鄂尔多斯盆地上三叠统瓦窑堡组层序-古地理及聚煤规律

摘要

依据钻孔岩芯及测井曲线资料对鄂尔多斯盆地东部晚三叠世瓦窑堡组沉积相、聚煤模式、层序地层格架、岩相古地理进行研究,提出该区聚煤规律.瓦窑堡组依据岩性共分为五段,其中底部的第1段和顶部的第5段以泥岩、粉砂岩为主,第2段、第3段、第4段下部以砂岩为主,上部以泥岩、粉砂岩为主.根据瓦窑堡组顶部区域不整合面、第1段、第3段以及第4段底部河流下切谷冲刷面、瓦窑堡组底部与其下伏地层间地层颜色及岩性突变面等层序界面,将瓦窑堡组划分为3个三级层序,其中层序Ⅰ对应瓦窑堡组一段和二段,层序Ⅱ对应瓦窑堡组三段,层序Ⅲ对应瓦窑堡组四段和五段.通过统计各个三级层序内的岩性数据,绘制地层厚度、砂泥比、煤层厚度等值线图,以三级层序为作图单元,恢复各层序的古地理格局,主要的古地理单元为河流、三角洲和湖泊,物源位于研究区东北部.层序Ⅲ的聚煤作用明显优于层序Ⅰ和层序Ⅱ,最有利于成煤环境为三角洲平原分流间湾,聚煤中心大致位于石湾镇—玉家湾镇—安塞县一带.%The sedimentary facies,sequence stratigraphy,lithofacies paleogeography and coal accumulation of the Late Triassic Wayaopu Formation in Ordos Basin were analyzed based on data from borehole cores and logging curves.The Wayaopu Formation was subdivided into five members based on the lithology,of which the first (bottom) and the fifth (top) members consist of mudstones and siltstones,the second,the third and the fourth members are mainly composed of sandstones in the lower part,and of mudstones and siltstones in the upper part.The sequence boundaries were identified,including the regional unconformity at the top of the Wayaopu Formations,the basal erosional surfaces of the fluvial incised valley fill sand bodies at the bottom of the first,third and fourth members,and abrupt changes in color and lithologies at the boundary between the Wayaopu Formation and its underlying formations.According to these sequence boundaries,the Wayaopu Formation was subdivided into three third-order sequences,namely sequence Ⅰ,sequence Ⅱ and sequence Ⅲ,which were corresponding to the intervals from member l to member 2,member 3,and member 4 to member 5,respectively.The lithological data of each third-order sequences were statistically counted,and the contours of a variety of these lithological parameters were drawn,including strata thickness,thickness ratio of sandstone and mudstone,and the thickness of coal seam.According to these contours,the lithofacies paleogeography maps of each third-order sequence were reconstructed.The major paleogeographical units were the fluvial facies,delta facies,and lacustrine facies,with a northeastern provenance.The coal accumulation were favored in sequence Ⅲ than sequence Ⅰ and sequence Ⅱ.The most favorable coal-forming environment was the inter-distributary bay of the delta plain,and the coal-accumulating center was located in the Shiwanzhen-Yujiawanzhen-Ansaixian area.