Page 9 - Practical Petroleum Geology, 2nd Edition

Introduction PRACTICAL PETROLEUM GEOLOGY

One common misconception about the nature of petroleum is that
it exists in large underground formations that are similar to flowing rivers
Petroleum Extension-The University of Texas at Austin
and lakes. Instead, most petroleum is found within rocks. Some rocks have
PORE
PORE a high porosity and allow for a large amount of petroleum to reside in the
pores. Other rocks have few pores, which allows for less petroleum (fig. i.2).
Over time, as the Earth shifted, folds, faults, and other formations
opened new channels through which the petroleum in the rock layers could
PORE
flow. Rock layers with high permeability allowed the petroleum to flow more
easily through the rock’s pores, whereas rock layers with low permeability
PORE
had the opposite effect (fig. i.3).
Figure i.2 Porosity within rock Eventually, the petroleum moved around and became trapped by
(magnified view) impervious layers of rock. These areas—called traps—kept the hydrocarbons
within porous layers of rock, thereby forming reservoirs. A reservoir’s size is
determined by the amount of oil and gas it contains. A reservoir might be
broad and shallow, narrow and deep, or any variation in size. And it is these
reservoirs that drillers want to find and tap.
Armed with this knowledge of the origins of petroleum, we can now
focus on the main topic of the book: petroleum geology.

Figure i.3 Connected pores
resulting in permeability
(magnified view)

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