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A The first step in exploration is to select an isolate area to study. The selection is usually based on a very general analysis of geologic data. When a study area has been reduced to a manageable size, it is called a prospective area, or prospect. Once a prospect is located, some means of evaluating its potential value is needed to determine if a well should be drilled. If a company decides to drill the well, it must determine exactly where the best drilling site would be.
The earliest site selection method was probably based on petroleum seeps. It was logical to assume that if one saw oil seeping at the surface, oil was probably located directly below. Undoubtedly, most of the earlier discovered deposits were located by this type of surface investigation. When all possible sites that could be picked in this manner were exhausted, new selection methods were necessary.
Another site selection technique was surface geology. This method involved visually examining the general lay of the land, looking for unusual surface features such as large mounds or faults that might reflect similar subsurface features. This is a completely valid technique for locating geologic anomalies, or unusual features, that are relatively close to the earth’s surface. A surface anomaly implies that a shallow subsurface anomaly also exists. If a petroleum seep happened to be in the area, the site was considered extremely promising.
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B There is little doubt in the minds of most petroleum geologists, from the evidence of association, that petroleum originates from organic matter of both plant and animal origin that accumulates in fine-grained sediment under quiet conditions relatively deficient in oxygen. These ideas receives firmer support from geochemical studies of sedimentary organic matter, indicating that petroleum is generated during burial of the sediment, primarily under the influences of heat and time, and that it is expelled from the source rocks during compaction.
Crude oils are complex mixtures of hydrocarbons and non-hydrocarbons. In the present context it is interesting to note that there is as much variation between crude oils of the same age—or, more precisely, crude oils in rocks of the same age—as between crude oils of different ages. Since there has been a continually changing pattern of life over the last 600 millions years or so, the general similarities of crude oils suggest that they are generated from the fundamental biochemical components of organisms.
Geochemical evidence suggests that conditions favorable for significant generation of petroleum do not normally occur above depths of about 1500 meters below the surface. There is some geological evidence, based on oil occurrence and inferred source rocks, suggesting a shallower “threshold” of about 600 meters.