Page 25 - Petroleum and Gas Field Processing

1.4 FINDING OIL AND GAS EXPLORATION

As explained in the previous sections, oil and gas exist in reservoirs located
thousands of feet below the Earth’s surface and ocean floors. These
reservoirs would exist only in certain locations depending on the geologic
history of the Earth. Therefore, determining the location of petroleum
reservoirs is a very difficult task and is, probably, the most challenging
aspect of the petroleum industry. Finding, or discovering, a petroleum
reservoir involves three major activities: geologic surveying, geophysical
surveying, and exploratory drilling activities. The following subsections
provide a brief background on each of these activities.

1.4.1 Geologic Survey
Geologic surveying is the oldest and first used tool for determining
potential locations where there would be a possibility of finding
underground petroleum reservoirs. It involves examination of the surface
geology, formation outcrops, and surface rock samples. The collected
information is used in conjunction with geologic theories to determine
whether petroleum reservoirs could be present underground at the
surveyed location. The results of the geologic survey are not conclusive
and only offer a possibility of finding petroleum reservoirs. The rate of
success of finding petroleum reservoirs using geologic surveys alone has
been historically low. Currently, geologic surveys are used together with
other geophysical surveys to provide higher rates of success in finding
petroleum reservoirs.

1.4.2 Geophysical Surveys
There are mainly four types of geophysical surveys used in the industry:
gravity survey, magnetic survey, seismic survey, and remote sensing.
The gravity survey is the least expensive method of locating a
possible petroleum reservoir. It involves the use of an instrument, a
gravimeter, which picks up a reflection of the density of the subsurface
rock. For example, because salt is less dense than rocks, the gravimeter
can detect the presence of salt domes, which would indicate the presence of
an anticline structure. Such a structure is a candidate for possible
accumulation of oil and gas.
The magnetic survey involves measurement of the magnetic pull,
which is affected by the type and depth of the subsurface rocks. The
magnetic survey can be used to determine the existence and depth of
subsurface volcanic formations, or basement rocks, which contain high

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.