POTENTIAL differences between strata of shale and sandstone have been recognized for about 15 years, and they form the basis of the electrical logging of oil wells. Hitherto these potentials have been ascribed solely to electrochemical reactions caused by the disturbing effects of the water in the drill hole through which they are generally measured. Observations by the author in wells empty of water and in a mine shaft sugge~t that potential differences between sandstones and shales are natura.! and specific properties of the rocks, although they are modified in measurement by the disturbing effects of the water in the drill hole, The cause of these potentials probably is to be sought in the relative polar adsorptive capacities of quartz and clay particles.
Five different chemical types of water are found in the Oklahoma panhandle area, each type characteristic of a particular geological zone. The Permian of the Hugoton-Panhandle field contains concentrated chloride-calcium brine, which resembles normal connate water except that it is unusually rich in sulfate (Type V). The Pennsylvanian sands above the Morrow (Lansing, Tonkawa, and Toronto) contain normal chloridecalcium connate water (Type Y). The Oswego sands contain similar water, somewhat less concentrated (Type W). The Morrow sands contain three different types: in Cimarron County, Okla., a high-sulfate, chloride-calcium brine resembling Type V (Type X); in Texas C01ll1ty, the normal chloride-calcium brine (Type Y); and in Beaver C01ll1ty eastward to Blaine County, a very strange, dilute water containing appreciable amo1ll1ts of carbonate and sulfate (Type Z).The origin of the different chemical types is quite puzzling. The dilute waters of Type Z are especially anomalous, for they resemble meteoric waters chemically, although there is no hydrodynamic circulation at the present time. They may be ancient meteoric waters trapped by the uplift of the Amarillo-Wichita m01ll1tains shortly after deposition of the Morrow sedimentõ r they may be "immature" connate waters similar to those reported from the abnormally pressured References and illustrations at end of paper. areas of the Gulf Coast Tertiary.
Oil and gas are fluids but our prospecting methods have been based mainly on observations of the reservoir rocks. Little use has been made of measurements of the physical and chemical properties of their fluid contents. There is now general agreement that petroleum originates in a very disperse state and is segregated into oil pools during the compaction and lithification of the sediments. The mechanism of this process is little understood, but it may be caused by the expulsion of pore water from the sediments and involve a capillary filtering action. If the routes of water migration and the location of capillary filters could be predicted, much more use could be made of our available geological data in predicting likely places for oil to accumulate. Usually the oil remains where it was originally trapped. Sometimes, however, the regional dip may be reversed and the oil will then run to the other end of the aquifer that contains it. In mountainous areas, reservoirs are severely disturbed and often breached so the oil can run out, which is the principal cause of surface oil seeps. In such areas an effort should be made to locate the sites of the original accumulations, and then consider where the oil might now be.
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