content of the reservoir rock. We can also examine the nature of the overlying "roof rock" and of potential source
beds.
(c) Aerial and satellite photography can be used by the geologist to get an overall layout of the
region being studied. The use of the stereoscope aids the geologist in determining where to take more detailed
studies based on the "lay of the land."
(d) Geophysical Exploration methods are becoming the preferred method of field mapping and
can take several forms.
1 Seismic. The Seismic method is most widely used. This method uses explosives to
create underground sound waves and a seismograph to record the waves. By correlating the time it takes for
sound to travel through the ground, geologists can determine the subsurface structure of the underlying rock
layers.
2 Gravimetric. The Gravimetric method uses a gravitometer to measure the variations in
gravity. Large masses of dense rock create a slightly higher gravity than lighter rock and can give the geologist a
better understanding of the region's subsurface structure.
3 Magnetic. The Magnetic method uses a magnetometer to measure the magnetic fields in
the rocks. This allows the geologist to determine a general idea of the structural formations of the region.
(e) All this information about the basin is put together on three types of diagrams: maps, cross-
sections, and three-dimensional models. Each of these diagrams portray the nature and physical relationships of
the rock units and rock fluids that seem important to the exploration effort. These diagrams must now be
interpreted in a four-dimensional space frame, which involves the reconstruction of the successive events that
gave rise to what we see today. These events include deposition of the sediments, their compaction, their changes
upon burial, and the deformation or deformations, which the rocks then suffered. The ultimate aim is to
understand the generation of hydrocarbons in the basin, the flow of fluids, and the accumulation of oil and gas.
Armed with this understanding, the geologist attempts to predict the existence of undiscovered oil and gas pools.
g. Petroleum Production. Once a well has been drilled and completed in the manner dictated by
experience and engineering design, it is ready to be produced. That is, the petroleum is removed from the ground.
There are two types of production: primary production and enhanced recovery.
(1) Primary Production. Primary production is caused by the natural forces existent in the reservoir
formation which drives the petroleum into the well bore. There are three types of "drives":
(a) Water drive. This drive is provided by the pressure of the water exerting an upward force on
the oil zone of the reservoir. This will push oil toward the producing well which has a lower pressure. With time,
water will occupy a larger portion of the reservoir as oil is produced and the pressure exerted by the water will be
depleted.
(b) Gas cap drive. The typical oil reservoir has a zone of reservoir rock above the oil zone,
which contains gas. This gas-bearing zone expands and exerts a downward force on the oil- bearing zone to force
the oil to the production well. With time, this to will become depleted.
19-5
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