Does Oil Composition Limit Biodegradation? Implications from a Biodegraded Reservoir

Yongge Sun¹, Zhiyong Wang², and Mingsheng Yuan^2
1 SKLOG, Guangzhou Institute of Geohemistry, Chinese Academy of Sciences, Guangzhou, China
² Petroleum Research Institute for Exploration & Development of Tuha Oilfield Branch Company, PetroChina, Hami, China

A suit of petroleum reservoir extracts from Turpan basin, NW china, was used to investigate whether petroleum composition limits biodegradation in deep reservoir. Screening analysis showed a positive relationship between porosity and oil saturation, suggesting that crude oil has experienced biodegradation before accumulation. The GCs of aliphatic fractions were characterized by almost complete loss of n-alkanes and acyclic isoprenoids with high level of UCM, indicating a heavy level of biodegradation. Systematic changes, seen as marked gradients in petroleum bulk composition and component concentration, have been observed within a single oil column. Surprisingly, no change in molecular parameters that are sensitive to biodegradation (e.g. 20S/20S+20R sterane) was observed across the profiles although the fact that the reservoir was buried in the depth of <1000m for at least 20 Ma after accumulation. This inconsistent phenomenon suggested that, except water leg size, there are other factors to control the degree of in-reservoir biodegradation. We think that oil composition probably makes a significant impact on in-reservoir biodegradation. Because the migrated oil in this study had a relatively high percentage of resins and asphaltenes due to biodegradation, it is possible for such a biodegraded oil to produce a sealing effect at the base of the column (oil-water contact) by the formation of semi-solid state bitumen or tar mats after charging, hereafter to prevent the diffusive transport of nutrients and electron acceptors in the aquifer to oil column, and further limit in-reservoir biodegraded process.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005