Diagenetic facies, volcanic tuffaceous sediment, and its diagenetic features in deeply buried tight sandstone, Shan 2 and He 1 Members, Ordos Basin, China

Abstract

Understanding diagenetic processes plays a crucial role in evaluating the quality of tight reservoirs. In this study, we aimed to examine diagenetic facies and thereby assess reservoir quality by conducting an integrated analysis of porosity, petrology, permeability, mineralogy, mercury injection, and stable isotopic data in core samples from the Permian Shan 2 and He 1 members (Shan 2–He 1 Mbr) in China's Ordos Basin. Early compaction and precipitation of diagenetic minerals have significantly reduced primary pore space in these members, although certain sandstones have retained anomalously high porosity. These high-porosity reservoirs have been shaped by many factors, with dissolution identified as the predominant mechanism. Detailed petrographic observations and assessments of the primary minerals, the minerals' genesis, and associated diagenetic processes revealed seven distinct diagenetic facies differentiated by their sand-grain populations, types of cementation, clay matrix contents, and volcanic tuffaceous sediment (VTS). The main interstitial fillings consist of VTS, clay minerals, quartz overgrowths, and calcite (ferrocalcite). Clasts and VTS-dissolved sandstone have mainly formed Class I reservoirs with highly unstable grains and good secondary pores resulting from the dissolution of the grains and VTS. VTS tight sandstone, siliceous cementation tight sandstone, illite-siliceous cementation tight sandstone, and calcite cementation tight sandstone have formed Class II reservoirs with high rates of interstitial filling and cementation. Sandstone tightly cemented by illite has poor reservoir properties and has mainly formed Class III reservoirs. This classification underscores the critical importance of understanding diagenesis, diagenetic facies, and especially the diagenetic features of VTS for accurately assessing reservoir quality. A comprehensive understanding of these factors may facilitate more effective hydrocarbon exploration and exploitation in this area.