Effects of biodegradation on diamondoid distribution in crude oils from the Bongor Basin, Chad

Abstract

The sensitivity of biodegradation on diamondoids was investigated using a series of biodegraded oil samples from the Ronier tectonic unit of Bongor Basin, Chad. The results suggest that diamondoids, including adamantanes (As) and diamantanes (Ds), are relatively resistant to biodegradation and obvious biodegradation was observed in oils with a Peters-Moldowan (PM) biodegradation rank of 6 or more. Overall, the sensibility of biodegradation on diamondoids is generally similar to hopanes and regular steranes. As biodegradation evolves, the changes in concentration and components of diamondoids show that the biodegradation process is selective and stepwise. The significant increase of MD/MA and DMD/DMA for oils with a PM ranking 6+ indicates that diamantanes are generally more resistant to biodegradation than adamantanes. The similar trends of DMA/MA, EA/MA, MD/D, DMD/MD and other relevant indexes, show that higher alkylation homologs are more resistant to biodegradation. The commonly used diamondoid ratios, such as MAI, EAI, MDI and DMID-1, are obviously affected by biodegradation at the stage of high-level biodegradation, which may indicate that these ratios should be used with caution in case of severely degraded oils.