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Controls and Distribution Features of Petroliferous Belts of Subtle Traps in Paleogene Strata of Bohai Bay Basin, Eastern China


Bohai Bay Basin is a Cenozoic petroliferous rift basin in eastern China. The most of the traditional and easy-to-find hydrocarbon traps have been found. Subtle traps such as lithologic traps, stratigraphic traps and structural lithologic combination traps are dominant exploring plays in abundant petroliferous depressions. Using interpretation of 3-D seismic profiles, analysis of well logs and observation of drill cores, organic geochemistry data, and analysis of stratal pressure in the abundant petroliferous depressions, controls and distribution features of petroliferous belts of subtle traps in Paleogene stratal of Bohai Bay Basin were investigated. Distributing in source rock of upper part of the fourth member to third member of Shahejie Formation, sand bodies within lowstand systems tracts comprising incised channels, sublacustrine fans, deltas controlled by valleys and syndepositional faults/slope break zones, and sand bodies of sandy debris and turbidites controlled by prodelta slope and normal faults within transgressive systems tracts to early highstand systems tacts are good reservoirs of lithologic traps and structural lithologic traps. Mudstones of deep lake subfacies in transgression systems tracts of sequences in upper part of fourth member of Shahejie Formation to the third member are high quality source rock. Abnormal high overpressure caused mainly by oil generating, undercompaction of mudstone, and montmorillonite dehydration developing in strata of upper part of fourth members to third member is the main dynamic of hydrocarbon from source rock to subtle traps. Normal faults and unconformities are conduit systems. Lowstand sand bodies controlled by valleys and syndepositional normal faults/ fault slope break zones and sealed by high quality source rock within transgrissive systems tracts are petroliferous belts of lithologic traps and structural lithologic combination traps. Unconformities control distribution of petroliferous belts of stratigraphic traps. The sand bodies controlled by syndepositional faults within lowstand systems tracts, high quality source rock in transgressive systems tracts, and abnormal high overpressure control petroliferous belts of subtle traps. Keywords: subtle traps, sand bodies within lowstand systems tract, syndepositional normal fault, abnormal high overpressure compartment, Bohai Bay Basin.