Sedimentary environment and distribution law of Carboniferous-Permian coal-bearing series in Bohai Bay Basin

Based on more than 100 drilling wells of various depressions in Bohai Bay Basin, combined with drill cores, logging and seismic data, this paper discussed coal-bearing series sedimentary environment of Carboniferous and Permian, established sedimentary model of coal-bearing series, revealed the distribution law of residual coal measure strata.Coal-bearing series of Carboniferous and Permian in Bohai Bay Basin mainly developed in Taiyuan Formation and Shanxi Formation.Rock types are coal and rock and dark mudstone (including carbonaceous mudstone).The sedimentary environment of coal-bearing series in Taiyuan Formation is characterized by peat flat under epeiric sea barrier sedimentary system.Sedimentary peat is mainly composed of humus group, which is generally rich in sulfur and minerals.After coal formation is characterized by vitrinite group, and chitinite group is relatively abundant.Shanxi Formation is affected by the increasing supply of detrital materials and the large-scale regression, mainly coal-forming environment such as all kinds of peat flat developed from north to south over time, and is gradually replaced by peat bog of shallow water delta plain.The thickness of coal seam formed by this kind of peat swamp is large, but it is subject to the scour action of distributary channel, so the lateral thickness changes greatly, the structure is complex, the sulfur content is generally low, and the ash content is high.According to the characteristics of coal-bearing series and coal-forming environment in north China late Paleozoic, peat flats can be divided into peat flat after barrier island, tidal flat peat flat, lagoon peat flat, tidal delta peat flat and shallow water delta plain peat bog.They have different lithofacies combination and coal-forming characteristics.The variety of coal-forming model can coexist and mutual evolve at the same time.According to the study, the carboniferous Permian coal-bearing rock series in Bohai bay basin has a large residual thickness and a wide distribution, but some areas suffer from denudation due to the influence of Mesozoic and Cenozoic tectonic faults.Through the analysis of carboniferous Permian coal-bearing rock series organic matter abundance and other hydrocarbon source rock indexes in Bohai bay basin, combined with different types of logging curves, the thickness of carboniferous Permian hydrocarbon source rocks in Bohai bay basin is estimated.It is considered that the effective coal source rocks of Carboniferous and Permian in Bohai Bay Basin is thicker in southern Huanghua Depression and Suqiao-Wen'an area of Jizhong Depression, and the thickness of the effective coal source rocks is 150-200 m, the maximum thickness can be up to 250 m.The thickness of the effective coal source rocks of Linqing Depression and Dongpu Depression is generally 50-100 m.Effective coal source rocks in Jiyang Depression are scattered, coal source rocks with a large thickness are developed in each secondary sag.