Abstract:

Authigenic kaolinite is developed in the tight sandstone reservoirs at the bottom of Xu 4 Member in the Qiongxi-Baimamiao area of southwestern Sichuan Basin. To investigate its formation and preservation mechanisms and their relationship with high-quality reservoir distribution,we conducted a comprehensive study integrating petrophysical analysis,rock thin sections,scanning electron microscopy(SEM),fluid inclusion analysis,cathodoluminescence(CL),and in-situ micro-scale carbon-oxygen isotope analysis. This research focuses on analyzing the microcharacteristics,formation stages,formation and preservation mechanisms of authigenic kaolinite,and its impact on reservoir quality of the Xu 4 Member tight sandstones. The study reveals that the tight sandstone reservoirs of Xu 4 Member are predominantly composed of lithic sandstones,and the reservoir space is dominated by intragranular dissolution pores,with lithic intragranular dissolution pores being the most prevalent type. Authigenic kaolinite in the reservoirs predominantly exhibits “worm”and “accordion”shapes,filling intergranular pores and intragranular dissolution pores. The reservoir contains two distinct stages of authigenic kaolinite formation,both derived from the dissolution of aluminosilicate lithics and feldspars within the reservoir. The early-stage dissolution was facilitated by humic acids sourced from adjacent coal-bearing strata,while the late-stage dissolution resulted from organic acids generated through decarboxylation of mature organic matter in mudstones. The potassium feldspar content in the sandstone reservoir of the Xu 4 Member is relatively low,providing only a minimal amount of K⁺. When late-stage organic acids are charged,the formation temperature is relatively low. As diagenesis progresses,the organic acids are gradually consumed. Meanwhile,a significant amount of Fe²⁺ and Mg²⁺ from mudstone participate in the precipitation of ankerite and are thus depleted. Consequently,the concentrations of K⁺,Fe²⁺,and Mg²⁺ in the diagenetic fluids are insufficient to facilitate large-scale illitization and chloritization of authigenic kaolinite. The formation temperature and the organic acid concentration can not simultaneously meet the conditions necessary for the transformation of authigenic kaolinite to dickite. As a result,authigenic kaolinite has not been able to convert in significant quantities into illite,chlorite,and dickite,and has been preserved to this day. Therefore,sandstones formed under high-energy hydrodynamic conditions with coarser grain sizes and better sorting exhibit stronger dissolution,resulting in developed authigenic kaolinite and superior reservoir properties. Research indicates that the unique preservation mechanism of authigenic kaolinite in Xu 4 Member has partly maintained reservoir porosity and permeability. Moreover,authigenic kaolinite content can reflect the intensity of dissolution,further indicating the quality of reservoir properties. Consequently,kaolinite content can be used as a reliable marker for identifying high-quality tight sandstone reservoirs in Xu 4 Member of the Qiongxi-Baimamiao area,southwestern Sichuan Basin.

Key words: Xu 4 Member, Xujiahe Formation, tight sandstone, authigenic kaolinite, preservation mechanism, Sichuan Basin, Qiongxi-Baimamiao area

摘要：

川西南邛西—白马庙地区须四段底部致密砂岩储集层自生高岭石发育,为探究其形成与保存机制及与优质储集层分布的关系,综合运用物性分析、铸体薄片、扫描电镜、流体包裹体、阴极发光、原位微区碳氧同位素等资料,对研究区须四段致密砂岩储集层自生高岭石微观特征、期次、形成与保存机制及其对储集层物性的影响进行分析讨论。研究发现,须四段致密砂岩储集层岩性为岩屑砂岩,储集空间以粒内溶孔为主,且主要发育岩屑粒内溶孔。储集层中自生高岭石多呈“蠕虫状”、“手风琴状”充填粒间孔和粒内溶孔; 储集层中发育早、晚2期自生高岭石,且均来源于储集层内含铝硅酸盐岩屑和长石的溶蚀,其中早期溶蚀流体为邻近煤系地层产生的腐殖酸,晚期溶蚀流体来源于泥岩中有机质成熟脱羧产生的有机酸。须四段砂岩储集层钾长石含量较少,仅能提供极少量K⁺,晚期有机酸充注时地层温度较低,而随着成岩作用的进行,有机酸逐渐被消耗,同时来自泥岩中的Fe²⁺、Mg²⁺大量参与铁白云石沉淀被消耗,因而成岩流体中的K⁺、Fe²⁺和Mg²⁺无法达到自生高岭石大规模伊利石化和绿泥石化所需的含量,地层温度和有机酸浓度无法同时达到自生高岭石地开石化所需的条件,导致自生高岭石无法大量向伊利石、绿泥石和地开石转化并得以保存至今。因此,在强水动力条件下形成的粒度较粗、分选较好的砂岩溶蚀作用较强,储集层内自生高岭石发育且物性较好。研究表明: 研究区须四段自生高岭石独特的保存机制,在一定程度上保持了储集层的孔隙度与渗透率,并使得其含量能够反映溶蚀作用的强弱,进而指示储集层物性的好坏,可作为川西南邛西—白马庙地区须四段致密砂岩优质储集层发育的标志。

关键词: 须四段, 须家河组, 致密砂岩, 自生高岭石, 保存机制, 四川盆地, 邛西—白马庙地区

CLC Number: 

• P618.13