Regional Tectonic Controls on Reservoir and Seal Quality of Mio-Pliocene Fluvial-Channel Sandstones, Bohai Basin, Eastern China

Castellanos, Hugo A.¹, Paul Mann² (1) The University of Texas at Austin, Austin, TX (2) Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

Subsidence analysis of 10 wells from the 200,000 km2 Bohai basin, eastern China; reveals an acceleration of subsidence from rates of 13 m/my during the Miocene to 53 m/my during the Pliocene. The zone of rapid subsidence occurs in a 200-km-wide belt flanking the presently active, right-lateral Tan-Lu fault zone that traverses the entire Bohai basin in a NNE direction. The mechanism for rapid subsidence flanking the Tan-Lu fault can be related to displacements along east-west-striking, secondary normal faults. We describe the stratigraphic effects of this tectonic event on non-marine, fluvial sedimentation using an integrated well log and seismic data set. The Miocene and Pliocene fluvial sag deposits of the western offshore portion of the Bohai basin have been subdivided into ten regional correlatable accommodation/supply cycles using well logs and seismic data. Fluvial-channel sandstones of the Guantao Formation older than 5.1 my laterally and/or vertically contact one another or are separated by a few feet of floodplain facies suggesting deposition under lower accommodation/supply conditions. Erosion usually preceded channel-fill deposition therefore only the lowermost elements of channel successions were typically preserved resulting in a high sand to mud ratio. Fluvial-channel sandstones of the Pliocene Minghuazhen Formation younger than 5.1 my are isolated and encased within floodplain facies and show well developed splays suggesting deposition under higher accommodation/supply conditions. Pre-5.1 my sandstones form better oil reservoirs than the younger unit due to their greater degree of continuity with adjacent sandstone bodies and the higher degree of sorting of the fluvial sand itself.