Geochemical Characterization Of Biodegradation Of Crude Oil In Liaohe West Sag

Biodegradation is the main cause of the formation of heavy oil and oil sands,the vertical gradient of biodegradation makes chemical composition and physical properties of the oil at the bottom and top of the reservoir completely different,so it is of great significance to further analyze and discuss the biodegradation in the western sag of Liaohe Basin to understand and discover the distribution,development and production of biodegraded oil in this area.In this paper,high resolution core samples at every 1–2 m apart from wells G3–6–18 and K037 in the western sag of the Liaohe Basin are selected and geochemically characterized by bulk compositions(SARA)and molecular compositions of total hydrocarbon gas chromatography–mass spectrometry(GC–MS)quantification.The results show that the n-alkanes,isoprenoid alkanes and dicyclic sesquiterpenes of saturated hydrocarbons have been affected by biodegradation,while the steranes and terpanes are almost not affected,and no 25 alkane norhopanes have been detected.More than 90% of the n-alkanes in the top oil of well G3-6-18 were degraded,and no n-alkanes and more than 90% of the isoprenoids were removed from the bottom oil.For well K037,the bottom oil is partially missing n-alkanes and the isoprenoid distribution is complete.The top isoprenoid is consumed,but the hopane distribution is still complete.It can be determined that well G3-6-18 is PM3 of Peters and Moldowan(1993)at the top and grade 4-5 at the bottom.Well K037,however,increased from PM2 at the bottom to PM5 at the top.The biodegradation level of well G3–6–18 was increased from PM3 at the top of the reservoir to PM4–5 at the bottom,showing a normal biodegradation gradient,while biodegradation influence in well K037 varies from PM5 at the top to PM2 at the bottom,different from normal biodegradation sequence.Lithological analysis,combined with oil–water contact zone location,suggested that the reversal biodegradation gradient in well K037 is caused by gradual coarsing upwards of grain size and the occurrence of water in the oil column.The discovery of compositional gradients in different formats deepened our understanding of oil biodegradation mechanism,which can be applied for heavy oil exploration and development optimization.The conceptual model of biodegradation presented in this paper covers all major processes including biodegradation and shed the light on favorable zones identification for heavy oil exploration and development.