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

Abstract

The 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.

Geochemistry, origin and accumulation of natural gases in the deepwater area of the Qiongdongnan Basin, South China Sea

“…9b) and the Cp value is predicted to be as high as 2.10 in the Yacheng Formation source rock layers . Overpressures developed in the source rocks in central basin appear to be the main driving-force for fluid migration (Xie et al, 2001). Notably, the diapiric structures (intermittently chaotic and blank seismic reflections) like those in Yinggehai Basin are also observed in the area (Fig.…”

Subsidence history and forming mechanism of anomalous tectonic subsidence in the Bozhong depression, Bohaiwan basin

“…The exact age of the lowermost strata could not be determined. Hence, we assume the age of basement to be 65 Ma based on the regional data [3,13] . As seen in Figure 5, the early rifting events last from 65 Ma to 32 Ma, and the later rifting from 32 Ma to 24.6 Ma.…”

Occurrence and Nature of Overpressure in the Sedimentary Section of the South Caspian Basin, Azerbaijan

“…So, for example, in the Dongying depression, located in the Bohaiwan basin (China), where the average geothermal gradient is high at about 3.6˚C /100 m (Zhou, 1981), the mature zone for OM is predicted to be at depths of about depth 2200-3000 m, and overmature OM is expected to occur in an interval of 3000-3800 m. In complete conformity with this estimation, excess pressure with gradients of 13-14 MPa/km already occurs here from a depth 2200 m, increasing up to 16 MPa/km below 3000 m (Xie et al, 2001).…”