from organic matter in
sedimentary rocks, dead vegetation in the absence of oxygen ceases to decompose. It
accumulates in the soil as humus and as deposits of peat in bogs and swamps.
Peat buried beneath a cover of clays and sands becomes compacted as the temperature,
weight and pressure of the cover increase, and water and gases are driven off. The
residue, ever richer in carbon, becomes coal.
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In the sea, a similar process takes place. Of the marine life that is eternally falling slowly to the bottom of the sea,
vast quantities of it are eaten, some is oxidized, but a portion of the microscopic animal and plant life escapes
destruction and is entombed in the mud on the seafloor. The organic debris collects at the bottom and is buried
within a growing buildup of sands, clays and more debris until the thickness of sediment attains thousands of feet.
Bacteria takes oxygen from the trapped organic residues, breaking them down molecule-by-molecule into
substances rich in carbon and hydrogen, and the extreme weight and pressure of the mass compacts the clay into
hard shale.
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The generation of hydrocarbons from the source material depends primarily on the temperature to which the organic
material is subjected.
Hydrocarbon generation appears to be negligible at temperatures less than 150°F (65°C) in the subsurface and reaches
a maximum within the range of 225° to 350°F (107° and 176°C), the “hydrocarbon window”. Increasing temperatures
convert the heavy hydrocarbons into lighter ones and ultimately to gas. However, at temperatures above 500°F
(260°C), the organic material is carbonized and destroyed as a source material. Consequently, if source beds become
too deeply buried no hydrocarbons will be produced.
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Type of Hydrocarbon migration
Primary Migration: Primary migration of petroleum from source to reservoir is caused by the movement of water, which
carries oil out of the compacting sediments. When the source mud is deposited it contains 70 to 80 percent water the
remainder is solids, such as clay materials, carbonate particles or fine-grained silica. As they build up to great thickness in
sedimentary basins, water is squeezed out by the weight of the overlying sediments. Under normal hydrostatic pressure
(approximately 0.446 psi/ft), the clays lose porosity and the pore diameters shrink.
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2- Migration from source to reservoir
After generation, the dispersed hydrocarbons in the fine-grained source
rocks must be concentrated by migration to a reservoir. Compaction of
the source beds by the weight of the overlying rocks provides the driving
force necessary to expel the hydrocarbons and to move them throughout
the more porous beds or fractures to regions of lower pressure (which
normally means a shallower depth.) Gravity separation of gas, oil and
water takes place in reservoir rocks which are usually water-saturated.
Consequently, petroleum is forever trying to rise until it is trapped or
escapes at the earth's surface. Vertical migration via faults and fractures
has led to many of the large oil accumulations, such as that found at
shallow depths in the northern Iraq.
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Fluids tend to move toward the lowest potential energy. Initially this is upward, but
as compaction progresses there is lateral as well as vertical movement. The lateral
movement results primarily from the tendency of the flat clay mineral particles to lie
horizontally as they are compressed.
This reduces the vertical permeability of the compacting sediments. In addition, the
long continuous sands on the edges of basins orient fluid movement laterally as
burial progresses, as illustrated in Figure 2-18. The migration of oil from source to
reservoir is as follows:
1- Water flows toward the lowest potential energy.
2- Clays often have abnormal pressure because they are slow to release water.
3- Avenues of migration during basin compaction are:
• Sandstones
• Unconformities
• Fracture-fault systems
• biohermal reefs
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Primary migration mechanisms
1. Migration by diffusion. Because of differing concentrations of the fluids in the source rock and the surrounding rock
there is a tendency to diffuse. A widely accepted theory.
2. Migration by molecular solution in water. While aromatics are most soluble in aqueous solutions, they are rare in
oil accumulations, therefore discrediting the general importance of this mechanism, although it may be locally
important.
3. Migration along micro fractures in the source rock. During compaction the fluid pressures in the source rock may
become so large that spontaneous “hydro-fracing” occurs. A useful if underestimated hypothesis.
4. Oil-phase migration. OM in the source rock provides a continuous oil-wet migration path along which the
hydrocarbons diffuse along pressure and concentration gradient. This is a reasonable but unproved hypothesis,
good for high TOCs.
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Secondary Migration: The process in which hydrocarbons move along a porous and permeable layer to its final
accumulation is called secondary migration
The position of the accumulating pool is affected by several - sometimes conflicting - factors. Buoyancy causes oil to
seek the highest permeable part of the reservoir; capillary forces direct the oil into the coarsest-grained portion first,
and into successively finer-grained portions later.
Any permeability barriers in the reservoir channel will drive the oil into a somewhat random distribution. Oil
accumulations in carbonate rock are often erratic because part of the original void spaces have been plugged by
minerals introduced from water solutions after rock is formed.
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The geological activities effects on petroleum accumulation:
- In large sand bodies, barriers formed by thin layers of dense
shale may hold the oil at various levels.
- When crustal movements of the earth occur, oil pools are shifted
away from the place in which they originally accumulated.
- Faults sometimes cut through reservoirs, destroying parts of the
pools or shift them to different depths.
- Uplift and erosion bring the pools near the surface where the
lighter hydrocarbons evaporate.
- Fracturing of the cap rock allows oil to migrate vertically to
shallower depth.
Wherever differential pressures exist and permeable openings provide a
path, petroleum will move.
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Tertiary migration: the migration of petroleum accumulation which trapped in the reservoir to the surface.
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3- Reservoir rocks
The petroleum liquids and vapors were emitted from these source rocks, moved upward through the sediment pores,
and accumulated between the grains of the sediment, or “reservoir rock.” The reservoir rocks often contained water,
which then pushed the lighter oil and gas upward until they hit an impermeable rock layer, such as mudstone or salt
rock, which becomes a “seal” or “cap rock.” The oil and gas are thus trapped in “reservoir rocks,” usually sandstone or
limestone.
So petroleum reservoirs aren’t underground pools as is commonly believed. They are actually rocks soaked in oil and
gas, just as water is held in a sponge. (The word petroleum comes from the Latin for “rock oil.”) And they’re not alone in
that sponge-like home. Other substances such as water, salt, carbon dioxide and hydrogen sulfide can get trapped in the
rocks too. Oil and gas, however, contain mostly two elements: hydrogen and carbon. How those elements are arranged
determines the form of the hydrocarbon. For example, natural gas contains the simplest hydrocarbon, methane, while
crude oils can be made up of more complex liquid and solid hydrocarbons.
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14. Reservoir Rocks Characteristic
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1- Permeability
Definition
The Permeability of a rock is the ability of the hydrocarbons to move from one pore to
another so the pores of the rock must be connected together.
Unless hydrocarbons can move and flow from pore to pore, the hydrocarbons remain
locked in place and cannot flow into a well. In addition to porosity and permeability
reservoir rocks must also exist in a very special way. To understand how, it is necessary
to cross the time barrier and take an imaginary trip back into the very ancient past.
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15. Texture Affecting Permeability (and Porosity as well)
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Increased roundness and sphericity lead to higher permeabilities, In what depositional settings do we find the
different grains shown here?
Typical occurrences of clay minerals in sandstones
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The clay type can also have a great influence on permeability.
Shown are kaolinite (a), chlorite (b), and fibrous illite (c).
How do their distributions and shapes affect permeability’s?
The different clay textures on the previous slide lead to dramatically different porosity/permeability relationships.
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Compaction
Compaction is particularly strong in rocks with lower grain fractions (the amount that
grains constitute of the total solid volume, shown here in fractions of unity, with the
rest being fine-grained matrix minerals).
Clays and other matrix minerals move under pressure into the pore spaces.
The softer grains in greywackes crumble and dislocate to clog the pores.
Cementation additionally leads to porosity reduction.
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Porosity
Definition
The porosity of a rock is the percentage of the total volume of the rock that is occupied
by interstices (or Porosity is the volume of the non-solid portion of the rock filled with
fluids, divided by the total volume of the rock).
Nearly all rocks and sediments contain openings called pores or voids, which come in all
shapes and sizes. Some of them are too small to be seen with the unaided eye, and the
smallest range in size down to the dimensions of molecules. In exceptional cases, they
may be many feet across. Materials containing a relatively large proportion of void
space are described as porous or said to possess "high porosity". That fraction of the
pores through which water can flow is called “effective porosity”. Total porosity may
range from near zero to over 50%, depending on the material, while effective porosities
are typically somewhat smaller. The picture shows examples of different types of
porosity
Because of the higher solubility and different geomechanical properties of carbonates
compared to sandstones, a much greater variety of pore types is found in them;
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Type of Porosities
Primary porosity is the porosity developed by the original sedimentation process by
which the rock was created.
Secondary porosity is created by processes other than primary cementation and
compaction of the sediments.
An example of secondary porosity can be found in the solution of limestone or dolomite
by ground waters, a process which creates vugs or caverns. Fracturing also creates
secondary porosity. Dolomitization results in the shrinking of solid rock volume as the
material transforms from calcite to dolomite, giving a corresponding increase in
porosity.
Example of different type of porosity as shown in picture:
Intergranular (lime and dolomite grainstone)
Intercrystalline (Sucrosic dolomites)
Moldic* (moldic oolitic limestone and dolomite grainstone)
Matrix or Chalky (Mudstone, Chalks)
Moldic or vuggy in addition to matrix (vuggy Packstone and Wackstone)
Fracture or fissure porosity
* moldic porosity (mouldic porosity) A form of secondary porosity, developed by the preferential dissolution of shell
fragments or other particles, to leave empty spaces previously occupied by the particles.
