Biodegradability of Diamondoid Hydrocarbons in Biodegraded Oils from the San Joaquin Valley: Impact on Oil Mixing Interpretation by Diamondoids and Biomarkers?

Wei, Zhibin¹, J. Michael Moldowan¹ (1) Stanford University, Stanford, CA

Bacterially and thermally altered petroleum hydrocarbons represent significant potential resources in subsurface reservoirs. Petroleum from multiple sources may charge a reservoir over geologic time, resulting in a mixture of hydrocarbons generated and expelled at varying degrees of thermal maturity. Information on cracked oil sources may be recorded in diamondoids that can survive oil cracking because of their ultra stability. Diamondoids have proven to be a useful proxy in estimating the thermal maturity of both oils and high maturity condensates and thus can be used for recognition of the oil deadline, calibration of expulsion efficiency models and identification of mixed sources. However the use of diamondoids for cracking estimation on biodegraded oil remains questionable.

The biodegradability of diamondoids in petroleum reservoirs was investigated using a collection of oil samples from the San Joaquin Valley that have been biodegraded in the reservoirs to various extents. Our results clearly show that some diamondoids are subjected to microbial degradation in petroleum reservoirs. However, the biodegradation of diamondoid species is selective as well as stepwise. In the present study, we report possible pathways for microbial degradation of adamantanes. While adamantanes are clearly susceptible to severe biodegradation, higher diamondoid species such as diamantanes, triamantanes and tetramantanes do not display significant biodegradability. Therefore, microbial alteration has a negligible impact on the quantification of oil cracking achieved by the diamondoid-biomarker method which utilizes methyldiamantanes.