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One episode in the geologic history of much of the United States and part of Canada consisted of three parts: (1) the formation of several great upfolds or arches at the end of Mississippian time, (2) the erosion of the elevated rocks until Precambrian rocks were exposed along the crests of these uplifts, and (3) the deposition of Pennsylvanian sediments across the eroded and peneplaned surface. A paleogeologic map of this surface of unconformity shows the geology of the eroded surface at the time it was overlapped.
A question that may have significance in exploration is how and where was the oil preserved in the pre-Pennsylvanian rocks during this episode? Obviously all of the "loose" or "free" oil and gas would be expected to have moved into the anticlines and arches and have been eroded or have escaped by seepages along the outcrops of that time. Yet a great amount of petroleum was preserved in the early Paleozoic rocks as evidenced by the oil and gas production in rocks of these ages today. How? and Where?
Some of the possible solutions include such phenomena as oil fields protected because erosion did not extend deep enough, buried wedge belts of permeability, favorable hydrodynamic gradients, late generation of petroleum, late transformation of organic matter to petroleum, late accumulation of petroleum into phase continuity and post-unconformity (Pennsylvanian) source of the petroleum. Any one, or combinations of several, may explain where and how the oil and gas now found in early Paleozoic rocks were preserved during such an episode. If the regions favorable for the preservation of oil and gas can be located, then these would be areas in which to concentrate detailed structural and stratigraphic work in order to locate specific traps. The same type of reasoning would apply to each urface of unconformity in the geologic section.
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