The characteristics and controlling factors of high-quality reservoirs of ultra-deep tight sandstone: A case study of the Dabei Gas Field, Tarim Basin, China

High-quality reservoirs with sufficient physical properties of ultra-deep tight gas reservoirs in the Lower Cretaceous Bashijiqike Formation exhibit significant relationship with gas production across the Dabei Gas Field. Clarifying the characteristics, controlling factors, evolution and distribution of the high-quality reservoirs is important for the deployment of "sweet spots". An integrated approach of petrography, SEM, cathode luminescence, XRD, physical property, NMR experiment, well logs and 3D pre-stack depth migration data was carried out. This study examined the sedimentation, diagenesis, tectonism, gypsum-rock and overpressure to reveal the formation and distribution of the high-quality reservoirs. The sandstones are very fine-to coarse-grained lithic arkose. The variation in detrital grains texture and mineralogical composition affects diagenetic evolution, pore structure and gas-bearing characteristic. Three diagenetic facies (diagenetic facies A to C) are classified. Different diagenetic facies underwent different evolution of reservoir quality. The medium–fine to medium-grained lithic arkose (diagenetic facies A) underwent medium vertical compaction, tectonic compression and cementation, and exhibited relatively porous, which makes partial primary pores preserved with the occurrence and persistence of overpressure. Conversely, the very fine to fine-grained and mud gravel-rich medium-grained lithic arkose (diagenetic facies B) and diagenetic facies C appeared premature densification by strong compaction and tight carbonate cementation, respectively, which makes them free from overpressure effects. Natural fractures increased the permeability by several orders of magnitude. Gypsum-rock weakened the vertical compaction and tectonic compression, and delayed cementation, promoted and preserved overpressure. With the injection of internal high alkaline fluids, anhydrite/dolomite cements/veins and dissolution of siliceous developed. Diagenetic facies A exhibited sufficient physical properties and gas volume, making it the high-quality reservoir in tight gas sandstone. Diagenetic facies B and C mainly were used as barriers for hydrocarbon charging. Diagenetic facies A was mainly distributed in the middle-lower part of distributary channel or the middle-upper part of mouth bar. This distribution model provides guidance for exploration in ultra-deep tight sandstone gas reservoir.