Evaluating the lacustrine Shahejie Formation source rock potential in the Bohai Bay Basin using a simplified regional basin model

Abstract

Often petroleum system modelers tend to build complex and time-consuming basin models to understand detailed aspects of hydrocarbon generation and migration within a sedimentary basin. This study built a simple basin model focused on the middle part of the organic-rich, Eocene-Oligocene Shahejie Formation which is the predominant lacustrine source rock in the Bohai Bay Basin. This map-based basin model was created using published regional cross sections, subsurface maps, geochemical data, geothermal gradient data, and structural elements. The integration of these datasets allowed for basin-wide, average TOC and thermal maturity maps to be modeled much more quickly than a geocellular model. These maps show that the majority of the known hydrocarbon fields are found above the Shahejie source rock kitchens. Some fields, however, are found slightly outside of the source rock kitchens indicating either contribution from deeper known source rocks intervals (e.g., the Kongdian Formation) or lateral migration from the middle Shahejie Formation.

The Bohai Bay Basin is a structurally complex rift basin with several strike-slip fault systems being present in the basin with the Tanlu fault system being the most prominent. Using potential field data, published cross sections, isopach maps, hydrocarbon fields, and the presence of Eocene evaporites, structural elements (lineaments) were interpreted. These structural elements influence depositional trends at various stratigraphic levels and the regional thermal maturity trends. Many of these structural elements likely represent thick-skinned faults and subtly influence the TOC trends within the Eocene. Within the middle Shahejie Formation, the average TOC is higher within the depocenters, which may show that lacustrine source rocks are driven more by accommodation space because of the inherent restricted nature associated with lacustrine depositional environments.