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The difficulties inherent in the prediction of potential reservoir conditions in basement rocks buried by a concealing mantle of sediments are apparent to all geologists and constitute the main cause for prejudice against basement exploration. Consequently, geologists have not played an important role in the discovery of oil accumulations in basement rocks. All the major discoveries have been accidental.
Our record can be improved by a critical examination of our working hypotheses concerning the nature of the geological processes responsible for the formation of basement oil pools. Among these hypotheses is the generally accepted "up-slope" theory of oil migration in basement reservoirs. The theory is closely patterned after our working concept of migration of oil in conventional sandstone carrier beds. The theory and its limitations as a guide for exploratory work are discussed.
An alternative proposition is advanced that fracturing of competent basement rocks involves dilatancy which in turn reduces hydrostatic pressures in focal areas of deformation. Pressure gradients are thereby established between the potential basement reservoir rocks and the overlying source and carrier beds containing oil, gas, and water. Thus a tendency to "suck in" fluids into the basement rocks is established.
Deposition of calcite in fractured facies of the basement complex above some oil accumulations may be due indirectly to dilatancy--reduced pressures allowing carbon dioxide to escape from waters associated with oil and gas thus causing precipitation of calcium carbonate.
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