Organic geochemistry, sedimentary environment, and organic matter enrichment of limestone-marlstone rhythms in the middle Permian northern Sichuan Basin, China
Organic geochemistry, sedimentary environment, and organic matter enrichment of limestone-marlstone rhythms in the middle Permian northern Sichuan Basin, China
复制标题
川北中二叠统石灰岩-泥灰岩韵律有机地球化学、沉积环境及有机质富集
DOI：
10.1016/j.marpetgeo.2021.105306
复制
发表时间：
2021
影响因子：
4.2
通讯作者：
You Jie
中科院分区：
文献类型：
--
作者：
Pang Qian;Hu Guang;Zhang Xihua;Chen Gong;Gao Zhaolong;Shan Shujiao;Chen Yangui;Hu Chaowei;You Jie
四川盆地中二叠统茅口组广泛发育灰岩-泥灰岩韵律，有机质含量高，是潜在的生油岩和重要的页岩气靶区。针对四川盆地长江沟段茅口组下部有机质热成熟度中等的特点，选择了其下部的LM-MR韵律进行研究。从沉积学、有机岩石学和地球化学学的角度对这些韵律的有机地球化学特征、形成过程、有机质富集机制和含油气性进行了一系列研究。根据有机岩石学结果，泥灰岩中的有机质主要由蓝藻、低规则甾烷/17α-Hopanes和高中链支链单甲基烷烃(MMA)贡献。相反，石灰岩中的有机质被认为主要来自底栖藻类。泥灰岩样品比灰岩样品具有更多的负碳、氧同位素和较高的Fe/Mn比值，表明沉积环境具有较浅的水深和较强的沉积约束。此外，泥灰岩样品中伽马蜡烷/C30αβ正庚烷的比值低于石灰岩样品，表明泥灰岩沉积过程中的水盐度较低。此外，泥灰岩样品的TOC含量高于石灰岩样品，这归因于较少的碳酸盐稀释、亚缺氧环境和泥灰岩沉积过程中生产率的提高，这些最初都是由高频的相对海平面变化控制的。生烃潜力分析表明，研究区的LM-MR韵律具有较高的TOC含量、II类有机质和中等的热成熟度。
Limestone-marlstone (LM-MR) rhythms widely develop in the middle Permian Maokou Formation of Sichuan basin, and they are regarded as potential source rocks and important shale gas targets due to their high organic matter contents. In this study, LM-MR rhythms in the lower part of the Maokou Formation in the Changjianggou section of the Sichuan basin are selected for research because of their moderate thermal maturity of organic matter. A series of studies are implemented to investigate the organic geochemical characteristics, formation processes, organic matter enrichment mechanism, and hydrocarbon potentials of these LM-MR rhythms, from perspectives of sedimentology, organic petrology, and geochemistry. Organic matter in marlstones is suggested to be mainly contributed by cyanobacteria according to organic petrographic results, low regular steranes/17α-hopanes, and high mid-chain branched monomethyl alkanes (MMAs). In contrast, organic matter in limestones is suggested to mainly result from benthic algae. Marlstone samples have more negative carbon and oxygen isotopes and higher Fe/Mn ratios than the limestone samples, suggesting sedimentary environments with shallower water depth and stronger restrictions during marlstone deposition. Moreover, marlstone samples have lower ratios of gammacerane/C30αβ hopane than limestone samples, suggesting lower water salinity during marlstone deposition. Furthermore, marlstone samples have higher TOC contents than limestone samples, and this is attributed to the less carbonate dilution, the suboxic-anoxic environment, and the increased productivity during marlstone deposition, all of which are initially controlled by the high-frequency relative sea-level change. The hydrocarbon potential analysis suggests that LM-MR rhythms in the study area are with high TOC contents, Type II organic matter, and moderate thermal maturity.