DIFFERENCE IN NATURAL GAS ACCUMULATION CONDITIONS BETWEEN YINGGEHAI AND QIONGDONGNAN BASINS AND ITS IMPLICATIONS FOR NATURAL GAS EXPLORATION

The Yinggehai and Qiongdongnan basins, often called Ying-Qiong basin, are quite different in their petroleum accumulations. The Tertiary and Quaternary sediments in the Yinggehai basin is up to 17 km thick, with the Oligocene Yacheng and Lingshui formations over-mature. Therefore, the effective source rocks are the Sanya and Meishan formations. The main source rocks in the Qiongdongnan basin are the Yacheng and Lingshui formations. Although both basins are overpressured, their fluid flows are greatly different. The combination of strong overpressure and tensile stress field caused by the right-lateral slipping movement has led both to the fractures in the top layers of the over-pressure systems and to the occurrences of diapirs, an expulsion pathway for the overpressured fluids including natural gases. The fluid flow in the Qiongdongnan basin, however, is compaction driven, and laterally dispersed from the deep depression towards its surrounding areas with a relatively low fluid potential. Traps associated with diapirs are regarded as important exploration targets in the Yinggehai basin, whereas the Neogene lithological-structural complex traps with the excellent pathways to the Oligocene source rocks or the Oligocene structural traps should be considered as the main exploration targets in the Qiongdongnan basin.