Diagenetic alteration and its control on reservoir quality of tight sandstones in lacustrine deep-water gravity-flow deposits: A case study of the Yanchang Formation, southern Ordos Basin, China

Abstract

Abstract The tight oil sandstone reservoir of the Upper Triassic Yanchang Formation is one of the main targets for hydrocarbon exploration in the Ordos Basin, China. Sandstones of the Upper Triassic Yanchang Formation within lacustrine deep-water gravity-flow deposits are characterized by ultralow porosity, ultralow permeability, and complex diagenetic alterations and sedimentary lithofacies. However, the diagenetic alterations and their controls on the reservoir quality of the Chang 6 tight sandstones remain poorly understood. This study integrated a variety of techniques, such as casting thin section analysis, scanning electron microscopy (SEM), back-scattered electron imagery (BSE), X-ray diffraction (XRD), cathodoluminescence (CL) microscopy, and fluid inclusion, to investigate the lithofacies, diagenetic history, and diagenetic intensity, and also evaluate their controls on reservoir quality. The research results indicated that the Chang 6 sandstones exhibit variable intensity of diagenetic alterations among the various lithofacies, which leads to variable reservoir quality. Mechanical compaction was the most important factor reducing primary porosity. Sandstones of the Ss lithofacies (siltstone to very fine-grained sandstones with graded bedding or ripple laminations) showed the highest loss of primary porosity. Carbonate cements were commonly present in sandstones of the Sc (fine-grained and cross-bedded sandstones) and Sm (fine-grained and massive sandstones) lithofacies near the sandstone–mudstone bounding surface. Sandstones with more authigenic illite rims were able to resist compaction to some extent and were favorable to the preservation of porosity; however, abundant pore-filling illite aggregates commonly bridged pore throats, causing a significant reduction in reservoir quality. Feldspar dissolution porosity leads to a slight enhancement in reservoir quality. Intergranular pores and feldspar dissolution pores are most abundant in the sandstones away from the sandstone–mudstone bounding surface of the Sc lithofacies, resulting in the best reservoir quality, which is followed by sandstones of the Sm lithofacies. Ultimately, four evolution patterns of diagenetic alterations that formed best, moderate, or poor reservoir quality were summarized by linking diagenetic alterations to lithofacies. The research results are conducive to the evaluation of reservoir quality and to understanding the genetic mechanisms of tight sandstones, so they should facilitate hydrocarbon exploration and production in similar lacustrine deep-water gravity-flow reservoirs.