Coupling thermal and maturity histories of the Gulong shale oil reservoir in the Songliao Basin: Insights from vitrinite reflectance and apatite fission track

Abstract

The coupling relationship between the thermal state and maturation during the geological historical period is critical for shale oil-gas exploration because their present distribution characteristics are not probably the same. In this study, the thermal–maturity history of the shale oil reservoir in the Qingshankou Formation of the Gulong Sag, Songliao Basin, was established, and their coupling relationship was analyzed using vitrinite reflectance (Ro) and apatite fission track (AFT) data. The results revealed that the Gulong Sag exhibits a high thermal state, with temperatures decreasing from central to peripheral regions. In contrast, the maturity of organic matter exhibits an opposite trend; for example, GY8 well in the central region has low temperatures but high maturity. The thermal–maturity history constructed using the Ro and AFT data indicates two critical developmental stages for shale oil reservoirs between 60 and 90 Ma: the late Mingshui and Nenjiang movement stages. The former stage had a considerably greater influence on shale reservoir formation than the latter stage, which accelerated organic matter maturation rapidly because of the greater burial depth and longer duration. Additionally, the Gulong Sag has a low height-to-width ratio, and the thermal state plane distribution characteristics are influenced to some extent by the concave–convex structures. Therefore, the thermal structure at the end of the Mingshui stage is particularly important, and high temperatures in concave areas are likely the most desirable areas for the formation of shale oil reservoirs during that period. This understanding can also guide shale oil exploration in regions with similar geological settings.