页岩黄铁矿成因类型及其沉积环境指示意义——以贵州岑巩CY-1井牛蹄塘组为例

Genetic Types of Pyrite in Shale and Implications for Sedimentary Environment:A Case Study of the Niutitang Formation from Well CY-1 in Cengong,Guizhou,China

黄铁矿广泛分布于页岩层系,对页岩沉积环境具有重要的指示意义。为明确页岩黄铁矿成因及其形成环境指示意义,以贵州岑巩区块CY-1井牛蹄塘组为研究对象,对其内部黄铁矿的形貌、大小、分布及地球化学特征等展开研究。研究结果表明,页岩黄铁矿主要以草莓状、自形—半自形及他形3种形态发育,草莓状黄铁矿成因类型分为沉积型与成岩型,自形—半自形黄铁矿成因类型为成岩型,他形黄铁矿成因类型为成岩—热液型;黄铁矿形貌特征、粒径统计结果及原位微量元素特征综合显示,该井牛蹄塘组页岩上段处于含氧—贫氧沉积环境,中段处于较缺氧的还原环境,下段所处沉积环境的还原性比中段更为强烈。将黄铁矿形貌与原位地球化学特征结合,可作为恢复牛蹄塘组页岩沉积环境氧化还原条件的有效手段,研究成果对页岩气富集规律、储层含气量预测及勘探开发等具有一定的指导意义。

Pyrite is widely distributed in shale strata,which is of great significance to indicate the sedimentary environment of shale.To clarify the origin and environmental significance of pyrite in shale,the morphology,pore size distribution and geochemical characteristics of pyrite in the Niutitang formation of well CY-1 in Cengong Block,Guizhou province were studied.The results show that pyrite is mainly developed in framboids,idiomorphic-hypidiomorphic and xenomorphic forms.The genetic type of pyrite framboids is divided into sedimentary type and diagenetic type.The idiomorphic-hypidiomorphic pyrite is diagenetic type,and the xenomorphic pyrite is diagenetic-hydrothermal type.The morphological characteristics,particle size statistics and insitu trace element characteristics of pyrite show that the upper shale of Niutitang formation is in oxygen-poor sedimentary environment,the middle shale is in a relatively anoxic reducing environment,and the lower shale is in a more intense reducing environment than the middle shale.The combination of pyrite morphology and in-situ geochemical characteristics is an effective means to restore the redox conditions of shale sedimentary environment in the Niutitang formation.The research results have certain guiding significance for gas enrichment,gas content prediction and exploration and exploitation of shale gas reservoirs.