Timeframe of hydrocarbon migration in the Paleogene Shahejie Formation in the Dongying depression, Bohai Bay Basin (northeastern China) based on fluid inclusions and oil geochemistry.

The Dongying depression is an important oil-producing area in northeastern China, where the Paleogene Shahejie Formation is the main oil-bearing feature. Although petroleum migration and accumulation in the Dongying depression have been extensively studied, the charging history is still unclear. This paper defines the timeframe of petroleum migration on the basis of a comprehensive investigation of fluid inclusions coupled with basin modeling. According to the petrography and microthermometry of the fluid inclusions, types of fluid inclusion with low-peak-range distributions of the homogenization temperature (T h) (85–95 °C for the Niuzhuang sag and 90–105 °C for the Lijin sag) and high-peak-range T h distributions (105–120 °C for the Niuzhuang sag and 115–130 °C for the Lijin sag) were identified. By recovering the burial and thermal histories of Niuzhuang and Lijin sags with pressure–volume–temperature–composition simulation and one-dimensional basin modeling, it can be inferred that the trapping pressures of two types of hydrocarbon inclusions were higher than the corresponding paleopressure. (42.4 and 42.0 MPa, respectively), indicating rapid hydrocarbon charging resulting from abnormally high pressure. Two hydrocarbon charging events associated with the regional tectonic evolution of the Dongying depression, namely, continuous deposition and tectonics in the Paleogene Dongying Formation (Ed) and rapid deposition and tectonics in the Paleogene Minghuazhen Formation (Em), were inferred. The direction of oil charging was traced from the center to the margin in the north, and from the center to the central fault in the southeastern part of the depression in general, as indicated by the molecular maturity parameters, which roughly coincided with the abnormal pressure. • The investigation of fluid inclusions showed two oil-charging episodes in Sha 3 reservoirs. • The basin modeling suggested each reservoir was charged during the Late Paleogene and Early Neogene respectively. • The oils are charged along with the central faults and from the center to the edge of the depression. [ABSTRACT FROM AUTHOR]