鄂尔多斯盆地北岔沟砂岩沉积特征及其对天然气成藏的控制作用*

北岔沟砂岩为华北地台二叠系山西组与太原组划分对比的主要标志层,在鄂尔多斯盆地呈大面积连片分布,是上古生界重要的天然气储集层。依据地面露头调查结果与勘探资料,结合前人有关古构造和岩相古地理研究成果,笔者对鄂尔多斯盆地北岔沟砂岩的沉积特征进行了详细分析。结果表明,受益于物源区大量高石英含量的粗碎屑物质在短时间内的集中供应,北岔沟砂岩在地层中位置稳定,以含砾粗粒石英砂岩为主,石英含量高达90%左右,砂体叠置紧密,单砂体之间很少见泥岩隔夹层。这些砂体分别隶属于冲积扇、砾质辫状河和辫状河三角洲沉积体系,在鄂尔多斯盆地北部高、南部次高、中南部低的古地理背景控制下,由盆地南、北边缘向中部依次推进,沉积中心区以北发育4个规模宏大的三角洲沉积主砂带,以南发育6个规模较小的主砂带。受各主砂带砂体规模、矿物组分含量及其成岩作用差异影响,各砂带的储集物性具有较大变化,其中砂体规模最大、石英含量最高、长石等易溶矿物在酸性地层水作用下溶蚀彻底的杭锦旗—榆林—子洲—清涧—永和—吉县主砂带,储集能力最好,最有利于天然气的大规模聚集成藏,是鄂尔多斯盆地北岔沟砂岩天然气进一步勘探开发的有利场所。

Sedimentary characteristics of Beichagou Sandstone in the Lower Permian Shanxi Formation in Ordos Basin and its effects on natural gas accumulation

Beichagou sandstone,which is the marker bed between Shanxi and Taiyuan Formations of Permian in North China Platform,is widely distributed in the Ordos Basin and acted as the significant Upper Paleozoic natural gas reservoir. Based on the outcrop investigation,exploration data,and previous studies of palaeostructure and lithofacies paleogeography,the sedimentary characteristics of Beichagou sandstone in the Ordos Basin were analyzed in detail. Due to the abundant supply of coarse clastics with high quartz content in the provenance area in a short time,the lithology of the Beichagou sandstone is mainly gravel bearing coarse-grained quartz sandstone with quartz content up to 90%. The sand bodies are closely overlapped and stable distributed,and there is rare mudstone interlayer between single sand bodies. The paleogeography is highest in the north,high in the south and low in the south central,making the alluvial fan,gravelly braided river,braided river delta plain,delta front and prodelta successively advanced from the north and south margin to the central area of the Ordos Basin. Four large scale sand belts of delta deposit developed in the north part of the deposition center,while six small scale sand belts developed in the south region. Influenced by the sandbody size,variation of mineral composition and diagenesis,the reservoir properties of each sand belt varies greatly. The largest sand body along Hangjinqi-Yulin-Zizhou-Qingjian-Yonghe-Jixian,which is characterized as the highest quartz content,and very high dissolution of feldspar and other soluble minerals under the action of acidic formation water,has the favorable reservoir properties and beneficial for the accumulation of natural gas. The Beichagou sandstone along Hangjinqi-Yulin-Zizhou-Qingjian-Yonghe-Jixian,is a favorable belt for further exploration and development of natural gas in the Ordos Basin.