Characteristics and hydrocarbon accumulation models of the whole petroleum system in the Dongpu Sag, Bohai Bay Basin

The Dongpu Sag represents a typical sedimentary sag of the saline lake facies. Presently, the hydrocarbon exploration in this sag is in a period of stagnation. By combining geological and geochemical methods, we systematically analyze the generation conditions, genetic mechanisms, and differential accumulation characteristics of hydrocarbons in this sag. The results indicate that the Dongpu Sag contains five suites of source rock sequences and four suites of evaporite sequences, exhibiting a structural framework characterized by multiple faults, uplifts, and sub-sags. The reservoirs in the sag feature significant heterogeneity and diverse hydrocarbon properties and genetic types, forming a large-scale whole petroleum system consisting of complete medium and shallow conventional hydrocarbon reservoirs, along with deep tight and shale hydrocarbon reservoirs with excellent accumulation conditions. Further investigation reveals that the properties and spatial distribution of hydrocarbons in the Dongpu Sag are governed by nine sub-sags, resulting in nine hydrocarbon types in three categories of sedimentary facies: saline, transitional, and freshwater facies in the northern, central, and southern parts, respectively. Accordingly, 11 hydrocarbon accumulation models are identified. The central uplift zone, or the intra-sub-sag uplift tectonic zone, in the salt-bearing area serves as the most hydrocarbon-enriched area, with conventional hydrocarbon reservoirs predominating. Based on the theory of the whole petroleum system, we establish a comprehensive hydrocarbon accumulation model subjected to varying dynamic fields and determine play fairways of conventional, tight, and shale hydrocarbons, thus laying a scientific basis for the future emplacement of hydrocarbon exploratory wells in the study area.