A model for faults to link the Neogene reservoirs to the Paleogene organic-rich sediments in low-relief regions of the south Bohai Sea, China

Fault acts as an important vertical migration conduit for hydrocarbons in rift basins and can transport hydrocarbons from deep-buried organic-rich sediments to shallow-buried coarse-grained sequences. Such migration process is expected, but generally needs to pass through thick mudstone intervals and its controlling factors are not well understood. The low-relief regions of the south Bohai Sea in the Bohai Bay rift basin, comprising both Paleogene and Neogene hydrocarbon traps, can act as a good case study for the factors controlling the hydrocarbon transporting capacity of faults. By comparing the hydrocarbon generation potential of source rocks in different formations, we confirm that the hydrocarbons trapped in the Neogene traps mainly originate from the Eocene Shahejie Formation, and need to pass through a thick Oligocene regional caprock. Hydrocarbon fluid inclusions within the Neogene reservoir sandstones indicate two major hydrocarbon charging events in the middle Pliocene to Quaternary, based on which we chose the T02 seismic marker bed to calculate fault displacement. Seismic profiles show that Neogene hydrocarbon-charged traps are commonly linked to faults with large-amount displacement, where the regional caprock has a limited thickness. Using statistics of the Neogene charged traps in a regional 3D seismic grid, we draw the minimum fault displacement for hydrocarbon migration in the Laizhouwan Sag at 53.6 m and in the Huanghekou Sag at 56 m, and the biggest caprock thickness for hydrocarbon migration in the Laizhouwan Sag at 240.7 m and in the Huanghekou Sag at 320.2 m. These observations emphasize the coupling of fault displacement and regional caprock thickness controlling the hydrocarbon transporting capacity of faults in a rift basin.