Abstract: The current study focuses on oil-gas exploration in the rhythmic layers of mudstone, argillaceous limestone and limestone in the first Member of Permian Maokou Formation in Sichuan Basin. The presence of talc with varying occurrences in these layers plays a crucial role in reconstructing the sedimentary patterns of the first Member of Maokou Formation and predicting unconventional reservoir qualities. This study systematically identifies the types of talc occurrences in the first Member of Maokou Formation using a combination of outcrop section survey, core observation, thin-section identification, scanning electron microscopy (SEM) analysis, X-ray diffraction (XRD) mineral analysis, major and trace element analysis, strontium isotope test, and carbon-oxygen isotope test. It also explores the genesis of talc with different occurrences, and establishes a sedimentary-diagenetic evolution model for the talc. The results show that the first Member of Maokou Formation is primarily composed of mudstone, argillaceous bioclastic limestone, bioclast-bearing micrite, micrite limestone, and siliceous rocks, and develops three depositional microfacies of low-energy bioclastic shoal, inter-shoal depression, and quiet-water lime-mud. There are four types of talcs with laminated, lenticular, spot- like, and bioclastic pattern. The first three types of talcs, which are formed in environments consistent with the surrounding lithology, are primarily developed in quiet-water lime-mud microfacies, inter-shoal depression microfacies, and low-energy bioclastic shoal microfacies, respectively. These talcs are interpreted to have formed through the diagenetic transformation of sepiolite. In contrast, bioclastic talc is characterized with bioclast metasomatism, suggesting its formation through later fluid metasomatism. The occurrence types of talc reflect the original depositional structure of sepiolite, leading to the establishment of a sedimentary-diagenetic transformation model from sepiolite to talc, controlled by sedimentary microfacies. Specifically, from the sedimentary stage to the early diagenetic stage A₁, the medium-to-thick-layered sepiolite formed in the quiet-water lime-mud microfacies, the thin-layered sepiolite appeared in the inter-shoal depression microfacies, and the spot-like sepiolite was deposited in the low-energy bioclastic shoal microfacies. After the early diagenetic stage A₂, sepiolite underwent further transformation into laminated, lenticular, and spot-like stevensite and disordered talc. During the middle diagenetic stage B₂ to late diagenetic stage, the stevensite and disordered talc continued to transform into various types of talc. Additionally, the metasomatism occurred in magnesium- and silica-rich fluids released during the transformation of sepiolite may result in the formation of bioclastic talc.

Key words: talc, occurrence types, genetic mechanism, first Member of Maokou Formation, Sichuan Basin

摘要： 四川盆地中二叠统茅口组一段(茅一段)泥岩-泥质灰岩-灰岩韵律层是近期油气勘探的热点层系,赋存于其中的不同产状的滑石对茅一段沉积格局的恢复和非常规储层的预测具有重要意义。结合野外剖面实测、岩心观察、岩石薄片鉴定、扫描电镜分析、全岩X射线衍射(XRD)矿物分析、主量/微量元素测定、锶同位素与碳/氧同位素测试等手段,对茅一段中滑石的赋存类型进行了系统识别,并探讨了不同产状滑石的成因,建立了不同产状滑石的沉积-成岩演化模式。研究结果表明:茅一段以泥岩、泥质生屑灰岩、含生屑泥晶灰岩、泥晶生屑灰岩和硅质岩为主,发育低能生屑滩沉积微相、滩间洼地沉积微相与静水灰泥沉积微相;滑石发育层状、透镜状、星点状和生屑状4种类型,前3种滑石的形成环境与围岩一致,分别发育在静水灰泥沉积微相、滩间洼地沉积微相和低能生屑滩沉积微相中,为海泡石成岩转化成因,而生屑状滑石具有交代特征,为后期流体交代作用形成。不同产状的滑石能够反映海泡石的原始沉积建造,由此建立了沉积微相主控下不同产状的海泡石—滑石的沉积-成岩转化模式:沉积期—中成岩A₁期,中层—厚层状海泡石形成于静水灰泥沉积微相中,薄层状海泡石出现于滩间洼地沉积微相中,而星点状海泡石沉积于低能生屑滩沉积微相中;中成岩A₂期后,不同产状的海泡石分别转化为层状、透镜状和星点状的斯蒂文石和无序滑石;中成岩B₂期—晚成岩期,继续转化为不同产状的滑石;此外,海泡石在转化过程中释放的富Mg、富Si流体发生交代作用可形成生屑状滑石。

关键词: 滑石, 赋存类型, 成因机理, 茅口组一段, 四川盆地