Differential Diagenetic Evolution and the Mechanism of Low-Porosity, Ultra-Low-Permeability Sandstone Reservoirs in the Jurassic Badaowan Formation, Central Depression of the Junggar Basin, China

The aim of this study is to investigate the diagenetic processes and their impact on reservoir quality within the Jurassic Badaowan Formation of the Junggar Basin. Various experimental techniques were employed, including thin section petrography, scanning electron microscopy, x-ray diffraction, and fluid inclusion micro-thermometry, to characterise the rock composition, pore types, and diagenetic alterations from core samples of nine wells in the study area. The results reveal that the compaction and cementation are the dominant factors contributing to reservoir densification, with porosity significantly reduced by mechanical compaction and carbonate cementation. A total of four facies were identified based on diagenetic strength and sedimentary cycle associated with different diagenesis stages and reservoir physical-property responses: (1) "Low-plastic lithic weak compaction facies (LPLF)" occurred the cementation of chlorite rims at the early diagenetic stage and multiple stages of cementation and dissolution at the middle diagenesis stage; (2) "Volcanic lithic-rich dissolution facies (VLDF)" was the characteristic of weak compaction at the early diagenetic stage and intense dissolution and weak cementation of middle diagenesis stage; and (3) "Calcareous cemented facies (CCF)" and (4) "carbonaceous-laminated strongly compacted dense facies (CLCF)" underwent strong cementation and compaction respectively at the early diagenetic stage resulting in a tight reservoir. Dissolution processes, primarily driven by acidic fluids from coal-bearing strata, enhance secondary porosity development, particularly in feldspar and volcanic rock fragments. However, in certain lithofacies, these improvements are offset by extensive calcite and siderite cementation, reducing permeability and hindering fluid flow. The study concludes that the diagenetic heterogeneity plays a critical role in reservoir quality variation across the Badaowan Formation, with the findings providing valuable insights into the differential diagenesis evolution of reservoir quality for improving hydrocarbon exploration strategies in similar geological settings.