【作者】 黄培培；

【导师】 黄思静；

【作者基本信息】 成都理工大学 ， 沉积学， 2010， 硕士

【摘要】 鄂尔多斯盆地上古生界天然气资源丰富,勘探开发潜力巨大,勘探前景良好,榆林地区山西组2段砂岩储层具有储集砂体厚度大、横向分布稳定、单井产能高及开发效果好等特点,是上古生界主力含气层段。山2期是鄂尔多斯盆地重要的成煤期,是重要的含煤地层,与之有关的酸性流体在砂岩的成岩演化过程中具有十分重要的作用。本文运用野外岩心观察、室内铸体薄片观察、X射线衍射、扫描电镜、化学元素分析、同位素分析等手段,对鄂尔多斯盆地榆林地区山西组2段砂岩的岩石学特征、自生矿物特征、孔隙构成与物性等进行了讨论,通过上述分析,对砂岩成岩作用类型及其对储层的影响、成岩阶段演化及次生孔隙形成机制进行了细致研究。研究表明,研究区山2段砂岩的岩石类型以石英砂岩为主,岩石中长石含量极低,成分成熟度极高,贫长石富石英是本区砂岩骨架颗粒构成的显著特征;粒度较粗,以粗砂为主,分选好,杂基含量低;砂岩中原生孔隙含量较高,主要为剩余粒间孔,次生孔隙以高岭石晶间孔和次生溶孔为主,微裂隙不发育;平均孔隙度约5.03%,绝大多数样品的渗透率小于1.0×10-3μm2,属低孔低渗储层,孔隙度与渗透率之间具有良好的正相关性,渗透率的变化依赖于孔隙的发育程度。储层砂岩在埋藏成岩过程中经历的成岩作用类型包括压实压溶作用、胶结作用、溶解作用和交代作用,依据自生矿物分布和形成顺序、混层粘土矿物转化、有机质成熟度等指标,研究区砂岩的成岩演化阶段处于中成岩阶段B期。压实压溶作用是本区最主要的破坏性成岩作用,是孔隙度丧失的主要原因。自生矿物类型包括硅质胶结物、粘土矿物(主要是高岭石)和碳酸盐矿物,在岩石中的平均含量分别为6.30%、3.72%和3.28%,极高的硅质含量、较高的高岭石含量和很低的碳酸盐含量是本区砂岩自生矿物构成的显著特点。溶解作用是本区最重要的加强性成岩作用,溶解介质主要来源于与煤系地层有关的酸性流体,被溶物主要是长石等铝硅酸盐矿物,酸性流体的溶解作用发生时间早,作用时间长,使砂岩中长石消耗殆尽,并继续消耗部分不稳定岩屑,造成了砂岩中极低的长石含量和极高的硅质和高岭石含量,并形成了大量的次生孔隙,大大改善了储层的物性。铸体薄片结合扫描电镜观察发现,硅质胶结物主要以石英次生加大形式产出,可形成于较早的成岩阶段;高岭石的赋存状态主要是孔隙充填,多具有较好的晶间孔隙,常与硅质胶结物伴生;方解石和白云石主要以粒状结构充填孔隙,结构观察发现白云石的沉淀时间较早,而方解石往往沉淀于高岭石和石英次生加大之后,多是成岩晚期的产物,对碳酸盐矿物的碳、氧同位素分析也支持了上述观察结果。煤系地层酸性水对长石、部分不稳定岩屑等铝硅酸盐矿物的溶解为上述自生矿物提供了最主要的物质来源。较早成岩阶段形成的硅质、高岭石和白云石胶结作用增强了岩石的机械强度和抗压实能力,对压实作用起到了一定的抑制作用,在一定程度上保护了原生孔隙,是研究区砂岩中最重要的保持性成岩作用,这也是山2段砂岩在深埋藏条件下仍能保持较高含量原生孔隙的主要原因。更多还原

【Abstract】 Upper Paleozoic sandstones contain abundant natural gas resource, among the formations, Member 2 of Shanxi Formation is the most important gas-bearing strata and has indicated a promising prospect for exploration. As an important coal-bearing stratum, Member 2 of Shanxi Formation is controlled by acid pore fliud during diagenesis evolution. Petrology, authigenic minerals, pore types and physical properties of reservoir are generally inbolved in the paper based on core description, thin sections under microcope, X-Ray diffraction (XRD), scanning electron microsope (SEM), chemical element analysis and isotopic analysis, and on the basis of the above research, diagenesis and formation mechanism of secondary pore are further discussed.Quartzarenite is the main rock type of sandstones in the study area which has very high composition maturity. It is framework end-member compositon of sandstone that is characterized by extremely deficient feldspar and quite rich quartz. Sandstones are coarse-grained, well-sorted and lower matrix. The primary pore, dominantly residual intergranular pore, holds an important position in the pore type. Kaolinite intracrystalline micropores and secondary dissolution pores take a dominant place in secondary pore types. The average porosity is 5.03%, and permeability of most samples is below 1.0×10-3μm2. There is favorable positive correlation between porosity and permeability, while the changes of the latter are controlled by the development of the former.Diagenesis types include compaction, cemetion, dissolution and metasomatism. According to the diatribution and paragenesis of authigenic minerals, clay minerals transformation and organic matter maturity, the reservoir sandstone is now in middle diagenetic phase B. Compaction is the most important destructive process, which is mainly responsible for porosity loss. Authigenic minerals in sandstones involve siliceous cement, clay minerals (mainly kaolinite) and carbonate minerals, whose contents in rock are respectively 6.30%, 3.72% and 3.28%. As the formost constructive process, dissolution forms abundant secondary porosity and thus improves physical properties of reservoir. Dissolution medium roots in acid pore fluid related with coal-bearing stratum, and dissoluble material are primarily feldspar and metastable rock fragments and other aluminosilicate.Under thin section and SEM, siliceous cement often behaves as overgrowth, which can form during earlier diagenetic stage; as pore-filling cement, kaolinite usually has preferable ntracrystalline micropores, which is associated with quartz cements; calcite and dolomite fill the pores as a granular texture, and the latter is formed in earlier stage, while the former isprecipitated in the late stage, which is also favored by the analysis ofδ18O andδ13C of carbonate cements. Mateiral source of all these cements is provided by dissolution of feldspar via organic acid fluid. Siliceous cement, kaolinite and dolomite formed in earlier stage can enhance mechanical strength and resistance to compaction, and thus compaction is restrained in some degree and primary pore is preserved, which is also the chief reason for abnormmaly high primary pore in deep buried Shan 2 sandstone.更多还原