Comprehensive evaluation of the Paleogene lacustrine source rocks within the Bodong Sag, Bohai Bay Basin using kinetics-based petroleum system modeling

Abstract

The hydrocarbon potential of the Bodong Sag remains unclear. Investigation and comprehensive evaluation of Paleogene lacustrine source rocks is essential to determine the exploration potential and direction. This study examined the Paleogene source rocks using organic geochemical methods, including total organic carbon (TOC), Rock-Eval pyrolysis, maceral composition, vitrinite reflectance (Ro), and kinetic analysis. For the first time in this study area, a three-dimensional (3D) kinetics-based petroleum system modeling approach was employed to reconstruct the burial history, thermal maturity, and hydrocarbon generation history of the source rocks. The resource mass was evaluated based on the calculated generation mass. Results indicate that the third member (E2s3) and first member (E3s1) of the Shahejie Formation are mainly Type II1 organic matter, exhibiting high generation potential. The third member of the Dongying Formation (E3d3) contains mixed Type II1 and II2 organic matter, with moderate potential. The E2s3 and E3s1 source rocks matured early, entering the oil window in the Early Oligocene (∼30.3 Ma) and Middle Oligocene (∼28 Ma), respectively, and are currently in the wet gas to dry gas stage. The E3d3 source rocks matured later, entering the oil window at the end of the Oligocene (∼24.6 Ma) and are currently in the late oil to the wet gas stage. Subsidence and burial have resulted in higher maturity in the southern subsag compared to the northern subsag, with the margins remaining in low maturity to immature stages. Resource estimates for the Bodong Sag source rocks are quantified at 8.33 × 108 t of oil and 1.68 × 1011 m3 of gas. E2s3, E3s1, and E3d3 contribute 41.62%, 33.95%, and 24.43% respectively, with E2s3 source rocks being the major contributor. The southern subsag, accounting for 76.92%, is the primary hydrocarbon generation kitchen. The significant increase in natural gas resources highlights the prospects for natural gas exploration in the Bodong area.