Lithofacies Division and Type Evaluation of Shale Oil Reservoir in Fengcheng Formation of Mahu Sag, Junggar Basin

Shale reservoir of Fengcheng Formation in Mahu Sag, Junggar Basin is another important discovery after Lucaogou Formation in Jimusar Sag. During the sedimentary period of Fengcheng Formation, Mahu sag is characterized by near–terrigenous source supply, frequent volcanic activity and arid and hot climate, which creates conditions for the unique saline and alkaline lake sedimentary environment and is also an important reason for the complex and changeable lithology of Fengcheng Formation. Volcanic rock, internal rock, terrigenous clastic rock and multi–source mixed rock are frequently stacked longitudinally, forming shale oil reservoirs of meter to centimeter size. Based on the characteristics of lithologic changes, the lithology and lithofacies of shale oil reservoirs in Fengcheng Formation in Mahu Sag, Junggar Basin were analyzed by means of sub-ion polishing electron microscopy, energy spectrum testing and fluorescence analysis, with the structural style as the breakthrough point, cm–scale core observation as the basis and high–frequency sedimentary structure analysis as the auxiliary. The shale oil reservoirs of Fengcheng Formation in Mahu sag can be divided into 4 types and 8 types. The reservoir properties of glutenite, volcanic rock, mixed rock and internal rock are gradually weakened. Due to the differences in structure and composition, the mixed mudstone reservoir can be divided into mixed shale, laminate–laminate–like mixed mudstone, root–reticular mixed mudstone, snowflake–star–point mixed mudstone. Foliation development, dolomite content and its occurrence pattern are the important controlling factors for the difference of reservoir of reservoir physical properties of mixed rock reservoirs.