Molecular Geochemical Constraints on Crude Oil Classification and Oil-Source Relationships in the Hexiwu Structural Belt of the Langgu Depression, Bohai Bay Basin, China

The Hexiwu structural belt is an important oil and gas exploration area in Langgu Sag, Bohai Bay Basin, China. The source of oil and gas has always been the focus of exploration. In this study, the crude oil samples from the rich oil layers in the north, middle and south of the Hexiwu structural belt were used to carry out the analyses of crude oil molecular geochemical characteristics and oil source correlation in the whole area. Combined with source rock evaluation and oil source analysis, the geochemical parameter characteristic chart was used to clarify the existence of two sets of source rocks in the Hexiwu structural belt. The hydrocarbon supply situation and specific classification of crude oil types in Hexiwu structural belt was determined. The crude oil in the Hexiwu tectonic belt can be divided into three types. Type I crude oil is the source rock of the lower Es3 section of Hancun Trough, which is mainly distributed in the north-central Hexiwu tectonic belt. Type II oils occur in the southern Hexiwu structural belt and are primarily sourced from mixed hydrocarbons generated by the Lower Es3 and Upper Es4 source rocks in the Hancun–Caojiawu area. The type III crude oil is the source rock of the upper Es4 section of Caojiawu–Hancun, which is mainly distributed in the south of the Hexiwu tectonic belt. The classification of reservoir types and the definition of source in the Hexiwu structural belt provide theoretical guidance for future oil exploration in the Hexiwu structural belt, thus providing a new framework of oil and gas exploration in the Hexiwu structural belt.