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Oil and gas occupy the pore spaces of sedimentary rocks in petroliferous basins to the extent of the order of 1 part per 100,000; below shallow depths the remainder of the pore space is filled with water. Hence, oil and gas originate, migrate, and become stably trapped in a rock-water environment.
From an initial state of dispersion elementary volumes of oil or gas are driven by physical forces to positions of concentration and entrapment. The direction of these forces is from regions of higher environmental energy for the given fluid to lower--energy regions; and traps for a given fluid are regions of local minimum potential energy.
The search for oil and gas thus reduces itself to a search for regions of minimum potentials for these two fluids. These, in turn, depend on the density and state of motion of the ambient water as well as on the geometrical configuration of the rocks. Petroleum geology, to the extent that it is to become a rational, rather than an empirical science, must therefore ultimately be based on a comprehensive knowledge of the mutual relations of the rock-water-oil (or gas) complex.
Out of such knowledge, it is seen that the conventional horizontal stratification of gas, oil, the water is true only for the special case of hydrostatics. For the general dynamical case, when the water is in motion, traps for gas and those for oil do not coincide. Furthermore, it is possible for such traps to exist in almost any structural position from the crests of anticlines to the troughs of synclinal basins.
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