Abstract:

Drilling data show that there is overpressure in the Yinggehai basin now. Vertically, from the shallow to the deep, the pressure zones can be divided into normal pressure zone, transitional zone and overpressure zone. Data of both acoustic travel time of well logging and seismic velocity have been used to determine the distribution of overpressure on the period of the most gas discharge (1.9 Ma ago) and at present with the method of compaction equilibrium, and to restore the pressure evolution of the wells in the Yinggehai basin. The result shows that, laterally, Ledong area has the biggest overpressure which decreases both from south to north and from west to east, but in Lingao uplift and Yinggehai eastern slope, most places are in normal pressure zone. In each well, the overpressure increases with time. This result is closely related to the centre of subsidence and different zone’s sedimentary environment. Because overpressure exists in the Yinggehai basin, gas migrates vertically under the influence of overpressure. Natural gas pools are distributed in Ledong area and Dongfang area where Huangliu Formation’s roofs and floors have large pressure differences and Yinggehai Formation’s cap rocks have big sealing ability of overpressure. The reservoirs which satisfy the above two conditions and near diapirs are favorable for the accumulation of natural gas.

Key words: migration and accumulation, natural gas, overpressure evolution, Yinggehai basin