贵州岑巩地区牛蹄塘组地层黄铁矿特征及其对页岩气的指示意义

Pyrite characteristics and shale gas significance of the Niutitang Formation in Cengong area,Guizhou Province

黄铁矿特征研究可为页岩沉积环境恢复与页岩气富集保存规律预测提供依据。以贵州岑巩地区岑页1井牛蹄塘组为研究对象,基于岩心、薄片、扫描电镜及微量元素、硫同位素地球化学,确定牛蹄塘组页岩沉积环境及页岩气富集地质条件。研究表明,牛蹄塘组页岩黄铁矿发育为草莓状、自形-半自形及他形(主要为胶状他形)三种形态,不同形态的黄铁矿形成于不同沉积环境,草莓状黄铁矿形成于缺氧还原环境,自形-半自形黄铁矿形成于贫氧环境,胶状他形黄铁矿指示沉积过程有热液活动的参与。测试井牛蹄塘组整体处于较开放的沉积水体,上段为含氧-贫氧环境,硫同位素整体变轻且发生负漂;中段为较缺氧环境,硫同位素跨度大且发生正漂;下段为还原性更强的沉积环境,硫同位素小幅度负漂。黄铁矿含量与有机质富集密切相关,与黄铁矿有关的孔隙为页岩气吸附、保存及运移提供载体,且草莓状黄铁矿指示的缺氧硫化环境更利于有机质保存。研究工作系统梳理了黄铁矿对牛蹄塘组页岩沉积环境及储层页岩气富集的指示作用,为页岩气勘探提供指示。

The study of pyrite characteristics in shale gas reservoirs can provide a basis for the restoration of shale sedimentary environment and the prediction of shale gas enrichment and preservation.Based on core and thin section observation,scanning electron microscope observation,trace element geochemistry and sulfur isotope geochemistry,core samples of the Niutitang Formation in Well Cenye-1,Cengong area,Guizhou Province were tested,and the sedimentary environment and geological conditions of shale gas enrichment in the formation were determined.All samples were well preserved and unweathered.The results show that pyrites in the Niutitang Formation are mainly developed into three types,framboid,idiomorphic-hypidiomorphic form and xenomorphic form,among which the xenomorphic one is commonly gelatinous pyrite.Different forms of pyrite were formed in different sedimentary environments.Framboid pyrite was formed in anoxic reduction environment,idiomorphic-hypidiomorphic pyrite was formed in anoxic environment,and gelatinous xenomorphic pyrite indicated that some hydrothermal activity was involved during the sedimentary process of shale.The Niutitang Formation of the Well Cenye-1 is located in a relatively open sedimentary environment.The upper section of the well is in an oxygen-poor environment,and the sulfur isotope becomes light and a negative drift occurs.The middle section of the well is in an anoxic environment with a large sulfur isotope span and a positive drift.The lower section of the well is in a more reducting sedimentary environment,and the sulfur isotope has a small negative drift.The pyrite content in shale is closely related to the enrichment of organic matter.Meanwhile,pores which are related to pyrite provide carriers for the adsorption,preservation and migration of shale gas,and the anoxic sulfide environment indicated by pyrite framboids is more conducive to the preservation of organic matter in shale gas reservoirs.The study systematically sorted out the indication effect of pyrite on the shale sedimentary environment and the shale gas enrichment in the Niutitang Formation.The results obtained in the study will provide theoretical guidance for shale gas exploration.