Abstract:  Enormous breakthrough have been made in shale oil exploration of Fengcheng Formation in Mahu sag, Junggar basin. However, according to generally accepted evaluation standard of shale oil source rock, the quality characteristics of source rocks in this area are not ideal. In order to scientifically evaluate the quality of source rocks in Mahu sag, the sedimentary environment of Fengcheng Formation was studied, and the source rocks was evaluated according to the mineral composition of fine-grained rocks on the basis of systematic core observation and organic geochemical analysis. Fine grained rocks of Fengcheng Formation in Mahu sag were mainly deposited in normal semi deep lake, salinized semi deep lake and semi-salinized semi deep lake, hydrothermal semi deep lake and shallow lake. The organic matter characteristics of fine grained rocks in each environment are different, and the semi-salinized semi deep lake was favored to enrich organic matter, and the averaged TOC was nearly 1%. Further combining with the discrimination diagram of organic matter types, it showed that the source of organic matter in salinized semi deep lake and hydrothermal semi deep lake was dominated by aquatic organisms, while there were mixed organic matter sources of lacustrine and land in other environments. Based on the hydrocarbon generation potential evaluation and maturity estimation of organic matter (Vre=0.74%), it was revealed that the source rocks of Fengcheng Formation were in the mature stage of hydrocarbon expulsion, and the free hydrocarbons in fine-grained rocks were identified as primary hydrocarbons. Finally, due to the difference of adsorption capacity of different mineral composition of source rocks, the classification evaluation is carried out according to continental argillaceous source rocks and carbonate source rocks. The results showed that the Fengcheng Formation deposited effective source rocks with a cumulative thickness of nearly 250 m, with organic matter dominated by algae.  

Key words: Mahu sag, Fengcheng Formation, characteristics of organic matter, shale oil, evaluation of hydrocarbon source rock