川南威荣地区龙马溪组草莓状黄铁矿形态及其古环境指示意义

Morphological characteristics of framboidal pyrite and their paleo-environmental significance in Longmaxi Formation of Weirong area, southern Sichuan Basin

赵国伟 邓模 汪远征 蒋小琼 曾华盛 张长江

ZHAO Guowei DENG Mo WANG Yuanzheng JIANG Xiaoqiong ZENG Huasheng ZHANG Changjiang

1. 中国石化 石油勘探开发研究院 无锡石油地质研究所, 江苏 无锡 214126; 2. 中国石化 油气成藏重点实验室, 江苏 无锡 214126

1. Wuxi Research Institute of Petroleum Geology, SINOPEC, Wuxi, Jiangsu 214126, China; 2. SINOPEC Key Laboratory of Petroleum Accumulation Mechanisms, Wuxi, Jiangsu 214126, China

草莓状黄铁矿的形态对页岩沉积环境和页岩气富集具有重要指示意义。以川南威荣地区WY1井龙马溪组一段页岩为研究对象，利用氩离子抛光扫描电镜观察和ImageJ图像处理等技术，获取了草莓状黄铁矿及其微晶形态与粒径特征，并结合微量元素、有机质丰度、含气性等参数，探讨了草莓状黄铁矿形态对古水体氧化还原环境和页岩气富集的表征作用。威荣地区WY1井龙马溪组一段页岩中草莓状黄铁矿平均粒径、微晶平均粒径及微晶晶型等参数在垂向上变化明显，草莓状黄铁矿平均粒径为4.02 μm，微晶平均粒径为0.45 μm，自下而上黄铁矿及其微晶粒径逐渐增大；下部微晶晶型主要为截角八面体，上部为截角八面体辅以八面体和截角四面体，自下而上微晶圆度逐渐减小。草莓状黄铁矿及其微晶形态参数特征表明，龙马溪组一段沉积水体还原性逐渐减弱，从硫化—缺氧环境转变为贫氧环境，这与微量元素所反映的氧化还原环境吻合较好。此外，有机质含量、解吸气含量与黄铁矿含量具有明显的正相关性，而与草莓状黄铁矿粒径及其微晶粒径具有明显的负相关性，指示硫化—缺氧环境有利于有机质富集和页岩气生成、储集和保存。

The morphological characteristics of framboidal pyrite have important indicative significance for the shale sedimentary environments and shale gas enrichment. Taking the shale in the first member of the Longmaxi Formation from well WY1 in the Weirong area of southern Sichuan Basin as the study subject, argon ion scanning electron microscopic analyses and ImageJ image processing were conducted to obtain the morphological characteristics and particle sizes of framboidal pyrite and its microcrystals. By analyzing parameters such as trace elements, organic matter abundance, and gas content, the study explored the indicative role of framboidal pyrite morphology in paleo-water redox environments and shale gas enrichment. The results demonstrated that parameters such as the average particle size of framboidal pyrite, the average particle size of microcrystals, and the crystalline structures showed significant variations vertically. The average particle sizes of pyrite and its microcrystals were 4.02 μm and 0.45 μm, respectively. The particle sizes of both pyrite and its microcrystal gradually increased from bottom to top. The crystalline structure of microcrystals from the lower part was mainly truncated octahedrons, and the upper part featured truncated octahedrons, supplemented by octahedrons and truncated tetrahedrons. The roundness of microcrystals gradually decreased from bottom to top. The morphological characteristics of framboidal pyrite and its microcrystals indicated that the reducibility of the sedimentary water body in the first member of the Longmaxi Formation gradually weakened, transitioning from an euxinic and anoxic environment to a suboxic environment. This aligned well with the redox environment reflected by trace elements. Furthermore, contents of organic matter and desorbed gas showed a significant positive correlation with pyrite content, but a clear negative correlation with the particle sizes of framboidal pyrite and its microcrystals. The findings show that an euxinic and anoxic environment is favorable for the enrichment of organic matter and the generation, accumulation, and preservation of shale gas.

国家自然科学基金企业创新发展联合基金集成项目(U24B6001)和中国石化科技部项目(P24146)联合资助。

https://doi.org/10.11781/sysydz2025020261