Geological characteristics and favorable area prediction of shale gas in Cambrian Wuxun Formation in the southwestern margin of Xuefeng uplift, China
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摘要: 【研究目的】 2020年，雪峰隆起西南缘下寒武统乌训组首次发现页岩气显示，但其成藏条件和勘探潜力未知。【研究方法】基于野外地质调查、钻探资料，结合有机地化分析测试等手段，对雪峰西南缘下寒武统乌训组页岩进行综合研究.【研究结果】研究表明：(1)乌训组沉积于碳酸盐岩外缓坡相，与川中龙王庙组为同时异相。(2)富有机质页岩发育在乌训组中下部，岩石组合为钙质炭质页岩夹薄层灰岩，厚度30~80m，呈北东-南西狭长分布。(3)页岩有机质含量TOC平均为1%，有机质类型为I型腐泥型，Ro平均为2.02%，热演化程度适中。(4)页岩脆性矿物含量在43%~69%，均值为59.2%，粘土矿物含量在16%~37%，均值为28.2%，硅质含量高，脆性较大，易于储层改造。(5)页岩为低孔低渗储层，有机质孔、晶间孔、层间孔、碳酸盐岩矿物溶孔为页岩气富集提供了储集空间，但至少4期构造裂缝以及构造作用造成的页岩劈理域大大改善了页岩储集能力。【结论】综合地化、储层物性及保存条件等评价指标，提出雪峰西南缘山前兴仁向斜东翼可作为页岩气勘探潜力区。Abstract: This paper is the result of shale gas geological survey engineering.[Object] In 2020, shale gas shows were first discovered in the Lower Cambrian Wuxun Formation on the southwestern margin of the Xuefeng uplift, but its accumulation conditions and exploration potential are unknown. [Methods] Based on field geological survey and drilling data, combined with organic geochemical analysis, a comprehensive study of the Lower Cambrian Wuxun Formation in the southwestern margin of Xuefeng was carried out. [Results] The results show that:(1) The shale of the Wuxun Formation was deposited in the carbonate outer ramp facies, which is different from the Longwangmiao Formation in central Sichuan at the same time. (2) The organic-rich shale is developed in the middle and lower parts of the Wuxun Formation, and the rock combination is calcareous carbonaceous shale intercalated with thin limestone, with a thickness of 30-80 m, and a long and narrow distribution in the northeast-southwest. (3) The average TOC content of shale organic matter is 1%, the organic matter type is I type sapropelic type, the average Ro is 2.02%, and the degree of thermal evolution is moderate.(4) The content of brittle minerals in shale ranges from 43% to 69%, with an average value of 59.2%, and the content of clay minerals ranges from 16% to 37%, with an average value of 28.2%.Therefore, shale has high siliceous content and high brittleness, which is easy for reservoir stimulation. (5) Shale is a low-porosity and low-permeability reservoir. Organic pores, intercrystalline pores, interlayer pores, and carbonate mineral dissolved pores provide storage space for shale gas enrichment, but at least four stages of structural fractures and The shale cleavage domain caused by tectonic action greatly improves the shale storage capacity. [Conclusion] Comprehensive evaluation indicators such as geochemistry, reservoir physical properties and preservation conditions suggest that the eastern flank of the Xingren syncline in the southwestern margin of Xuefeng can be a favorable area for shale gas exploration.
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