Fault-fracture mesh petroleum plays in the Jiyang Superdepression of the Bohai Bay Basin, eastern China

Abstract

Based on the results of laboratory simulation experiments and field observations, the concept of ‘fault-fracture mesh petroleum play’ is proposed to explain the secondary petroleum accumulations in the Neogene reservoirs of the Jiyang Superdepression, Bohai Bay Basin, Eastern China. A well-defined fault-fracture mesh petroleum play usually contains three basic elements, an oil source network at the base, a transient storage for migrating hydrocarbons in the middle, and a petroleum accumulation network at the top. The oil source network provides fairways for hydrocarbons generated in a deep source to migrate vertically upward to shallow strata, often consisting of primary faults and fracture zones and/or regional unconformities in direct contact with mature petroleum source beds, and secondary faults intercepting lenticular fluvial sandstones. The transient storages for migrating hydrocarbons include the thick, porous, mat or sheet-like sandstones deposited under the fluvial, braided stream setting and occurring over relatively large geographic areas. These sandstones serve as temporary storage spaces, through which deep-sourced petroleum fluids migrate further, either laterally or vertically into shallower reservoirs. The upper petroleum accumulation network provides suitable habitats for oil and gas accumulation, comprising structural, lithologic, stratigraphic and combination traps. The Jiyang Superdepression of the Bohai Bay Basin hosts several excellent examples of the fault-fracture mesh petroleum play, where hydrocarbons in the Neogene reservoirs were derived either directly from Eocene-Oligocene source rocks, or from the redistribution of pre-existing oil accumulations in the deeper reservoirs. Reactivation of basement-involved faults and regional unconformities provided conduits for channeling the pre-Neogene hydrocarbons into the shallow strata. The magnitudes and frequencies of the faulting activities and their relative timings to those of peak oil generation and expulsion were critical factors for controlling the quantities of hydrocarbons transmitted to the Neogene strata. The thick sandstones and conglomerates in the lower Guantao Formation acted as excellent transient storages for the deep sourced oils, which were then trapped to form accumulations in the shallow structural, lithologic, stratigraphic and combination traps of the Guantao (Ng)–Minhuazhen (Nm) formations. With the recent discovery of several giant oilfields in the shallow Neogene reservoirs both onshore and offshore Bohai Bay Basin, the proposed ‘fault-fracture mesh petroleum play’ concept helps the exploration geologists to modify and clarify long held traditional concepts of petroleum play types in this basin, and expand upon the new exploration horizons of the shallow subtle petroleum traps.