e. The Trapping Mechanism.
(1) As oil and gas are lighter that the ground water which permeates the porous rocks below the
water table, it is evident that the upward movement of petroleum must be restricted in order for accumulations to
form. A natural barrier, or trap, is the mechanism that allows a petroleum reservoir to form.
(2) There are many types of structural features that result in conditions favorable to the accumulation
of hydrocarbons. The most common traps associated with petroleum reservoirs are: anticlines, salt domes, faults,
and stratigraphic traps.
(a) In an anticlinal structure, the rocks comprising the crust of the earth are folded upward. The
oil and gas are usually found on the crest of an anticlinal structure. An impervious cap rock must be present to
seal the reservoir and prevent the escape of the gas and oil into higher layers. This cap rock, in one form or
another, must be present in all reservoirs to contain the oil and gas within the structure.
(b) The salt dome is the result of the intrusion of large masses of salt into the sediments where
they are found. This salt is believed to flow as a viscous semisolid when subjected to high pressure. This
intrusion creates an upward pressure and results in the doming of the overlying sedimentary rocks. In this type of
structure, oil accumulates within the upturned porous beds about the summit and flanks of the salt core.
(c) A fault is a region where two or more of the earth's tectonic plates meet. As the plates slide
against each other, they may form a trap that allows a petroleum reservoir to form when a porous rock is brought
into contact with an impervious layer.
(d) In the stratigraphic trap, the producing formation gradually pinches out for one reason or
another and is overlaid by impervious rock. As a result, the oil, gas, and water can no longer migrate upward and
is trapped.
f. The Search for Petroleum. While there are many methods of searching for petroleum to include
geologic and geophysical means, the major concern of oil company exploration is the study of the sedimentary
strata in a producing or potential petroleum province (basin).
(1) The petroleum geologist studies the composition, texture, fossil content, and thickness of the
strata and the way they relate to one another. That is, their vertical succession and horizontal (lateral) extent and
variability.
(2) On top of this, the geologist must know the way in which the strata were folded and faulted after
deposition and the geologist must understand any flow of fluids now taking place in the rocks. The most
important rocks, of course, are the known or potential reservoir rocks and the strata directly associated with them.
(3) The information for this "field mapping" comes from several sources.
(a) Surface geologic surveying can determine the subsurface structure of the rocks based on the
geologic age of the rock, layers of exposed rock, and the angles at which the beds are formed.
(b) Subsurface geologic surveying is obtained from actual samples of the reservoir rocks
obtained from wells already drilled, deep core wells, and sometimes, mining records. The samples can be studied
under the microscope; comparing samples from different places will reveal lateral variability in the texture,
mineral composition, organic components, and fluid
QM5200
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