上扬子地区下寒武统牛蹄塘组页岩中黄铁矿特征及其地质意义

Existence and geological significance of pyrite in the organic-rich shale of Lower Cambrian Niutitang Formation in Upper Yangtze region

黄铁矿的研究不仅为沉积、成岩环境的恢复提供依据,也为富有机质页岩的预测提供参考。以上扬子板块3个不同沉积位置钻井(宜昌EYY1井、威远W001-4井、汉中SNY1井)的下寒武统牛蹄塘组页岩为例,通过X射线衍射全岩矿物分析(XRD)、显微镜、扫描电镜和碳硫分析等实验手段对样品及其黄铁矿进行对比研究。研究区样品中黄铁矿主要发育草莓状、立方体自形、条带状及不规则非自形等4种类型。其中草莓状黄铁矿发育最多,其粒径整体较小且变化范围较窄,反映以同沉积成因为主,一般与有机质含量有良好的正相关性,其他类型黄铁矿则可能为成岩成因。3口钻井页岩样品中黄铁矿形貌特征和粒径特征差异显示,SNY1井沉积水体安静、还原性最强,W001-4井和EYY1井沉积水体下部具有贫氧—厌氧环境的转变,其中W001-4井高丰度黄铁矿可能受绵阳—长宁拉张槽热液活动的影响。岩相对黄铁矿含量具有一定的控制作用,硅质页岩和硅质—钙质过渡型页岩比钙质页岩更有利于黄铁矿的形成。黄铁矿对有机孔的发育和保存具有积极影响,有利于页岩储层中天然气的富集和储存。

The existence of pyrite provides a basis for the restoration of sedimentary and diagenetic environment,also,it can be regarded as a reference for the prediction of organic-rich shale.Shales of the Lower Cambrian Niutitang Formation drilled at three different sedimentary locations(wells Yichang EYY1,Weiyuan W001-4 and Hanzhong SNY1)in the Yangtze region have been taken as samples in this study.The pyrites founded within these samples have been analyzed with XRD,microscope,scanning electron microscope and carbon and sulfur contents.By the comparison of analytical results,it was indicated that there were four types of pyrite in these samples including framboidal,cubic automorphic,strip-shaped and irregular non-self-formed.The framboidal pyrite was mostly developed,and its particle size was relatively smaller and varied in a narrow range,reflecting the synsedimentary origin,generally has a good and positive correlation with the content of organic matter,while other types of pyrite may be diagenetically originated.The morphological differences of pyrite in shale samples from three wells indicated that the sedimentary water of well SNY1 is quiet and has the strongest reductivity.The lower part of the sedimentary water of wells W001-4 and EYY1 experienced a transformation from oxygen-poor to anaerobic,and the high-abundance pyrite in the shales of well W001-4 might be affected by hydrothermal activities in the Mianyang-Changning tension trough.Compared with calcareous shale,siliceous shale and silicic-calcareous transitional shale are more favorable for the formation of pyrite.Pyrite has a positive effect on the development and preservation of organic pores,which is beneficial to the enrichment and storage of natural gas in shale reservoirs.