Page 269 - Standard Handbook Of Petroleum & Natural Gas Engineering

240 General Engineering and Science

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GEOLOGICAL ENGINEERING
Geology is the study of the earth, its internal and surface composition, structure, and
the earth processes that cause changes in composition and structure. The earth is constantly
changing. The processes within the earth and the history of these processes are important
factors in determining how minerals deposits were formed, where they accumulated, and
how they have been preserved. The geology of the present composition and structure of
the earth and, secondarily, the history of the processes that resulted in the present geology
have become very important in the prediction of where accumulations of economically
valuable hydrocarbons (oil and gas) may be found. Studies of surface geological fractures
and the past processes coupled with such surface geophysical investigation techniques as
seismic, gravity, magnetic, radioactive, electrical, and geochemical are used to locate
probable subsurface target regions that might contain economically valuable accumulations
of hydrocarbons. However, only by drilling a borehole from the surface to these subsurface
regrons is it possible to definitely assess whether hydrocarbons exist there. The borehole
provides a direct fluid communication from these subsurface regions to the surface,
where the fluid (if present) can be assessed for its economic value.
Not only is geology important in exploring for hydrocarbons, but also engineers
must study the present composition and structure of the earth to successfully drill
the borehole itself. Further, once hydrocarbons have been found and have proven to
be economically recoverable, studies of the physical and chemical aspects of earth in
such regions are important to the follow-on production and reservoir engineering.
These studies help ensure that the accumulated hydrocarbons are recovered in an
economic manner [24].

General Rock Types
The earth is composed of three general rock types: igneous, sedimentary, and
metamorphic [24].

Igneous rocks are the original rocks of the earth and were solidified from the
molten mixture of materials that made up the earth prior to its cooling. Igneous
rocks are very complex assemblages of minerals. Usually such rocks are very
dense and have very few pores (or voids) which can accumulate or pass any type
of fluid.
Sedimentary rocks are aggregates of particles broken away from other rock masses
which are exposed at or near the earth’s surface to weathering processes. These
particles are then transported by water, wind, or ice (glaciers) motion to new
locations where they assemble eventually into a new rock mass. The origin of
the rock mass prior to action by the weathering process can be igneous rock,
another sedimentary rock, or metamorphic rock (see next rock type). During
the process of weathering and transportation (particularly by water) compounds
are precipitated chemically from the original rock mass or from materials within
the water itself. In lake or seawater environments organisms provide such com-
pounds. .4s the particles are deposited in slower moving waters, the chemical
compounds provide a type of cement that ultimately binds the particles into a
sedimentary rock mass. Sedimentary rock masses that are formed from water
erosion and deposition by water in lake or ocean environments were originally
laid down (Le., deposited) in horizontal or near horizontal (deltas) layers.
Metamorphic rocks are formed from either igneous, sedimentary, or possibly other
metamorphic rock masses. These original rock masses are subjected to heat,