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In southeastern Kansas and northeastern Oklahoma, geological evidence indicates that rocks of the Cherokee group (Desmoinesian) were the source of most of the petroleum which has accumulated in sandstone members and in porous zones along the pre-Pennsylvanian unconformity upon which the Cherokee group was deposited. The Cherokee group in the subsurface is divisible into several cyclothems characterized by non-marine and marine facies including coal, underclay, sandstone, greenish gray shale, gray (fossiliferous) shale, black shale, and limestone. Although lateral continuity of specific members and cyclothems is evident, the proportion of marine facies is greater in the Cherokee basin than in the adjacent shelf areas.
Organic geochemical studies of the Cherokee group included determination of organic C, hydrocarbons, and saturate-aromatic ratios of the hydrocarbon mixtures. Organic composition, like lithology, shows extreme vertical variability, although principal lithologies have characteristic organic compositions. Greenish gray shales are low in organic C (<0.5%) and hydrocarbons (<50 ppm) with high saturate-aromatic ratios (>1.0); gray shales have intermediate values of organic C (1-3%) and hydrocarbons (100-500 ppm) with low saturate-aromatic ratios (1.0); black shales are high in organic C (5-18%) and hydrocarbons (<2,000 ppm). Despite this internal variability, the organic composition of the Cherokee group as a whole appears to remain uniform over a wide area.
Implications of these results are: (1) organic composition is an inherent property of sedimentary rocks and reflects depositional environment; (2) migration of fluids through shales during compaction has apparently not created compositional gradients or smoothed out primary differences in organic composition; (3) although the uniform character of Cherokee basin crude oils is explicable if the Cherokee group is their common source rock, lack of knowledge on the origin and migration of hydrocarbons poses problems with respect to details of source evaluation.
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