Depositional and Diagenetic Controls on Pore Structure of Tight Gas Sandstone Reservoirs: Evidence from Lower Cretaceous Bashijiqike Formation in Kelasu Thrust Belts, Kuqa Depression in Tarim Basin of West China

Deeply buried Lower Cretaceous Bashijiqike sandstones are important gas exploration targets in the Kelasu thrust belt, Kuqa Depression of Tarim Basin in China. The sandstones are characterized by low porosity, low permeability and strong microscopic heterogeneity due to diagenesis during their geologic history. Mineralogical, petrographic, and geochemical analyses combined with high-pressure mercury injection analysis have been used to investigate the diagenesis, diagenetic minerals, and their impact on reservoir quality. This article addresses the controls exerted by depositional parameters and diagenetic modifications on pore-network characteristics (porosity, pore types, sizes, shapes, and distribution), with the aim to unravel the formation mechanisms of this complex pore structures, and to improve the characterization and classification evaluation for the Bashijiqike sandstone reservoirs. The Bashijiqike sandstones are dominated by lithic arkoses and feldspathic litharenite. The pore system consists of intergranular macropores, intergranular micropores, and intragranular pores. Framework grains are generally heavily compacted. Authigenic quartz, authigenic feldspar, clay minerals and carbonates are the major pore-filling constituents. The pore structure is characterized by small pore radius and poor interconnectivity. Entry pressure reflects the microscopic pore network and macroscopic reservoir property characteristics. Pore structure characteristics are linked to the depositional parameters, type and degree of diagenesis. Clays do not control reservoir pore networks alone, and pores and pore throats are wider in coarser grained sandstones. Entry pressure decreases with the content of the rigid quartz. Compaction and cementation continue to decrease the pore-throat size, while dissolution enlarges pores and pore-throats radius. Considerable amounts of microporosity associated with clay minerals and altered grains contribute to the high entry pressure. Comprehensive Coefficient of Diagenesis (CCD), which considers the integrative effect of diagenesis, shows strong statistical correlations with entry pressure. CCD is an integrative modulus of diagenesis and physical property, and generally the higher the values are, the better the pore structure. It is suitable for quantitatively characterizing pore structure in tight gas sandstones. The results of this work can help assess pore-network characteristics like the Bashijiqike sandstones which had experienced strong diagenetic modifications during their geological history.