Organic Geochemistry of Cherokee Group in Southeastern Kansas and Northeastern Oklahoma: ABSTRACT

Abstract

Geological evidence indicates that rocks of the Cherokee Group (Desmoinesian) were the source of the petroleum which has accumulated in the Cherokee petroleum province of southeastern Kansas and northeastern Oklahoma. In order to test this source-rock hypothesis, two subsurface sections of the Cherokee Group were subjected to a geological and organic-geochemical study. One section is located in the Burbank oil field, Osage County, Oklahoma, and the other in the Thrall oil field, Greenwood County, Kansas. Both fields are productive from shoestring-sand reservoirs and yield petroleums of virtually identical composition. The Cherokee Group at these two localities is divisible into cyclothems which are characterized by a variety of lithologic types including coal, underclay, sandstone, greenish gray shale, gray shale, black shale, and limestone. It is evident that the Cherokee Group was deposited in sedimentary environments which fluctuated from non-marine to marine. Organic-geochemical studies included the determination of organic-carbon and hydrocarbon contents, and saturate-to-aromatic and hydrocarbon-to-organic-carbon ratios of the rocks. Organic composition, like lithology, shows extreme vertical variability, although principal lithologic types have characteristic organic compositions. Despite this internal variability, the organic composition of the Cherokee Group as a whole is very similar at Burbank and Thrall. This observation substantiates the hypothesis that the non-reservoir facies of the Cherokee Group were the source of the petroleum in the shoestring sands, because it would be expected that crude oils of similar type would be derived from sedimentary sequences of similar organic composition. Other implications of these results are: (1) organic composition and source rock quality are inherent properties of sedimentary rocks and are a function of depositional environment; (2) composition of crude oil is controlled by the parent source-rock section; (3) saturate-type hydrocarbons migrate from the source rock more readily than aromatic types; (4) the amount of indigenous hydrocarbons in shales is a function of the amount of organic carbon (organic matter) in the rock; and (5) primary migration is a very inefficient process and causes the removal of only a small part of the hydrocarbons from the source rock.