Characteristics and hydrocarbon generation mechanism of early low-mature oil in Baoding sag,Bohai Bay Basin
Early low-mature oil is formed by specific organic parent materials with low activation energy for hydrocarbon generation.under low temperature and early mature conditions[vitrinite reflectance(Ro)of 0.3％to 0.6％,geothermal temperature of 60 ℃ to 100 ℃].It has higher maturity and better oil quality than immature oil,and is easy to form commercial oil reservoirs and natural in-dustrial capacity.This type of oil was discovered early in China,but there is less understanding of its hydrocarbon generation mecha-nism and reservoir potential.In recent years,the Exploration and Development Research Institute of PetroChina Huabei Oilfield Company has been exploring early low-mature oil and obtained high-yield oil flows and large-scale cost-effective reserves in the mid-dle-shallow strata of Dongying Formation of Baoding sag in Jizhong depression,thus achieving a breakthrough in traditional cogni-tion.The research on the hydrocarbon generation mechanism,including the organic geochemical analysis of crude oil and source rocks,analysis of hydrocarbon inclusions in reservoirs,and thermal simulation experiments of hydrocarbon generation and expulsion in Baoding sag,reveals that under the environment of strong-reduced saline water bodies and high geothermal field,the bacteria and algae with low activation energy and soluble organic matter in the organic-rich strata of the lower submember of Member 1 of Shahe-jie Formation have generated large amounts of early low-mature oil by biochemical action and low-temperature thermocatalysis in the late diagenesis stage,with the oil rate of 190 mg/g and oil expulsion of 86 mg/g,respectively.They account for 46.4％and 37.3％of the oil generation and expulsion of kerogens in source rocks during the peak stage of oil generation(Ro of 0.8％to 1.3％,geo-thermal temperature over 120 ℃),providing sufficient oil sources for the formation of early low-mature oil reservoirs.The early gen-erated oil completed the migration and accumulation process from source to reservoir by fault networks and sand bodies,showing a regularity of near source enrichment.
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