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The anticlinal or so-called "gravitational" theory, despite its effectiveness as a basis for petroleum exploration, represents but a special case of oil and gas accumulation, and is valid only when the associated ground water is in hydrostatic equilibrium. Since this need not be the case, a more general formulation, valid for both hydrostatic and hydrodynamic conditions, is required.
Oil and gas possess energy with respect to their positions and environment which, when referred to unit mass, may be termed the potential at any given point of the fluid considered. When the potential of a specified fluid in a region of underground space is not constant, an unbalanced force will act upon the fluid, driving it in the direction in which its potential decreases. Hence, oil and gas in a dispersed state underground migrate from regions of higher to those of lower energy levels, and come ultimately to rest in positions which constitute traps, where their potentials assume locally minimum or least values. In nearly all cases traps for petroleum are regions of low potential which are enclosed jointly by regions of higher potential and impermeable barriers.
Oil and gas migration occurs through a normally water-saturated environment. If the water is at rest, the oil and gas equipotential surfaces will be horizontal, the impelling forces will be directed vertically upward, and the traps will be the familiar ones of the anticlinal theory. If the water is in motion in a non-vertical direction, the oil and gas equipotentials will be tilted downward in the flow direction with those for oil inclined at an angle greater than those for gas. The impelling forces for oil and for gas will not be parallel and the two fluids will migrate in divergent directions to traps which in general will not coincide and may, in fact, be separated entirely, a trap for oil being incapable of holding gas, and vice versa.
Under hydrodynamic conditions accumulations of oil or gas will invariably exhibit inclined oil- or gas-water interfaces with the angle of inclination given by
where dz/dx is the slope of the interface, ^rgrw the density of the water and ^rgr0 that of the oil (or gas), and dh/dx the component of slope of the potentiometric surface of the water in the horizontal direction x. Stable oil and gas accumulations may be found in anticlines but they may equally well occur in structural terraces, noses, monoclines, and other unclosed structures entirely devoid of lithologic barriers to updip migration.
Not only are these effects theoretically expectable but they have been found to occur, with tilts ranging from tens to hundreds of feet per mile, in almost every major oil-producing area. If many such accumulations are not to be overlooked, we must supplement our customary knowledge of structure and stratigraphy with the three-dimensional ground-water hydrology of every petroliferous basin.
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