Page 8 - Practical Petroleum Geology, 2nd Edition

Petroleum Extension-The University of Texas at Austin

ny examination of the subject of petroleum geology must first begin with a Introduction
Abrief consideration of where the petroleum came from in the first place.
The popular organic theory states that the hydrogen and carbon that
make up petroleum come from the remains of microscopic organisms that
lived in the rivers and seas covering the Earth’s surface millions of years
ago. As these organisms died, they fell to the ocean floor and mixed with
silt, sand, and mud. Eventually, a thick body of sediments enriched by the
organic remains accumulated on the bottom of the ocean.
Over a very long period of time, the great weight of the overlying
sediments pushed the lower layers deep into the Earth and changed the
bottom beds into rock. In such an environment, the high heat and intense
pressure—along with bacteria, chemical reactions, and other forces—had
a profound effect on the organic remains. The remains were transformed
into petroleum, which subsequently found a home within the rock’s small
porous spaces (fig. i.1).

PETROLEUM AND NATURAL GAS FORMATION
Source: U. S. Department of Energy, Energy Information Administration
OCEAN OCEAN
300-400 MILLION YEARS AGO 50-100 MILLION YEARS AGO

SAND AND SILT
ROCK
SAND AND SILT

PLANT AND ANIMAL REMAINS OIL AND GAS DEPOSITS

Figure i.1 Petroleum formation

1