Orderly distribution and differential enrichment of hydrocarbon in oil-rich sags: A case study of Dongying Sag, Jiyang Depression, Bohai Bay Basin, East China

Based on fine geological modeling and taking the evolution of key reservoir-forming elements as the main line, the mechanism of orderly distribution and differential enrichment of hydrocarbon was revealed by studying the joint evolution and coupling effect of pressure, fluid and reservoir properties. Orderly development of sedimentary systems in a rift basin is the base of orderly distribution of reservoirs, and the continuity of pressure structure is the key to controlling orderly distribution of reservoir. From the sag center to the margin, in the sag, second-order sequence and large-scale sedimentary system, the reservoirs appear in an orderly distribution from lithologic reservoir to structural reservoir to stratigraphic reservoir. Alternative acid and alkaline actions controlled the development of high quality reservoirs in the mid-fan sandy conglomerate bodies in the steep slope, resulting in the oil and gas accumulation pattern of sealing at the root fan and enrichment at the mid-fan, in which the breakthrough pressure difference between root fan and mid-fan determines the reservoir enrichment level. The action of acidic fluid controlled the development of high quality reservoirs in beach bar sand and turbidite. The pressure difference between high-pressure source rock caused by pressurization of hydrocarbon generation and low-pressure reservoir caused by reservoir improvement provided driving force for oil and gas charging, giving rise to the pressing-absorbing oil and gas charging mechanism controlled by source rock overpressure. The research results have guidance and reference significance for fine exploration in mature exploration areas. Key words: rift basin, reservoir orderly distribution, oil and gas differential enrichment, pressure-fluid-reservoir coupling effect, Bohai Bay Basin, Dongying Sa