Pool-formation and secondary change of biodegraded viscous oils: Case study of reservoir section in the Zhengjia-Wangzhuang Oilfield, Jiyang Depression

It is demonstrated by various geochemical indexes that the Zhengjia-Wangzhuang Oilfield with viscous crude oil in the Jiyang Depression has been sourced from the contribution of matured source rocks in the upper Es4. The principal cause leading to the densification of crude oils would be biodegradation, with the degradation level of crude oils being ranked as 2-8; vertically, the biodegradation level increases from the top to bottom of the oil column, with a distinctive biodegradation gradient occurring. Calculated parameters of sterane, terpane and methyl-phenanthrene have indicated that the source-rock's maturity of crude oils and asphaltic sands ranges from 0.7 to 0.9, and based on the calculation of Easy Ro model, the temperature of hydrocarbon generation in the source rock would be within 120-140℃, which coincides with the measurements of reservoir inclusions. The measured homogenization temperature would represent the generation temperature of the source rock, and be fairly different from that of reservoir while being charged with hydrocarbons, which reflects the hydrocarbon-charging to be a fast process, and the period of pool-formation to be consistent with that of peaked generation. According to the evaluation of generation history, the pool-formation could have been occurring in 7-15 Ma. And the biodegradation of crude oils in the study area would be considered to take place in 4-15 Ma based on the examination of biodegradation order and dynamic calculations.