Characteristics of overpressure distribution and its implication for hydrocarbon exploration in the Qiongdongnan Basin

The Qiongdongnan Basin is a strongly overpressured basin with the maximum pressure coefficient over 2.2. Two types of vertical overpressure configuration can be identified by electronic logs and mud pressure, based on the calibration with the test pressure. The first is a double overpressure configuration, in which the middle low overpressure zone divides the entire overpressure zone into two zones. The double overpressure configuration lies primarily in the western part of the basin. The second is a single overpressure configuration, in which overpressure increases with depth. The single overpressure configuration lies primarily in the eastern part of the basin. Distribution maps of the overpressure top and overpressure horizons show three important characteristics: (1) The distribution of the pressure coefficient is not uniform. There are many low overpressure zones against a background of high overpressure. (2) The pressure coefficient in the western area is greater than in the eastern area. The maximum pressure coefficient in the western area is greater than 2.2. (3) There is a low overpressure interval between the high overpressure zones in the western area. Based on the overpressure distribution, some important implications for hydrocarbon exploration can be drawn. In the Qiongdongnan Basin, it has been shown that normal pressure zones are favorable for hydrocarbon accumulation, and strongly overpressured zones (pressure coefficient greater than 1.8) are unfavorable for hydrocarbon accumulation. Accordingly, the NW low overpressure belt around wells R, Q and S should be beneficial for hydrocarbon accumulations, and should be considered as the next exploration play in the Qiongdongnan Basin. Theoretical hydrofracture calculation and interpretation of the sand injectites indicate the presence of widespread hydrofractures in the basin. Comparison between the sealing capacities of the double overpressure and single overpressure configurations shows that the former is superior for hydrocarbon accumulation and preservation. Because of the strong sealing capacity provided by the displacement pressure and pore pressure difference between the seal rock and reservoir in the double overpressure configuration zone, hydrocarbons barely penetrate the seal rock in the middle low overpressure zone. Therefore, the exploration interval should be within and below the middle low overpressure zone in the western basin.