Evolution of channel patterns and its geological significance for oil and gas exploration in the Lower Member of Minghuazhen Formation in Laibei low uplift,Bohai Bay Basin

Multiple types of in-channel deposits with obvious characteristics are developed in the Lower Member of Minghuazhen Formation in Laibei low uplift, Bohai Bay Basin. Based on the comprehensive analysis of 3D seismic data, cores, logging, ancient organisms and clay minerals, this paper quantitatively studies the features of channels, and then discusses the evolution rule, controlling factors and their significance for oil and gas accumulation. Based on analyzing the plane shape, development scale, hydrodynamic conditions and sedimentary environment, the channels in the Lower Member of Minghuazhen Formation can be divided into five types:large high-curvature meandering channel, small low-curvature meandering channel, micro meandering channel, braided channel and underwater distributary channel. The evolution of channel patterns is mainly driven by tectonic subsidence rate and paleoclimate change. As the tectonic subsidence accelerated, the type of channel was transformed from large meandering river to small meandering river and braided river successively. The East Asian monsoon enhancement event occurred around 8.5 Ma ago resulted in increases in both rainfall and source supply. With the rapid lake transgression, the study area was transformed from the sedimentary environment of braided river to shallow water delta; taking underwater distributary channels as the skeleton and sheet sand body as the inter-channel sediments, the large continuous sandbody was developed in this area. Guided by this understanding, the Neogene lithologic traps of large continuous sandbody were discovered for the first time in Bohai oilfield, and KenLi6-1 oilfield with hundred million tons was successfully discovered, initiating a new model of "lithologic exploration for large continuous sand body".