The northern Guizhou area, located near the southwestern margin of the Yangtze Block, is a promising area for shale gas exploration and development. The Lower Silurian Shiniulan Formation as a new discovery stratum of natural gas marks an exciting breakthrough in natural gas exploration in northern Guizhou area. Based on several field investigations and samples analyses, the lithology and fracture characteristics were systematically analyzed in the lower Shiniulan Formation, and the reservoir specificity and its influence on natural gas accumulation were determined. The characteristics of the relatively fractures and lithology assemblages were identified as key factors controlling the natural gas accumulation. The lower Shiniulan Formation is deposited as calcareous shale and marlstone with frequent centimeter-scale interlayers. This is reflective of a shallow sea shelf strata with decreasing sedimentary rhythm and gradual weakening of sedimentary changes and developed calcareous shale and marlstone with frequent centimeter scale interlayer changes. The gas reservoir is dominated by calcareous mudstone, controlled by the interbedded rock association (calcareous mudstone and limestone), characterized by the raw-storage and the accumulation-reservoir interbedded system. The reservoir is located in the central part of the syncline and is characterized by strong sealing of the stratum, large proportion of free gas, and high abnormal pressure. The Lower Shiniulan Formation is formed between the shale layer with horizontal fractures and dense limestone with underdeveloped fractures. Among them, the shale section generally develops diagenetic shrinkage fractures, which provide good storage space for natural gas and act as the main body of natural gas. The pore sizes in limestone (2.8 nm) are significantly smaller than those in mudstone (7.5 nm), which results in a good capping and preservation of shale gas. This paper reports on results that are of significance for supplementing the theory of unconventional natural gas accumulation and guiding shale gas exploration in similar areas.

中文翻译：

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黔北下志留统石牛栏组互层岩组合及裂缝特征对天然气成藏的影响

贵州北部地区位于扬子区块西南边缘附近，是页岩气勘探和开发的有前途的地区。下志留统石牛栏组作为天然气新发现地层，标志着黔北地区天然气勘探取得了令人振奋的突破。在多次野外调查和样品分析的基础上，对新牛栏组下段的岩性和裂缝特征进行了系统分析，确定了储层特征及其对天然气成藏的影响。相对裂缝和岩性组合的特征被确定为控制天然气成藏的关键因素。石牛栏组下部沉积为钙质页岩和泥灰岩，常有厘米级夹层。这反映了浅海陆架地层沉积节律减弱，沉积变化逐渐减弱，钙质页岩和泥灰岩发育，厘米级层间变化频繁。气藏以钙质泥岩为主，受钙质泥岩和灰岩互层控制，具有生储、成藏互层系统特征。储层位于向斜中部，具有地层封闭性强、游离气比例大、异常压力高的特点。下石牛栏组形成于水平裂缝页岩层与裂缝欠发育的致密灰岩层之间。其中，页岩段普遍发育成岩收缩裂缝，为天然气提供良好的储存空间，是天然气的主体。石灰岩孔径（2.8 nm）明显小于泥岩孔径（7.5 nm），页岩气封盖和保存良好。本文的研究成果对补充非常规天然气成藏理论和指导类似地区页岩气勘探具有重要意义。