淮南煤田下二叠统山西组煤系页岩气储层特征及物性成因

Reservoir Characteristics and Its Physical Origin of Shale Gas in Coal Measure in the Lower Permian Shanxi Formation in Huainan Coal Field

煤系页岩气是煤系非常规气的一种,煤系泥页岩具备较好的聚集和成藏潜力,研究煤系泥页岩储层特征对于进一步确认煤系非常规气共生成藏与共采具有重要意义。以淮南煤田下二叠统山西组为例开展研究,结果表明:山西组厚约72m,泥岩约占25%,泥岩中石英矿物含量在29.2%~36.7%之间,主要为陆源碎屑外源成因。山西组有机碳含量高,TOC含量为0.37%~8.87%,有机质以Ⅱ2型干酪根为主;RO值变化范围为0.83%~1.32%,整体为中成熟阶段。泥岩压汞孔隙度为0.87%~4.29%,脉冲渗透率区间为(3.16~8.88)×10-7μm2,平均为4.82×10-7μm2,二者线性关系较好。扫描电镜下山西组泥岩微裂缝较为发育,包括石英颗粒边缘延伸微裂缝、颗粒内部平行线形裂缝和黏土矿物内部发育的弯曲状不规则裂缝,微孔隙主要包括粒间孔和粒内孔,孔隙直径为2μm^50nm。有机碳含量、热演化程度对微孔隙发育的促进作用不明显,伊/蒙混层在黏土矿物组成中占有较大比例,可能是微孔隙发育的主要贡献者。不同的沉积微相类型发育了不同的岩相类型,是研究区山西组煤系页岩气储层物性形成的沉积基础。

Shale gas in coal measures is one of the important types,and the shale in coal measures holds favorable ability of gas accumulation and enrichment potential.It has important significance of research for shale reservoir characteristics in coal measure strata to confirm the coexistence and co-exploration of unconventional gas.As a case of the Lower Permian Shanxi Formation in Huainan Coal Field,the results show that the thickness of the Shanxi Formation shale is about 72 m,and the thickness of mudstone accounts for about 25%.The quartz mineral content varies from 29.2%to 36.7%in the mudstone with terrigenous origin.Shanxi Formation contains 0.37%to 8.87% of TOC.The organic matter is moderate mature with ROof 0.83%-1.32% and dominated by typeⅡ2-kerogen.Porosity of the mudstone measured by Hg intrusion ranges from 0.87%to 4.29%,and pulse permeability ranges from 0.31×10^-3μm^2 to 0.91×10^-3μm^2 with an average of 0.54×10^-3μm^2.The linear relationship of porosity and pulse permeability is generally better.The development of micro-fracture observed in the mudstone includes micro-fracture extended at the edge of quartz grain,particle internal parallel linear fracture and bent irregular fracture of the internal clay minerals.The micro-porosity dominated by inter-granular and intra-granular pores,with pore size ranging from 2μm to 50 nm.The promoting effect of TOCand thermal evolution to micro pore is not obvious,but the mixed-layer minerals of illite and smectite is the main contributor to pores with a large proportion in the composition of clay mineral content.Different types of sedimentary micro-facies could develop different lithic facies,and it is the sedimentary basis of the physical properties of shale gas reservoir from Shanxi Formation in the study area.