--> Abstract: Origin and History of Bitumen in Geodes of the New Albany Shale, by Mirela Dumitrescu, David Finkelstein, Ovidiu Remus Lazar, Juergen Schieber, and Simon Brassell; #90039 (2005)

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Origin and History of Bitumen in Geodes of the New Albany Shale

Mirela Dumitrescu, David Finkelstein, Ovidiu Remus Lazar, Juergen Schieber, and Simon Brassell
Indiana University, Bloomington, IN

Centimeter-scale, bitumenous geodes associated with vertical to sub-vertical quartz-calcite-dolomite veins occur within the Late Givetian-Early Frasnian Trousdale/Blocher Member of the New Albany Shale in a roadcut exposure from central Kentucky (on the Cincinnati Arch between the Illinois and Appalachian Basins). The molecular organic geochemistry of bitumens from four geodes was investigated to determine their origin and formation history. Molecular characteristics such as n-alkane distributions, and pristane/phytane and C27/(C27+C29) sterane ratios provided evidence of the sources of organic matter. Maturity was assessed using carbon preference indices for n-alkanes, 20S/(20S+20R) and ββ/(ββ+αα) ratios for C29 steranes, triaromatic steroid distributions, methylphenanthrene indices, and vitrinite reflectance. The distribution of n-alkanes and the Pr/n-C17 and Ph/n-C18 ratios were examined to evaluate the degree of biodegradation. Comparison of these molecular characteristics for the bitumens with those obtained for a shale sample and two Devonian basinal oils enabled assessment of regional geochemical affinities of petroleum sources.

Results suggest that the source of organic matter of the bitumens in the geodes is different from that of the Trousdale/Blocher Shale. Also, these bitumens and the two Devonian oils have experienced different levels of biodegradation. The Trousdale/Blocher Shale could have been hydro-fractured during oil generation from older Ordovician or Silurian strata, and Ordovician/Silurian bitumen could have migrated along open fractures and accumulated within the geodes. Further, investigation of bitumens and shale using compound-specific carbon isotopic analyses will better constrain potential sources for hydrocarbons and help understand generation history and migration paths for petroleum in the Illinois and Appalachian Basins.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005