鄂西恩施地区中二叠统孤峰组黑色页岩古环境恢复与有机质富集因素

Paleoenvironmental Reconstruction and Organic Matter Enrichment of Middle Permian Black Shale:Insights From the Gufeng Formation,Enshi Area,Western Hubei

鄂西恩施地区中二叠统孤峰组发育了一套以盆地相为主的黑色岩系,是鄂西恩施地区页岩气勘探开发的新层系,但目前对该套黑色岩系的沉积古环境及有机质富集机制的认识尚且不足。本文以鄂西恩施地区两条孤峰组剖面(田凤坪和渔塘坝)为研究对象,开展了薄片鉴定、总有机碳(TOC)测定、全岩矿物组分分析、元素地球化学分析等测试,旨在对鄂西恩施地区孤峰组页岩沉积期古环境及有机质富集主控因素进行研究。结果表明:研究区孤峰组页岩主要为硅质页岩相,具有高TOC含量(5.79%~41.59%),矿物组成以石英(平均值为81.53%)和黏土矿物(平均值为12.63%)为主,还含有少量的长石、黄铁矿及碳酸盐矿物;化学蚀变指数(CIA)表明孤峰组页岩沉积期表现为炎热潮湿气候特征;V/(V+Ni)、U/Th等参数表明沉积期底层水体为缺氧-硫化环境;生物硅(Si_(bio))、Cu/Al及Ni/Al等指标揭示了沉积期具有较高的初级生产力水平;TOC与U/Th、Al、Ti/Al的弱相关性表明底水缺氧硫化环境和陆源碎屑对孤峰组黑色页岩有机质富集影响较小,而与Ni/Al的正相关性则表明初级生产力水平是影响孤峰组黑色页岩有机质富集的主要因素。孤峰组页岩沉积环境表现为深水缺氧、气候炎热潮湿、初级生产力水平较高,从而建立了一个受氧化还原条件、古生产力以及热湿型气候影响的深水台内盆地沉积环境模式。

A series of black rocks dominated by basin facies developed in the Middle Permian Gufeng Formation in the Enshi area,Western Hubei Province.This formation was recently identified for shale gas development and exploration;however,little is known about the sedimentary paleoenvironment and the mechanisms of orga-nic matter enrichment in the black rock series.In this study,we focus primarily on two sections of the Gufeng Formation,Tianfengping and Yutangba,located in the Enshi area,Western Hubei.Our aim is to elucidate the paleoenvironment during the sedimentary phase of the Gufeng Formation shale and to identify the key factors governing organic matter accumulation in this area.A comprehensive suite of analytical techniques was used in this study,including petrography,mineralogy,total organic carbon(TOC)quantification,and elemental geochemistry.The results reveal that the Gufeng Formation shale predominantly consists of siliceous shale facies,with a notable TOC content ranging from 5.8%to 41.6%.Quartz(81.5%on average)and clay minerals(12.6%on average)constitute the bulk of the mineral assemblages,with minor contributions from feldspar,pyrite,and carbonate minerals.The Chemical Alteration Index(CIA)suggests that the sedimentary environment of the Gufeng Formation shale was primarily characterized by hot and humid climate.The V/(V+Ni)and U/Th ratios indicate anoxic to sulfidic conditions in the bottom waters.The biogenic silicon(Si_(bio)),Cu/Al,Ni/Al values indicate higher primary productivity during the depositional phase.The weak correlation between TOC and U/Th,Al,Ti/Al suggests that bottom water anoxic sulfidic conditions and terrigenous detrital input have a limited influence on organic matter enrichment in black shales of the Gufeng Formation.Conversely,the positive correlation with Ni/Al suggests the primary productivity is a pivotal factor driving organic matter enrichment.The sedimentary environment of the Gufeng Formation shale is characterized by deep-water anoxia,a hot and humid climate,and high primary productivity.We therefore established a depositional environmental model for deep-water intraplatform basins,constrained by redox conditions,paleo-productivity,and a hot-humid paleoclimate.