Biomarker signatures of the Ediacaran-Early Cambrian origin petroleum from the central Sichuan Basin, South China: Implications for source rock characteristics

Hydrocarbons with an Ediacaran-Early Cambrian origin from the central Sichuan Basin, South China were investigated for biomarker compositions. They were obtained by solvent extraction from the Upper Ediacaran reservoir core samples, which contain abundant solid bitumen. The solid bitumen is widely believed to be formed by thermal cracking of early-charged oils in the reservoirs, which have presumed source rocks of an Ediacaran-Early Cambrian age. Thermal maturity parameters based on phenanthrenes and aromatic steroids suggest a very high thermal maturity for the extracted hydrocarbons, which is consistent with the severe maturation undergone by their host reservoirs. Age-diagnostic biomarkers including C30 24-isopropylcholestanes and mid-chain monomethyl alkanes support source rocks of an Ediacaran-Early Cambrian age. The extracted hydrocarbons have very similar biomarker signatures, which include high relative abundance of acyclic isoprenoids and pristane/phytane <1; high abundance of 2α-methylhopanes; high abundance of C27 to C29 steranes with a slight preference for either C27 or C29 homologues; high relative abundance of diasteranes; the concomitant presence of 24-n-propyl and 24-isopropylcholestanes and 24-isopropyl/24-n-propylcholestanes > 1; and the presence of C26 norcholestanes, methyl- and ethylsteranes. The presence of hopanoids including extended series and methylhopanes indicates contributions from prokaryotic bacteria while the presence of various steroids including regular steranes and methylsteranes indicates contributions from the eukaryotic algae. Parameters based on tricyclic terpane and hopane, dibenzothiophene/phenanthrene ratios, and C27 diasteranes/(diasteranes + regular steranes) ratios suggest a major shale source, but a mixed shale-carbonate source cannot be completely excluded. Presence of C30 demethylsteranes, low pristane/phytane ratios, high gammacerane index, and presence of 28,30-dinorhopane and 2-methyhopanes indicate that the source rocks were deposited in a stratified, anoxic marine depositional environment with enhanced salinity conditions.