Geothermal regime and hydrocarbon kitchen evolution in the Jianghan Basin

Abstract

The present geothermal gradient and terrestrial heat flow was calculated of 18 wells in the Jianghan Basin. Thermal gradient distribution of the Jianghan Basin was obtained based on data of systematical steady-state temperature and oil-test temperature. The basin-wide average thermal gradient in depth interval of 0–4000 m is 33.59°C/km. We report nine measured terrestrial heat flow values based on the data of detailed thermal conductivity and systematical steady-state temperature. These values vary from 41.9 to 60.9 mW/m2 with a mean of 52.3 ± 6.3 mW/m2. However, thermal history analyses based on vitrinite reflectance (VR) and apatite fission track (AFT) data indicate that thermal gradient in the northern and southern Qianbei Fault reached its peak of ∼36 and ∼39°C/km respectively in the Middle Jurassic and the Oligocene, and it descended during the early Miocene to the present-time value. Furthermore, tectonic subsidence analysis reveals that the tectonic subsidence of the Jianghan Basin in the Cretaceous to early Miocene was characterized by synrift initial subsidence followed by the subsequent thermal subsidence. The thermal history and tectonic subsidence history of Jianghan Basin are of great significance to petroleum exploration and hydrocarbon source assessment, because they bear directly on issues of petroleum source rock maturation. Based on the thermal history and tectonic subsidence history, with the combination of geochemical and thermal parameters, the maturation and the hydrocarbon generation intensity evolution history of the P2 d source rocks are modeled. The results show that the P2 d source rocks are in a higher degree of maturation at present, and the Yuan’an and Herong sags are the two most important kitchens in the Late Jurassic, Xiaoban Sag is another most important kitchen during the Late Cretaceous to late Paleogene, and the Zhijiang and Mianyang sags are other two important hydrocarbon kitchens in the Late Cretaceous. The Mianyang Sag and Yichang Ramp are the favorable exploration targets in the future. This study may provide new insight for the understanding of the oil and gas exploration potential for the Jianghan Basin.