Mechanism of Putaohua oil layer's overpressure in the Gulong Sag,Songliao Basin

Drilling data revealed there occurring overpressure in Putaohua oil layer in the Gulong Sag,Songliao Basin, hut no undercoinpaclion being discovered. On the contrary, the undereom-paction existed in the overlying shales of Members 1 and 2, Nenjiang Formation and Qingshank-ou Formation. According to the comparison between reservoirs' testing pressure and the overpressure calculated by interval transit time, the reason of the overpressure in oil layer might mainly be from the pressure transference of internal overpressure system. The statistics of oil-production capacity and test pressure data indicated that the oil cracking had some contribution to the overpressure of oil layer, and the pressure gradient of oil layer which existed overpressure was higher than that, of hydrostatic pressure gradient. It comprehended that there might be flowing pressure in the reservoirs, and Qingshankou Formation and Members 1 and 2, Nenjiang Formation are still providing fluids to Putaohua oil layer nowadays.