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.

Lamellar calcite veins are prevalent in carbonate-rich, lacustrine dark shale. The formation mechanisms of these veins have been extensively debated, focusing on factors such as timing, depth, material source, and driving forces. This paper examines dark lacustrine shale lamellar calcite veins in the Paleogene strata of Dongying Depression, using various analytical techniques: petrography, isotope geochemistry, cathodoluminescence, inclusion thermometry, and electron probe micro-analysis. Two distinct types of calcite veins have been identified: granular calcite veins and sparry calcite veins. These two types differ significantly in color, grain structure, morphology, and inclusions. Through further investigation, it was observed that vein generation occurred from the shallow burial period to the maturation of organic matter, with a transition from granular calcite veins to sparry calcite veins. The granular calcite veins exhibit characteristics associated with the shallow burial period, including plastically deformed laminae and veins, the development of strawberry pyrite, the absence of oil and gas, weak fractionation in oxygen isotopes, and their contact relationship with sparry calcite veins. These granular calcite veins were likely influenced by the reduction of sulfate bacteria. On the other hand, sparry calcite veins with fibrous grains are antitaxial and closely linked to the evolution and maturation of organic matter. They contain oil and gas inclusions and show a distribution range of homogenization temperature between 90 ℃ and 120 ℃ and strong fractionation in oxygen isotopes, indicating formation during the hydrocarbon expulsion period. The carbon isotope analysis of the surrounding rocks and veins suggests that the material for vein formation originates from the shale itself, specifically authigenic micritic calcite modified by the action of methanogens. The opening of horizontal fractures and vein formation is likely driven by fluid overpressure resulting from undercompaction and hydrocarbon expulsion. Veins may form rapidly or through multi-stage composite processes. Early veins are predominantly formed in situ, while late veins are a result of continuous fluid migration and convergence. Furthermore, the veins continue to undergo modification even after formation. This study emphasizes that the formation of lamellar calcite veins in shale is a complex diagenetic process influenced by multiple factors: biology, organic matter, and inorganic processes, all operating at various stages throughout the shale's diagenetic history.