Distribution Characteristics and Exploration Practice of Mixed Siliciclastic-Carbonate Sediments in Bohai Sea Area

Mixed siliciclastic-carbonate sediments are widely developed in Bohai Sea area. With successive discoveries of mixed reservoir in oil exploration recently, the distribution characteristics of mixed sediments need to be addressed. Based on abundant geological data and comprehensive analyses, the time and spatial distribution characteristics of Palaeogene sediments are figured out in this study, promoting geological cognition about mixed sedimentation. Three types of E₂s_1-2 mixed rock in Bohai Sea area are identified as siliciclastic-dominated mixed rock, biological-carbonate-dominated mixed rock and chemical-carbonate-dominated mixed rock. Controlled by arid climate and saline water environment, the mixed sedimentation mainly develops in the first to second members of Shahejie Formation (E₂s_1-2) and is spatially distributed around uplift inside the basin, being far away from outside regional provenance. The favorable background of underwater positive paleotopography includes low uplift, pitching end of uplift, near-shore uplift and offshore uplift. According to development position, paleotopography setting and relationship with siliciclastic supply, the genetic pattern of mixed sedimentation could be grouped into four categories:(1) near-shore mixed fan delta; (2) near-shore mixed beach bar between fan deltas; (3) mixed beach bar on near-shore uplift; and (4) mixed beach bar on offshore uplift. Synthetic evaluation indicates that the near-shore mixed fan delta, near-shore mixed biological beach bar between fan deltas, mixed siliciclastic beach bar on near-shore uplift and mixed biological beach bar on offshore buried hill are specific favorable types of exploration since the mixed reservoir possesses large thickness and scale, coarse grains, high content of carbonate particles, less matrix and dolomite, which results in good physical properties and significant productivities. These geological cognitions have led to several discoveries of mixed reservoirs and reveal great exploration potential of mixed sediments field. The above theoretical understanding and exploration practices obtained are of great significance for promoting deep exploration in Bohai Sea area.