Lecture 2 con't part (3)

 

2.1.10 Petroleum accumulations

A petroleum accumulation must have (1) a source of oil and gas, (2) a porous and

permeable bed or reservoir rock and (3) a trap that acts as a barrier to fluid flow so that

accumulation can occur.

2.1.10.1 Origin of petroleum

Oil and gas probably originated from organic matter in sedimentary rocks. The origin of

coal on land is a process similar to the origin of petroleum in the sea. In the formation of

coal, dead vegetation in the absence of oxygen ceases to decompose and accumulates as

humus in the soil and as deposits of peat in bogs and swamps. Peat buried beneath a

cover of clays and sands becomes compacted. As the weight and pressure of the cover

increase, water and gases are driven off. The residue, very rich in carbon, becomes coal.

In the sea a similar process takes place. An abundance of marine life is eternally falling

in a slow, steady rain to the bottom of the sea. Vast quantities of matter are eaten or

oxidized before they reach the bottom but a portion of this microscopic animal and plant

residue escapes destruction and is entombed in the ooze and mud on the sea floor. The

organic debris collects in sunken areas at the bottom and is buried within an

ever-increasing accumulation of sands, clays and more debris until the sediment is

thousands of feet thick. As the sediment builds, the pressure of deep burial begins to

work. Bacteria take oxygen from the trapped organic residues and gradually break down

the matter, molecule by molecule, into substances rich in carbon and hydrogen. The

extreme weight and pressure of the mass compacts and squeezes the clays into hard

shales. Within this deep. unwitnessed realm of immense force, oil is born.

2.1.10.2 Reservoir rocks

A petroleum reservoir is a rock capable of containing gas, oil, or water. To be

commercially productive, it must be big enough, be thick enough, and have enough pore

space to contain an appreciable volume of hydrocarbons. Also, it must give up the

contained fluids at a satisfactory rate when the reservoir is penetrated by a well.

Sandstones and carbonates (such as limestone and dolomite) are the most common

reservoir rocks.

Besides porosity, a reservoir rock must also have permeability; i.e., the pores of the rock

must be connected. Connected pores allow petroleum to move from one pore to another.

2.1.10.3 Traps

Migration is a continuing process once the hydrocarbons have been generated and

expelled from the source rock. Hydrocarbons will move ever upward until they escape at

the surface unless something stops the movement. Therefore, a barrier, or trap, is needed

to impede this migration in order to get subsurface accumulation of petroleum.

A trap is produced by geological conditions that cause oil and gas to be retained in a

porous reservoir. Reservoir traps for hydrocarbons have two general forms: (1) an arched

upper surface, commonly called structural and (2) an up-dip termination of porosity,

called stratigraphic (Figure 2-8).

2.1.10.4 Structural traps

A structural trap is formed by the folding or faulting of the rock layer that contains the

hydrocarbons (Figure 2-9). Structural traps vary widely in size and shape. Some of the

more common structural traps are anticlinal traps, fault traps and dome and plug traps.

2.1.10.5 Stratigraphic Traps

A stratigraphic trap is caused either by a nonporous formation sealing off the top edge of

a reservoir bed or by a change of porosity and permeability within the reservoir bed itself

(Figure 2-10). Two general kinds of stratigraphic traps are the disconformity and the

angular unconformity, both resulting from unconformities.

 

2.1.10.6 Combination traps

Another common type of reservoir is formed by a combination of folding, faulting,

changes in porosity and other conditions - some structural and some stratigraphic in

origin. Examples of reservoirs of this nature are the many reservoirs found in the

Seeligson field in Southwest Texas or parts of the East Texas field.

Lecture 2 con't part (2)

2.1.9 Structural geology

2.1.9.1 Introduction

At destructive plate margins, the sediments and the top part of the crust are compressed

and deformed by the process of collision. The rocks are bent and fractured. The study of

the structures that result and the processes that form them is called Structural Geology.

2.1.9.2 Earth movements

Most rocks are fractured during earth movement, resulting in cracks called joints. If the

rock layers on one side of a fracture have moved in relation to the other side, the fracture

is called a fault (Figure 2-4). Displacement - or how far apart the sides of the fault have

moved - may range from only a few inches to many miles, as along the San Andreas fault

in California.

2.1.9.3 Faults

A simple classification system outlines four kinds of faults: normal, reverse, thrust, and

lateral (Figure 2-4). The names are derived from the movement of adjacent blocks.

Movement is up or down in normal and reverse faults but is mainly horizontal in thrust

and lateral faults. A combination of vertical and horizontal movements is also possible in

all faults.

Rotational faults and upthrusts (Figure 2-5) are variations of normal and reverse faulting.

They are most important to the petroleum geologist because they affect the location of

oil and gas accumulations.

Earth movements often bury or prevent the depositing of part of a sediment series that is

present elsewhere. Such buried erosion surfaces are called unconformities. Two general

kinds of unconformities are the disconformity and the angular Figure 2-6). Earth

movements are most important to petroleum geology because they produce barriers that

cause a large proportion of petroleum accumulations.

2.1.9.4 Folds

Folds can be classified in many ways, one of the simplest is into anticlinal and synclinal

folds.

As compressional forces increase, the folds become tighter and the limbs drop more

steeply. Assymetric folds are ones in which one limb dips more steeply than the other.

These dips can eventually become greater than vertical and folds become overturned.

Axial plane cleavage can develop which is caused by alignment of platey minerals

parallel to the fold axis. With increasing deformation this cleavage can dominate the

structure of the rock, obliterating the original bedding. Fold axes need not be horizontal,

in which case they are said to plunge.

If more than one episode of the folding takes place, then the axial planes cleavage

developed by the first phase may itself be folded. This is then known as superimposed

folding and can often be recognized by statistical analysis of several fold axes in one

area.

Folding in sedimentary rocks is important as it creates the potential for oil traps on the

Crest of folds, and these are a major cause of hydrocarbon accumulations.

2.1.9.5 Joints

These are fractures in the rock which are not associated with any significant movement

of the rock. They typically occur in Limestones and Dolomites due to solution along

natural planes of weakness by percolating underground waters, or by removal of

overlying weight of rock by erosion which allows the rock to expand slightly from stress

release, and fracture. They normally develop in three planes, all at right angles, and often

have a strong control on the geomorphology of the area. Jointing in the rocks can lead to

large volumes of porosity and is an important reservoir type, particularly in carbonate

rocks. It can also give lost circulation problems when drilling a highly jointed or

cavernous area.

2.1.9.6 Unconformities

Although these are not strictly structural features, we will look briefly at unconformities.

An unconformity is any break in the geological sequence.