Characteristics and prediction of overpressure in continental strata, YB area, northeastern Sichuan Basin

In YB area of the northeastern Sichuan Basin,several sets of pressure systems exist.From the upper Triassic to Jurassic,especially in continental strata,overpressure and strong overpressure are common.Based on DST(drill stem test)and logging data,the present pressure features in YB area and the logging responses of mudstone overpressure sections in 3 typical wells are analyzed.The pressure coefficient of continental strata from the Upper Triassic to Jurassic in YB area ranges from 1.3 to 2.2,indicating overpressure or strong overpressure.The top burial depth of overpressure zones locates at 3 600-4 000 m.The logging responses of mudstone overpressure sections show that the resistance of mudstones changes obviously while DT and mudstone density do not change very much.It might be related to the obvious difference between ancient and present burial depth and the high saturation of gas.The formation pressures converted from mudstone DT by means of Eaton formula are close to field testing results.A pressure prediction model to revise mudstone acoustic logging based on tested pressure has been made and might achieve good prediction results.