Provenance of organic matter in an intracratonic rift basin: Insights from biomarker distribution in Palaeogene crude oils of Cambay Basin, western India

Abstract

The biomarker distributions of crude oils are a valuable and complementary tool for understanding the provenance of source organic matter (OM) in intracratonic rift basins such as the Cambay Basin, India, which has five tectonic blocks from north to south. Distinctive biomarkers recorded in Cambay oils included tetrapolyprenoids, C29 28-nor-spergulane, diaryl isoprenoids and methylated chromans. Other diagnostic markers including oleanane, A-ring degraded hexanortriterpanes, bicadinanes and aromatic oleanoid/ursanoid triterpenoids, suggest substantial inputs to the OM from tropical angiosperm families. The biomarker distributions and various plots of the data, including a principal component analysis, indicate two distinct OM facies present in the basin. The depositional environment of the source OM that prevailed in the north blocks (NB) and south blocks (SB) of the basin are different, despite similar ages and geography. The results suggest that the NB oils were generated from mixed algal/microbial and higher plant-derived organic biomasses, deposited in suboxic to anoxic conditions, probably in a restricted marine or lacustrine environment. Contrarily, the oils of SB were generated from OM substantially derived from tropical lowland angiosperm rainforest trees, which were deposited under a fluvio-deltaic environment with a suboxic to oxic setting. The various maturity parameters reveal that northern oils are marginally less mature than the southern oils.