Page 121 - 4714

porous rock has “porosity”. Reservoir rocks must be porous, because
hydrocarbons can occur only in pores.
A reservoir rock is also permeable – that is, its pores are
connected. If hydrocarbons are in the pores of a rock, they must be
able to move out of them. Unless hydrocarbons can move from pore to
pore, they remain locked in place, unable to flow into a well. A
suitable reservoir rock must therefore be porous, permeable, and
contain enough hydrocarbons to make it economically feasible for the
operating company to drill for and produce them.

Origin and Accumulation of Oil and Gas
To understand how hydrocarbons get into buried rocks,
visualize an ancient sea teeming with vast numbers of living
organisms. Some are fishes and other large swimming beasts; others,
however, are so small that you cannot see them without a strong
magnifying glass or a microscope. Although they are small, they are
very abundant. Millions and millions of them live and die daily. It is
these tiny and plentiful organisms that many scientists believe gave
rise to oil and gas.
When these tiny organisms died millions of years ago, their
remains settled to the bottom. Though very small, as thousands of
years went by, enormous quantities of this organic sediment
accumulated in thick deposits on the seafloor. The organic material
mixed with the mud and sand on the bottom. Ultimately, many layers
of sediments built up until they became hundreds or thousands of feet
(metres) thick. The tremendous weight of the overlying sediments
created a great pressure and heat on the deep layers. The heat and
pressure changed the deep layers into rock. At the same time, heat,
pressure, and other forces changed the dead organic material in the
layers into hydrocarbons: crude oil and natural gas.
Meanwhile, geological action created cracks, or faults, in the
earth’s crust. Earth movement folded layers of rock upward and
downward. Molten rock thrusted upward, altering the shape of the

120