Overpressure and petroleum generation and accumulation in the Dongying Depression of the Bohaiwan Basin, China

Abstract

AbstractThe occurrence of abnormally high formation pressures in the Dongying Depression of the Bohaiwan Basin, a prolific oil‐producing province in China, is controlled by rapid sedimentation and the distribution of centres of active petroleum generation. Abnormally high pressures, demonstrated by drill stem test (DST) and well log data, occur in the third and fourth members (Es3 and Es4) of the Eocene Shahejie Formation. Pressure gradients in these members commonly fall in the range 0.012–0.016 MPa m−1, although gradients as high as 0.018 MPa m−1 have been encountered. The zone of strongest overpressuring coincides with the areas in the central basin where the principal lacustrine source rocks, which comprise types I and II kerogen and have a high organic carbon content (>2%, ranging to 7.3%), are actively generating petroleum at the present day. The magnitude of overpressuring is related not only to the burial depth of the source rocks, but to the types of kerogen they contain. In the central basin, the pressure gradient within submember Es32, which contains predominantly type II kerogen, falls in the range 0.013–0.014 MPa m−1. Larger gradients of 0.014–0.016 MPa m−1 occur in submember Es33 and member Es4, which contain mixed type I and II kerogen. Numerical modelling indicates that, although overpressures are influenced by hydrocarbon generation, the primary control on overpressure in the basin comes from the effects of sediment compaction disequilibrium. A large number of oil pools have been discovered in the domes and faulted anticlines of the normally pressured strata overlying the overpressured sediments; the results of this study suggest that isolated sandstone reservoirs within the overpressured zone itself offer significant hydrocarbon potential.