Lithofacies Characteristics And Enrichment Model Of Shale Oil In Fengcheng Formation Shale In Mahu Sag

The shale oil of Fengcheng Formation in Mahu Depression of Junggar Basin belongs to Alkali Lake Shale Oil,which is one of the key areas of shale oil exploration in western China and has great resource potential.The research on shale lithofacies and shale oil enrichment model of Fengcheng Formation in Mahu Sag is of great significance for enriching the oil and gas exploration of alkaline lake shale in my country.In this paper,the shale lithofacies of the Fengcheng Formation in the Mahu Sag were classified through organic geochemical experiments,XRD,rock thin section observation,scanning electron microscopy,high pressure mercury compression,gas adsorption,micron CT scanning and Soxhlet extraction,etc.The hydrocarbon generation potential,storage capacity and crude oil transport capacity of the shales of different lithofacies were compared,the differences in oil content of the shales of different lithofacies and the main controlling factors were analysed,and the shale oil enrichment model of the Fengcheng Formation was established.Studies have shown that four lithofacies,felsic shale,mixed shale,lime shale and dolomitic shale,develop in the Fengcheng Formation shale in the Mahu Sag.The TOC content ranges from0.07%to 2.95%,the organic matter type is mainly type II₂and II₁,and the degree of thermal evolution reaches the mature-high maturity stage,which is at the peak of oil production,indicating that the shales of the Fengcheng Formation have strong hydrocarbon production potential.The Fengcheng Formation shale has various types of storage space,mainly inorganic pores and micro-fractures,and few organic pores.The pore shapes are mainly H3-type slit pores and a small amount of H2-type ink bottle pores.Some hydrocarbons are retained and accumulated in the shale of Fengcheng Formation,which has good oil-bearing properties.Shale oil mainly occurs in pores with a pore size range of>10 nm.Based on the relationship between S₁and TOC,the shale oil resources of the Fengcheng Formation was graded and evaluated,and it was revealed that the oil-bearing shale is controlled by three aspects:hydrocarbon generation capacity,storage capacity and migration capacity.Finally,the shale oil enrichment model of different lithofacies was established.Felsic shale is an enriched shale oil with strong hydrocarbon generation potential and strong storage capacity,which can accept foreign hydrocarbons.Mixed shale is self-generating and self-reserving shale oil with weak hydrocarbon-generating capacity and weak storage capacity.Lime shale is a partial reservoir type shale oil with weak hydrocarbon generation capacity and good storage capacity,which can receive foreign hydrocarbons.Dolomite shale is a partial source rock type shale oil with strong hydrocarbon-generating capacity,weak storage capacity,and external hydrocarbon expulsion.The OSI of the Fengcheng Formation shale can reach 176mg HC/g TOC,far exceeding the lower limit of benefit,indicating that the Fengcheng Formation shale has good shale oil potential.Felsic shale has great hydrocarbon generation potential,strong storage capacity,easy to accept foreign hydrocarbon migration,high content of brittle minerals,and easy fracturing production.It is the most favorable exploration target for shale oil in Fengcheng Formation in Mahu Sag.