| dc.description.abstract | 18α(H)-Oleanane is a biomarker that is proposed to be derived from β-amyrin, which is produced by angiosperm plants. As such, the presence of 18α(H)-oleanane is often used as an indicator that the petroleum source is of Cretaceous age or younger. Pre-Cretaceous occurrences of 18α(H)-oleanane are rare but have been identified. 18α(H)-Oleanane, as well as an aromatic degradation product of β-amyrin, 1,2,7-trimethylnaphthalene, have been tentatively identified in the Chesterian Limestone in the Anadarko Basin in northwest Oklahoma. The presence of these compounds suggests the presence of either angiosperms themselves or another plant group capable of synthesizing precursors of oleanane and oleanane-type compounds in the depositional environment. The present study analyzed 26 source rocks from the Chesterian Limestone and 14 oil samples from other Mississippian Limestones in the Anadarko Basin. The observed tentative distribution of 18α(H)-oleanane and 1,2,7-trimethylnaphthalene in the Chesterian Limestone are presented and possible explanations for the presence of these compounds are discussed.
Unusual distributions of 17α(H)-diahopanes and 9,15-dimethyl-25,27-bisnorhopanes (another series of rearranged hopanes) are also observed in the source rock samples. Both series of rearranged hopanes closely co-vary with each other, which is likely because of their similar structures and suggests that they have similar formation processes. The rearranged hopanes are thermally more stable than the regular hopanes and are known to increase in concentration with increasing maturity. The tricyclic terpanes are also thermally more stable than the regular hopanes and are observed in unusually high abundances in the study area and even dominate the regular hopanes in three source rock samples and in six oil samples from the study area. The corresponding 17α(H)-diahopane/17α(H)-hopane values suggest that thermal maturity plays a role in the increased abundance of the tricyclic terpanes.
The distributions of rearranged hopanes and tricyclic terpanes suggests that the oils in the Mississippian Limestones in the Anadarko Basin are not sourced from the underlying Woodford Shale. This is further confirmed by the observed presence and distribution of head-to-head isoprenoids in both the source rock and the oil samples used in the present study. | en_US |
