Variations in organic geochemistry of anoxic-oxic black shale-carbonate sequences in the Pennsylvanian of the Midcontinent, U.S.A.

Abstract

Two cores containing thin organic-rich phosphatic black shale members, the Excello and Little Osage shale, and transitional lithologies from Pennsylvanian cyclothems of southeast Kansas and northeast Oklahoma were characterized by geochemical and organic petrographic methods by very detailed sampling. Geochemical analyses included organic carbon determination, Rock-Eval pyrolysis, bitumen extraction and chromatographic separation into saturated, aromatic and nonhydrocarbon fractions, gas chromatography of saturated hydrocarbons, carbon isotope analysis of coexisting saturated and aromatic fractions, and whole rock V and Ni analyses. Organic petrography included maceral identification on polished whole rock fragments and vitrinite reflection and thermal alteration index (TAI) determinations of kerogen concetrates. The analytical results demonstrate large and systematic variations in geochemical properties within the black-shale units. In both cases an abrupt increase in TOC (20–25% level) and HI (400 level) at the initiation of phosphatic black-shale deposition is followed by a gradual decline to values of less than 1% TOC and 100 HI. Organic constituents include both liptinitic (algal derived) and humic (terrestrial) components. Variations in their relative amounts control the nature and quality of the organic profile of the Little Osage shale. Level of anoxia varied within this sequence and influenced several geochemical parameters. Pr/Ph ratios decrease; nonhydrocarbons increase relative to hydrocarbons; saturated hydrocarbons decline relative to aromatics; carbon isotopic composition of coexisting saturated and aromatic hydrocarbons converge; and V and Ni and the V/Ni + V ratio increase with increased anoxia. Eustatic rise of sea level and rapid marine transgression over the continental craton, with consequent flooding of adjacent extensive brackish-marine (peat) swamps and the resultant influx of a large supply of nutrients and humic detritus, appear to be the key geologic control of black-shale deposition.