Geothermal regime of the Bohai Offshore Area, Bohai Bay Basin, North China

The Bohai Offshore Area is located in the northeastern marine area of the Bohai Bay Basin in northern China. It is one of the most productive petroleum basins in China. In this study, thermal gradient and heat flow distribution results for the Bohai Offshore Area (Bohai Subbasin) were obtained based on new oil-testing temperatures, drill-stem test temperatures (DSTs), bottom-hole temperatures (BHTs) and documented thermal conductivity data. The average geothermal gradient is 32.0±4.6°C/km for the entire offshore area. Heat flow in the Bohai Subbasin ranges from 33.5 mW/m2 to 84.0 mW/m2, with an average value of 60.8±8.7mW/m2. Thermal gradient evolutions of several sags in the Bohai Subbasin were obtained using constraints for single well modeling from vitrinite reflectance (Ro) and apatite fissin track (AFT) data. Modeling results show that there were different thermal histories for different parts of the subbasin. Generally, the thermal gradient gradually decreased with geological evolution. The thermal gradient was 57∼53°C/km in the Paleocene epoch and 53∼43°C/km during the deposition period of the Shahejie Formation (e.g., Eocene epoch). This value slowly decreased after the Oligocene (38∼43°C/km). It was 38∼34°C/km in the Miocene, and it is 32°C/km at present-day. This thermal gradient evolution relates well to activities in the Tanlu Fault Zone and to tectonic subsidence. The first episode of the syn-rift initial subsidence of the basin shows a higher thermal gradient during the Paleocene∼Eocene epochs. The second episode of thermal subsidence corresponds to a lower thermal gradient after the Oligocene epoch. The geothermal regime of the Bohai Offshore region will provide some useful data to study regional tectonic characteristics of the basin and the evolution of hydrocarbon source rocks.