Abstract: The Member 1 of Middle Permian Maokou Formation represents the new series of strata for natural gas exploration in Sichuan Basin. Its complex lithological assemblages, heterogeneous rock fabrics and microscopic pore-facture structures pose challenges for accurately predicting high-quality reservoirs. Based on petrology, mineralogy and reservoir physical properties, this paper comprehensively investigates the types, controlling factors and spatial distribution of the Member 1 of Maokou Formation reservoirs in southern Sichuan Basin. The results show that the Member 1 of Maokou Formation can be divided into 8 sublayers from bottom to top, where three types of reservoir rocks are developed, including argillaceous (clay-rich)limestones, bioclastic packstones, and bioclastic wackestones, all characterized as low-porosity and low-permeability fracture-pore type tight carbonate reservoirs. Among these, argillaceous limestones feature a matrix microscopic pore-fracture system dominated by organic matrix pores, clay mineral (talc)pore-fractures, and grain-edge fractures, whose relatively higher porosity, permeability, and total organic carbon (TOC)contents are mainly controlled by the periodic enrichment of clay minerals and organic matters in high-frequency transgressive cycles. Argillaceous limestone has a source-reservoir integrated structure, stably and continuously distributed in the 1st, 2nd, 4th and 6th sublayers of Member 1 of Maokou Formation, representing the most favorable reservoirs in the Member 1 of Maokou Formation. Bioclastic packstones and bioclastic wackestones are dominated by dissolution pore-fractures, and mainly occur in the 7th and 8th sublayers. The formation of these reservoirs is controlled by penecontemporaneous dissolution in local uplift areas of the inner ramp. Although the dissolution can enhance the petrophysical properties of bioclastic packstones and bioclastic wackestones, their TOC contents are still relatively low, exhibiting a suboptimal condition for natural gas accumulation. A fracture network system was formed due to the multi-stage tectonic modifications during syndepositional to burial periods, significantly improving the accumulation/permeability capacity of tight carbonate reservoirs and providing favorable migration pathways for natural gas enrichment. For natural gas exploration in the Member 1 of Maokou Formation, the focus should be put on the area where the continuous thick argillaceous limestones (matrix pore-fracture developed zones)overlap with structural fault/fracture zones (fracture-enhanced transformation zones).

Key words: limestone-marlstone rhythmic layers, tight carbonate rocks, reservoir controlling factors, Member 1 of Maokou Formation, Sichuan Basin

摘要： 四川盆地中二叠统茅口组一段(茅一段)为天然气勘探的新层系,受其复杂的岩相组合、多变的岩石组构和微观孔缝结构影响,优质储层的精确预测面临显著挑战。基于岩石学、矿物学和储层物性等资料,系统厘定了川南地区茅一段的储层类型,并阐明了其主控因素与空间展布规律。研究结果表明,茅一段自下而上可划分为8个小层,发育含泥(黏土矿物)灰岩、生屑泥粒灰岩和生屑粒泥灰岩3类储集岩,具有低孔、低渗裂缝-孔隙型致密碳酸盐岩储层特征。含泥灰岩以发育有机质孔、黏土矿物(滑石)孔缝和粒缘缝等基质微孔缝系统为特征,其相对高的孔渗性和高总有机碳(TOC)含量主要受控于高频海侵旋回背景下黏土矿物和有机质的周期性富集;含泥灰岩呈源-储一体,稳定、连续分布于茅一段1小层、2小层、4小层和6小层,是茅一段的最有利储集岩。生屑泥粒灰岩和生屑粒泥灰岩的储集空间以溶蚀孔缝为主,主要赋存于茅一段7小层和8小层,其储集性能主要受内缓坡带局部高部位的准同生期溶蚀作用控制;溶蚀作用虽然可使得生屑泥粒灰岩和生屑粒泥灰岩的岩石物性得到提升,但其TOC含量仍较低,天然气富集条件一般。茅一段经历了同沉积期—埋藏期的多期构造叠加改造,形成了网状裂缝系统,显著改善了致密碳酸盐岩储层的储-渗性能,这为天然气的富集提供了优势输导条件。川南地区茅一段天然气勘探应重点关注厚层连续含泥灰岩(基质孔缝发育带)和构造断层/断裂带的叠合区(裂缝强化改造区)。

关键词: 灰岩—泥灰岩韵律层, 致密碳酸盐岩, 储层控制因素, 茅口组一段, 四川盆地