Biodegradation and water washing within a series of petroleum reservoirs of the Panyu Oil Field

A suite of crude oils across 10 discrete reservoirs at different depths in the Panyu Oil Field (Pearl River Mouth Basin, South China Sea) were geochemically appraised as having a common lacustrine shale source and of similarly low thermal maturity. The δ13C values and biomarker composition indicate that the Wenchang Formation is the major source of these oils. However, a progressive variation in the distribution of some hydrocarbon compounds (e.g., n-alkanes, isoprenoids, polycyclic aromatic hydrocarbons) across several of the oils is indicative of secondary alteration by biodegradation and possibly also water washing. Biodegradation contributed to the progressive removal of increasingly susceptible hydrocarbons, largely in accordance with previously defined conventions, and was most pronounced in the shallowest oils. A decline in extent of biodegradation with increasing production unit depth may reflect their increase in temperature (72 to 98 °C), with the higher temperatures of the deeper oils more hostile to microbial activity. Minor changes were also observed in the hydrocarbon composition of the deeper oils, where biodegradation impacts were negligible. This could be evidence of secondary alteration with water washing a potential influence, but might also reflect a variance in the secondary source contribution of the coaly Enping Formation. There was a notable decline in the relative abundance of several compounds with a potentially high susceptibility to water washing, including dibenzofuran (cf. non-heteroatom aromatics) and β-methylated polycyclic aromatic hydrocarbons, in contrast to α-methylated forms.