川东地区中二叠统茅口组沉积微相与环境演变

Sedimentary microfacies and environmental evolution of the Middle Permian Maokou Formation in the eastern Sichuan Basin： A case study of the Yangjiao section in Wulong District， Chongqing， China

张赫驿 杨帅 张玺华 彭瀚霖 李乾 陈聪 高兆龙 陈安清

Heyi ZHANG Shuai YANG Xihua ZHANG Hanlin PENG Qian LI Cong CHEN Zhaolong GAO Anqing CHEN

近年来，川东地区中二叠统茅口组持续发现工业气流，研究该区域茅口组的沉积微相与环境演化对油气勘探具有重要意义。以重庆市武隆区羊角剖面为研究对象，在野外沉积特征观察的基础上，通过岩石薄片鉴定及地球化学分析，研究了沉积微相及该地区茅口组的沉积环境演化特征。研究表明羊角剖面茅口组主要发育5种沉积微相。茅口组下部主要为含生物碎屑泥晶灰岩和泥晶灰岩沉积微相，是水体能量低的较深水环境下的沉积产物。茅口组中部以亮晶生物碎屑灰岩、泥晶生物碎屑灰岩、生物碎屑泥晶灰岩和生物碎屑泥晶灰岩为主，亮晶生物碎屑灰岩的出现表明高能的沉积环境，水体较浅。茅口组上部以泥晶生物碎屑灰岩、生物碎屑泥晶灰岩和含生物碎屑微晶灰岩为主，此时水体再次变深。茅口组下部δC值较低，平均3.00 ‰，中-上部δC值主要在4.00 ‰以上。显示茅口组一段为缺氧环境，随后氧化性增强转换成贫氧环境，茅口组沉积晚期再次回到缺氧环境。川东地区茅口组沉积微相特征及海平面变化受冰期影响，全球海平面下降是沉积环境转变的主控因素，浅水沉积环境是高能颗粒滩发育的有利地质条件，高能滩发育是川东地区茅口组中-上部优质储层形成的主要原因。

In recent years， industrial gas flow has been obtained in the Permian Maokou Formation in the eastern Sichuan Basin， underscoring the great significance of exploring the sedimentary microfacies and environmental evolution of the formation for oil and gas exploration. This study focuses on the Yangjiao section in Wulong District， Chongqing， China. Based on the field observations of sedimentary characteristics， coupled with thin section observation and geochemical analysis， we delve into the sedimentary microfacies and sedimentary environmental evolution characteristics of the Maokou Formation. The results reveal the predominant distribution of five sedimentary microfacies. Bioclast-bearing micrites and micrites， deposits in a deep-water， low-energy environment， prevail in the lower formation. The middle formation exhibits the primary distribution of sparry bioclastic limestones， micritic bioclastic limestones， bioclastic micrites， and bioclastic micrites， and the presence of sparry bioclastic limestones suggests a shallow-water， high-energy sedimentary environment. The upper formation predominantly contains micritic bioclastic limestones， bioclastic micrites， and bioclast-bearing micrites， and the water becomes deeper again. Geochemical analysis reveals that the lower Maokou Formation exhibits low δC values with an average of 3.00 ‰， while its middle and upper portions show δC values mostly exceeding 4.00 ‰. These findings suggest that the formation’s first member was deposited in an anoxic environment， which converted into a dysoxic environment with an increase in oxidation and then recovered to an anoxic environment in the late depositional stage. The sedimentary microfacies characteristics and the sea-level changes in the Maokou Formation in the eastern Sichuan Basin were affected by glacial periods， with the global sea-level drop serving as the dominant factor in the sedimentary environment transitions. The shallow-water sedimentary environment created favorable geological conditions for the emergence of high-energy grainstone shoals， which is， in turn， the main cause of the formation of high-quality reservoirs in the middle and upper Maokou Formation in the basin.

10.11743/ogg20240211