低可容空间浅水三角洲沉积层序及聚煤模式:以渤海湾地区下二叠统山西组为例

Sequence stratigraphy and coal accumulation of shallow water delta systems under low accommodation conditions:a case study from the Lower Permian Shanxi Formation in Bohai Bay region

为研究低可容空间浅水三角洲体系沉积层序及聚煤模式,以渤海湾地区下二叠统山西组为主要研究对象,开展层序地层研究,在层序地层格架内揭示低可容空间浅水三角洲体系聚煤模式。根据沉积相转换面以及下切谷冲刷面两类不整合面(SU)可将山西组划分为3个三级层序Sq1、Sq2及Sq3,每个三级层序代表1期三角洲。通过最大洪泛面(MFS)与最大水退面(MRS)将每个三级层序划分为LST、TST及HST 3个体系域,代表9个四级层序。根据岩性参数等值线绘制一系列特定于层序的沉积相图,在可容空间增长速率及泥炭堆积速率的控制下,三角洲平原泛滥平原及三角洲前缘泥炭沼泽为聚煤中心,位于冀中坳陷东部、黄骅坳陷中北部以及济阳坳陷部分地区。泛滥平原中形成的煤层厚度大,且聚煤中心具有随三角洲推进逐渐南移的趋势,三角洲前缘聚煤环境相对较差,形成的煤层厚度较薄,且易被分流河道砂体冲刷。渤海湾地区下二叠统浅水三角洲层序地层学及沉积学分析为聚煤模式提供了基础,该模式包括LST、TST早期、TST中期、TST晚期以及HST 5个时期的演化。煤层主要聚集在Sq1与Sq2的TST以及HST时期。这些成果对渤海湾地区煤炭资源勘探及提高钻探工程地质效果具有现实意义。

The Lower Permian Shanxi Formation in the Bohai Bay region was studied to analyze the sequence stratigraphy and coal accumulation of shallow water delta systems under low accommodation con-ditions.According to the two types of subaerial unconformities(SU)including sedimentary facies transi-tion surface and incised valley scour surface,the Shanxi Formation can be divided into three 3rd order se-quences(Sq1,Sq2 and Sq3),each of which represents one stage of delta.Each 3rd order sequence is divided into LST,TST and HST system tracts by maximum flooding surface(MFS)and maximum regres-sion surface(MRS),representing nine 4th order sequences.A series of sequence-specified sedimentary facies diagrams have been constructed based on the contours of lithological parameters.Under the controls of the accommodation space growth rate and peat accumulation rate,flood plain in delta plain and peat swamp in delta front are favorable coal accumulation centers,which are located in the eastern part of Jizhong depression,the middle northern part of Huanghua depression and part of Jiyang depression.Coal seams in the flood plain is thick,and the coal accumulation center tends to move southward with the ad-vance of the shallow water delta.The coal accumulation environment in the delta front is relatively poor,and coal seams are relatively thin,which is easy to be washed by the underwater distributary channel sand bodies.The sequence stratigraphic and sedimentological analysis of the Lower Permian coal-bearing strata of the Bohai Bay region provides a basis for a comprehensive coal accumulation model establishment,which involves a five-period evolution from the LST,early TST,middle TST,late TST to HST.The major coal seams were accumulated in the TST and HST of the sequences Sq1 and Sq2.These results are of prac-tical significance for coal resources exploration and enhance the geological effects of prospecting engineer-ing in the Bohai Bay region.