Diagenetic Evolution of Deep Tight Sandstone Reservoirs in the Lower Jurassic Strata of the Kuqa Depression, Tarim Basin

Abstract Deep sandstone reservoirs are generally characterized by low permeability; nevertheless, effective reservoir rocks can still develop within them. However, the types of effective reservoir rocks and tight rocks require further characterization, and the differences in their diagenetic processes warrant further investigation. This study examines the diagenetic evolutionary processes of different petrofacies in the tight sandstone reservoirs of the Jurassic Ahe Formation in the Kuqa Depression of the Tarim Basin, based on analyses of effective reservoir rocks and tight petrofacies. Research indicates that the sandstones of the Jurassic Ahe Formation are predominantly medium- to coarse-grained lithic sandstones with minor feldspathic litharenite. These sandstones are classified into three petrofacies: petrofacies A (ductile lithic-lean sandstone), petrofacies B (ductile lithic-rich sandstone), and petrofacies C (tightly carbonate-cemented sandstone). These three petrofacies exhibit significant differences in terms of diagenesis and diagenetic pathways. Petrofacies A is an effective reservoir rock, characterized by a relatively high content of rigid grains. During burial, fluid activity was frequent, causing this petrofacies to experience multiple episodes and types of diagenesis. Cement types are diverse but occur in low abundance, and secondary dissolution pores are relatively well developed. Petrofacies B and C are classified as tight petrofacies. Petrofacies B contains a high abundance of ductile grains; during the early diagenetic stage, severe porosity loss occurs because of mechanical compaction. Late diagenesis is dominated by clay mineral transformation, with almost no other fluid–rock interactions occurring. Similarly, petrofacies C experiences extensive pore filling by calcite cements during early diagenesis, making pore spaces exceptionally rare and leading to weak fluid–rock interactions. This study contributes to the analysis of deep tight sandstone reservoir quality and the identification of effective reservoir rocks capable of hosting hydrocarbon accumulation, thereby providing a geological basis for deep hydrocarbon exploration and development.