Biodegradation of tar-sand bitumens from the Ardmore and Anadarko Basins, Carter County, Oklahoma

Abstract

A series of tar-sand bitumens collected from a single shallow well in the Ardmore Basin, Carter County, Oklahoma has been characterized by various geochemical techniques. Particular emphasis has been placed on determination of variations in biomarker distributions, both hydrocarbons and porphyrins, as a function of biodegradation. The tar-sand bitumens have been severely biodegraded and most of the n-alkanes, low molecular weight cycloalkanes, isoprenoid alkanes, C 27C 29 steranes, light aromatics and sulfur compounds have been removed. In addition, the hopane distributions have been altered to differing degress with the pseudohomolugoues above C 30decreasing in concentration prior to the C 20–21 The triaromatic steroid hydrocarbons were also altered with preferential removal of 20R epimers (C 20–21 and C 27–28). Diasterane and C 30-sterane distributions appear to be relatively unaffected by biodegradation. The high resistance of tricyclic terpanes, The C 24-tetracyclic terpane and monoaromatic steroid hydrocarbons to biodegradation indicate that the distributions of these compounds are well suited to serve as correlation parameters in heavily altered samples. It was also observed in this study that the main ring structure of the porphyrin molecule was not affected by these very high levels of biodegradation. Previously hypothesized changes in porphyrin structure, due to biodegradation, have included n-alkyl side chain cleavage and possible conversion from DPEP- to ETIO-type porphyrins. An investigation has also been made with an active seep collected from Sulphur in the Anadarko Basin, Oklahoma and the results show that the relative rate of biodegradation of various classes of biomarkers differs between these two locations, although it was proposed that the original oil responsible for these two seeps is derived from the same source rock formation.