Reconstruction of oil charging history in the multi-source petroleum system of the Beidagang buried-hill structural belt in the Qikou Sag, Bohai Bay Basin, China: Based on the integrated analysis of oil-source rock correlations, fluid inclusions and geologic data

Abstract

The Beidagang buried-hill structural belt (BBHSB), located in the central part of the Qikou Sag, Bohai Bay Basin, hosts a multi-source and structurally complex petroleum system, in which the interpretation of oil-source relationships and the definition of hydrocarbon accumulation features are challenging due to the highly variable composition of oils and complex oil migration pathways. In this study, we use biomarkers, fluid inclusions, and geologic data to address the complexities of petroleum system analysis in BBHSB. Based on hierarchical cluster analysis, four genetic groups of oils (A, B, C and D) with distinct molecular compositions were identified. Group A, B, and C oils are originated from the source rocks in the third member (Es 3 ) of the Eocene Shahejie Formation (Es), the lower (Es 1 x ) section of the first member (Es 1 ) of Es, and the middle (Es 1 z ) section of Es 1 , respectively. Group D oils are mixed oils contributed by the source rocks of Es 1 x and the second member (Es 2 ) of Es. Bounded by geological settings, the charging histories of the four groups of oils were different. Fluid inclusion analysis suggests that Group A oils have two charging stages, i.e., the late Oligocene and the Pliocene-present. During the first stage, Group A oils mainly accumulated in Es 3 with relatively low maturity. During the second stage, the Group A oils vertically migrated along the fault in a large scale, resulting in its accumulation in the middle-shallow formations near the fault. Due to the late maturity of source rocks, Group B, C, and D oils have one accumulation stage that occurred during the Pliocene-present. Due to the blocking effect of the reverse drag anticline on the migration of oils to the fault, the accumulation of Group B and C oils were limited to inner source-layers and the distribution of Group D oils in the northeastern BBHSB was limited in Es 1 x . • The crude oils can be separated into four genetic groups (A, B, C, and D) with distinct sources. • Group A oils have two charging stages and the other three group oils have only one. • Constrained by geological conditions, the four group oils have different migration features. • The charging history of the four groups of oils has been constructed.