Sedimentary Environment And Source Rock Characteristics Of The Late Paleozoic Coal-bearing Strata In The Bohai Bay Basin Region

With the continuous improvement of oil and gas exploration in the eastern part of China,there are fewer and fewer geological reserves in the middle and shallow parts of the Bohai Bay Basin.Therefore the deep strata have gradually become exploration hot spots.In order to insight into the sedimentary environment and hydrocarbon source rock characteristics of the Late Paleozoic coal-bearing strata,Bohai Bay Basin is taken as the main research objects in this paper,with regional geology,field profile,core,logging,other data and previous research results to research the stratigraphic characteristics,types of sedimentary facies,types of source rocks and sedimentary environment of source rocks in Benxi,Taiyuan and Shanxi Formations of the Bohai Bay Basin.Combined with the investigation and observation description of typical field profiles around the Bohai Bay Basin,and the observation of downhole drilling cores and the analysis of logging data,Benxi Formation,Taiyuan Formation and Shanxi Formation are a set of coal-bearing formation deposits dominated by clastic rocks.The depositional period of Benxi Formation and Taiyuan Formation is characterized by tidal flat facies,lagoon facies,barrier island facies and platform facies.The depositional period of Shanxi Formation is characterized by the development of shallow water delta deposits,covered by delta plain subfacies and delta front subfacies.Two main types of coal measures hydrocarbon source rocks developed in the Bohai Bay Basin including coal and dark mudstone.Taiyuan Formation and Shanxi Formation are the most developed.The thick areas of residual coal measures hydrocarbon source rocks are mainly Huanghua Depression,Jiyang Depression and Jizhong Depression.Coal seams are mainly formed during the transition period from sea level decline to rise.And dark mudstones are mainly formed during the period of continuous sea level rise.The coal seams in the eastern part of the basin are thin in thickness and large in quantity,while in the western part,the coal seams are small in number,but the coal seams in the eastern part are large in thickness and good in lateral continuity.The total thickness of dark mudstone in the western part of the basin is smaller than that in the eastern part.But the thickness of single layer of dark mudstone in the eastern part of the basin is not large due to small-scale transgression.According to the association of source rocks and adjacent upper and lower strata,three types of sedimentary environments of source rocks are summarized including peat flat,lagoon and peat marsh.Peat flat is dominated by coal seam development.It can be subdivided into tidal peat flat,peat flat behind the island and peat flat behind the lagoon.Lagoon is mainly composed of thick dark mudstone.Peat marsh is dominated by coal seam.It can be subdivided into peat flat of delta plain subfacies and peat flat of delta front subfacies.