Mixing and biodegradation of hydrocarbons in the Daerqi oilfield, Baiyinchagan Depression, northern China

Crude oils in the Daerqi oilfield vary greatly in viscosities, including normal, viscous and highly viscous oils. They are also dramatically different in thermal maturity, from immature to mature based on biomarker parameters. According to bulk and molecular geochemical characteristics, crude oils can be classified into three groups in this oilfield. Group I oils are mature and derived from the deeply buried and relatively old source rocks within the Tengger Formation and the second member of the Arshan Formation of early Cretaceous age. Group II oils are immature to marginally mature and derived from the relatively young source rocks that experienced shallow burial within the first member of the Duhongmu Formation of earlier Cretaceous age. Group III oils are mixtures of group I and group II oils, with dominance of the group I oils (>70%). The viscosity is low for the non-biodegraded group I and group III oils (7.1–24 mPa s) but very high for the non-biodegraded group II oils (e.g., 433,104 mPa s for the representative oil Da12O1). The group III oils are similar to the group I oils in gross properties (e.g., gross composition and viscosity), but are similar to group II oils in terms of their molecular maturity parameters. The oil reservoirs are currently at a shallow depth (<800 m), and some oils are heavily biodegraded up to level 4 on the [Peters, K.E., Moldowan, J.M., 1993. The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Englewood Cliffs, Prentice-Hall, NJ] scale in this oilfield. A peculiar phenomenon is that some of the group I and group III oils have been heavily biodegraded, whereas the group II oils, interbedded with the biodegraded group I and/or group III oils within a narrow interval in the same wells, are not altered by biodegradation. This is possibly due to the low diffusion coefficient of hydrocarbons and isolation to meteoric water percolation for the group II oil columns.