A reversed compositional pseudo-gradient in biodegraded oil column from Liaohe Basin, NE China

Most heavy oils or tar sands are formed by microbial biodegradation and show a compositional gradient toward the base of oil column. However, a reversed pseudo-gradient has been observed from the fourth member of the Shahejie Formation in Shuguang oilfield, Liaohe Basin, NE China, on the basis of bulk and molecular compositions in a series core extracts from well K037. The bulk compositions show systematic depletion of saturated hydrocarbons and enrichment of polars (resins + asphaltenes) toward the top of the oil column. Molecular compositions illustrate that these extracts have similar source input, depositional environment and thermal maturity, and their compositional differences are mainly caused by varying degrees of biodegradation. Biodegradation removes all n-alkanes, most isoprenoid alkanes, partial alkylnaphthalenes and alkylphenanthrenes at the top of oil column, whereas only n-alkanes are affected at the bottom. Reservoir temperature has no crucial impact on biodegradation levels as the whole oil column in the temperature range of 41–44 °C is ideal for microbial activities. The studied reservoir is not a continuous oil column, which is separated by a few barriers and baffles. The occurrence of locally high-water saturation layers caused multiple small gradients in the mid and lower part of oil column with a normal biodegradation gradient in the lowermost compartmentalized interval. The overall reversed compositional pseudo-gradient is controlled by a generally upward coarsening sedimentary sequence and the existence of top water situated at a lithology change boundary from coarse sandstones and conglomerates to fine grained sandstones. This study provides a new perspective to assess the reservoir fluid heterogeneity and sheds the light on sweet spots identification for heavy oil exploration and development.