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.
