Formation and Evolution Mechanism of High-quality Reservoirs in the Lacustrine Hyperpycnal System: The 2nd Member of the Lower Jurassic Sangonghe Formation, Qiudong Subsag, Turpan–Hami Basin, NW China

Abstract

Hyperpycnal deposits has gradually emerged as a current research hotspot, with a special focus on lacustrine hyperpycnal deposits. However, our understanding of the formation mechanism and distribution of high-quality reservoirs in hyperpycnal deposits remains insufficient. In this study, the formation mechanism and distribution of high-quality reservoirs in the second member of the Sangonghe Formation are studied through sedimentology, petrology, and geochemistry, and 10 types of lithofacies were identified in the depositional sequences of the hyperpycnal flow. These can be summarized as gravelly bed to suspended load lithofacies association (GBS), gravelly reverse to normal compound-graded (GNR), coarse-grained sandy suspended load (CSS), and fine-grained sandy suspended load (FSS) lithofacies associations. The hyperpycnal system can be divided into four individual units: restricted channel, unrestricted channel, lobe, and levee. The reservoir quality varies with lithofacies. Gravelly bed load lithofacies has coarse grain sizes, high content of rigid minerals, and soluble components, such as magmatic rock fragments. Therefore, the gravelly bed load lithofacies reservoir has developed dissolution pores and well-preserved pore throats. Its reservoir quality is good. GBS and GNR are the main lithofacies associations in the restricted channel, where type I and type II reservoirs developed. The unrestricted channel is dominated by the CSS lithofacies association, with type III reservoirs developed. The lobe is dominated by the CSS and FSS lithofacies associations, with type III and type IV reservoirs developed. High-quality reservoirs mainly develop in the restricted and unrestricted channels, whereas reservoirs in the lobe have poor quality. The levees are dominated by siltstone and gray mudstone and are generally nonreservoirs.