Reason Analysis of High Geothermal Temperature of Bohu Depression and Relations with Hydrocarbon Generation

After measuring geothermal temperature today and analyzing rock thermal conductivity, it is discovered that Bohu Depression in Yanqi Basin has a higher geothermal gradient, being highest in the middle and gradually reduced in a south-north direction, which is attributed to higher terrestrial heat flow values since a higher geothermal gradient results from deep land inside. Various methods, including apatite fission track and inclusion temperature measuring, are adopted to recover paleogeothermal gradient and analyze thermal evolution for Bohu Depression. The recovered paleogeothermal gradient, namely 4.00-4.36℃/100m represents the geothermal gradient before the basin was uplifted and cooled in Early Cretaceous Epoch, being higher than the geothermal gradient today. Moreover, the recovered paleogeothermal temperature revealed that there were two oil-generation in northern sag Jurassic strata, namely late Jurassic to early stage of Early Cretaceous Epoch and the second since Tertiary period; and one oil generation for southern sag and fracture belt in Zhongmachang Area, namely late Jurassic to early stage of Early Cretaceous Epoch