内蒙古二连盆地巴彦宝力格煤田下白垩统赛汉塔拉组层序—古地理与聚煤作用*

二连盆地巴彦宝力格煤田蕴藏着丰富的煤炭和煤层气资源。利用钻孔和测井资料对赛汉塔拉组层序格架下的古地理演化特征和聚煤作用进行研究,识别出3种层序界面类型: 区域不整合面、河道下切谷冲刷面和对应整合面,将赛汉塔拉组划分为2个三级层序,厚煤层主要发育于层序Ⅰ湖侵体系域早期和晚期。采用单因素分析多因素综合作图法,恢复了层序格架下的古地理格局,古地理单元为冲积扇、扇三角洲平原和滨浅湖。有利聚煤环境为扇三角洲平原与滨浅湖过渡带、滨浅湖,具有合适的基底沉降速率并且受到较小的陆源碎屑影响。从层序Ⅰ到层序Ⅱ,煤层厚度减小,聚煤作用减弱;多层厚煤层的形成受基底沉降震荡性、周期性的影响,受可容空间增加速率与泥炭堆积速率比值不断变化的控制。层序Ⅰ发育时期,研究区中部为下一步煤炭资源和煤层气勘探的有利区。

Sequence-palaeogeography and coal accumulation of the Lower Cretaceous Saihantala Formation in the Bayanbaolige Coalfield,Erlian Basin,Inner Mongolia

The Bayanbaolige Coalfield of Erlian Basin is rich in coal and coalbed methane resources. In this paper, we studied the palaeogeographic evolution and coal accumulation in sequence stratigraphic framework of the Lower Cretaceous Saihantala Formation based on data of boreholes and logging curves. Three types of sequence boundary, including regional unconformities, basal erosional surfaces of the incised valley fills and correlative conformities, were recognized. Accordingly, the Saihantala Formation was divided into two third-order sequences, and thick coal seams were developed in the early and late transgressive system tract of Sequence Ⅰ. The palaeogeographic pattern were reconstructed based on the single factor analysis and multifactor comprehensive mapping method, and the palaeogeographic units include alluvial fan, fan delta plain and shoreline-shallow lake. The favorable coal-forming sites were in the lakeshore-shallow lake, the transitional zone between fan delta plain and lakeshore-shallow lake where moderate subsidence was maintained and there was little input of coarse-grained sediments. Seam thickness and coal accumulation decreased from Sequence Ⅰ to Sequence Ⅱ. The development of the thick coal seams of multilayers, which were affected by intermittence and periodicity of subsidence, were controlled by the continuous change of ratio between the accommodation increase rate and the peat accumulation rate. For the Sequence Ⅰ,the central study area is the favorable area for the exploration of coal and coalbed methane resources in the future.