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Petroleum: a primer for Kansas, Page 6 of 15
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Three conditions must be present for an oil or gas field to exist: 1) a source rock, such as shale, that is rich in organic material; 2) a reservoir rock, such as porous and permeable limestone, dolomite, or sandstone; and 3) a trapping mechanism, such as an anticline, faulted strata, or any of the myriad kinds of stratigraphic traps. Petroleum geologists must do everything possible to search for areas where all of these conditions are met. The task is very much like that of the private detective in a murder mystery. One must gather as many clues as possible (there are never really enough), the clues must be studied and interpreted individually, and then with a great deal of data compilation and imagination a recommendation to explore can be made. Usually patterns will develop, everything falls into place at an unexpected time, and the "drilling prospect" is ready to sell to management or investors. Yet the very best prospect in the world, where everything seems to be perfect, is suspect and risky until it is actually drilled. The cost of drilling and completing a well can be in excess of a million dollars, and the probability of success in a wildcat well is only about one in nine.

There are as many ways to search for oil as there are petroleum geologists. Usually a geologist begins by searching for an area with a thick section of sedimentary rocks. The more layers of rock present, the better the opportunities for porous and permeable rocks to exist; the source rocks must be deeply buried for long periods of time for hydrocarbons to be generated from organic matter. Thickness of the rock column can be determined by studying rock exposures in stream gullies and roadcuts (fig. 8). Wells drilled for water, or previously drilled oil and gas wells also provide information. In Kansas, where more than 350,000 wells have been drilled in search of petroleum, the amount of available data is staggering. At the same time geologists are studying rock thickness, they are also searching for organic-rich shales (source rocks) and porous sandstones or carbonate rocks (reservoir rock). If a thick sequence of sedimentary rocks with possible source and reservoir rocks is found, the search for traps begins. In most parts of the world, answers to these questions have already been determined by previous studies, and one needs only to visit a technical library to find the answers. In Kansas, the source rocks are relatively well known; the most prolific are the Chattanooga Shale, a Late Devonian-Early Mississippian black, organic-rich shale; the Middle Ordovician Simpson Group containing algal-rich brown, waxy, shales; and where they are more deeply buried, dark shales of Pennsylvanian age.

Figure 8--Geologic formations that underlie parts of Kansas may be exposed at the surface elsewhere. Geologists study the rocks where they are exposed for information that may aid in the discovery of oil and gas in Kansas.

rocks buried in Kansas may be exposed in Colorado

Locating traps is not easy, but structural traps are the most obvious kind. The easiest and least expensive method is to map the location and attitude of rocks exposed at the surface. Folds and faults can often be detected in surface exposures, and aerial photos and satellite images may give clues to the presence of structures too large or too obscure to be noticeable at the surface. In eastern Kansas, much of the oil discovered early was found by studying structures exposed at the surface; the El Dorado oil field is a good example. However, most of these more obvious structures have been previously mapped and drilled. Consequently, other techniques must be used to find structures beneath the surface of the earth that cannot be seen at the surface.

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Kansas Geological Survey, Education
Placed online April 2001
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