Full
LI Wencai,born in 2000,a master degree candidate,is mainly engaged in oil and gas reservoir geology. E-mail: 15184493494@163.com.
Outline
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.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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