Diagenetic facies identification and distribution study of ultra-low permeability reservoir in Su77 block of Sulige Gas Field

Braided channels developed well in the lower part of the 8th member of middle Permian Shihezi Formation in Su77 block of Sulige Gas Field. Lithic quartz sandstone and lithic sandstone are the main lithology of reservoir. After compaction, pressure solution, authigenic mineral cementation and later dissolution, most of re-servoirs that are low porosity and ultra-low permeability ones have entered middle diagenetic stage B. According to the main diagenetic minerals, fillings, diagenesis and pore combination, reservoirs of the lower part of the 8th member of middle Permian Shihezi Formation are divided into 4 diagenetic facies as followed: Ⅰ. quartz overgrowth and dissolved pore facies; Ⅱ. clay minerals filling and intergranular pore facies; Ⅲ. carbonate minerals cementation facies; Ⅳ. compaction and pressure dissolution dense facies. Facies Ⅰ are the best reservoirs and facies Ⅱ are second-best ones. Facies Ⅲ and Ⅳ are poor reservoirs. According to logging facies, diagenetic facies are combined as concentric pattern, lateral variation pattern and cross-layer pattern.