Characteristics and origin of high-quality gravity-flow sandstone reservoirs in the 3^rd member of the Dongying Formation, southern Liaozhong Sag, Bohai Sea

Gravity-flow sandstone reservoirs in the medium to deep parts of the Bohai Sea exhibit significantly varying physical properties, making it difficult to predict high-quality reservoirs. This study aims to address this oil/gas exploration challenge. Based on the analyses and tests of conventional physical properties, casting thin sections, clay mineralogy, and geochemistry, as well as data from well logging, core observations, and seismic survey, we investigate the sedimentary characteristics of the deep-water gravity-flow sandstone reservoirs in the 3^rd member of the Dongying Formation (also referred to as the Dong 3 Member) in the Liaozhong Sag, as well as primary factors influencing the development of high-quality reservoirs. The results show that the gravity-flow deposits in the Dong 3 Member in the study area are of the sandy debris flow type, with reservoirs primarily characterized by medium-to-low porosity and permeability. The storage spaces in the reservoirs are dominated by intergranular dissolution pores, followed by primary pores. Among the reservoirs, medium- and coarse-grained sandstones exhibit the optimal physical properties, followed by fine-grained sandstones, while siltstones show poor physical properties. The key factors influencing the development of high-quality reservoirs include: (1)Good sorting, low clay content, and feldspar enrichment act as the material basis for the formation of high-quality reservoirs; (2)In the early diagenesis stage of the Dongying Formation, the rapid subsidence led to undercompaction-induced overpressure, preserving numerous primary pores. During the hydrocarbon generation in the late stage, overpressure prolonged the generation period of organic acids, producing sufficient acidic fluids for the dissolution of minerals like potassium feldspar and calcite in sandstones; (3)Faults connected the reservoirs and source rocks, leading to enhanced dissolution in medium-deep reservoirs, which facilitated the formation of dissolution pores in quantity; (4)Tectonic uplift and lateral compression caused reservoir fracturing, as well as the formation of extensive fractures and dissolution zones, improving the physical properties of the reservoirs.