渝东南地区下寒武统页岩气高氮低烃成因

The reason of high nitrogen content and low hydrocarbon content of shale gas from the Lower Cambrian Niutitang Formation in southeast Chongqing

聚焦渝东南下寒武统牛蹄塘组页岩高有机碳、微含烃、高含氮的复杂特性,从气体组分、成因类型等有机地球化学角度出发,利用同位素和流体包裹体分析结合区域构造演化特征,追踪氮气来源,挖掘牛蹄塘组高含氮的原因,最终结合勘探实践建立渝东南地区下寒武统页岩气破坏散失模型,为评价选区和勘探部署提供参考。牛蹄塘组页岩气属高温裂解气,氮气含量普遍高于90%,烃类不足3%,其中氮气是有机质热演化氮气和大气氮气的混合成因气;上震旦统灯影组顶部的不整合面与周围深大断裂沟通,构成了水体侵入和气体散失的通道,底板封盖能力差、深大断裂发育和流体活动频繁是渝东南地区页岩气保存条件的最主要的3个破坏因素,是导致牛蹄塘组页岩气"高氮、低烃"的关键。

Focus on the complexity of the Lower Cambrian Niutitang Formation that high total organic con tent, low hydrocarbon content and high nitrogen content, based on gas compound and genetic type, isotope, fluid inclusion and tectonic evolution analysis were done to trace nitrogen origin,which could help to under- stand why Niutitang Formation has high nitrogen content. Finally combined with exploration practice, the destruction model of shale gas was built,which could be the reference of evaluation and deployment.Nitro- gen content of shale gas from the Lower Cambrian Niutitang Formation was above 90% and hydrocarbon is below 3%.The nitrogen gas was from air and organic evolution.The regional decollement zone at the bot tom of the Lower Cambrian Niutitang Formation and the unconformity above the Dengying Formation were connected with the deep-large fault, which was the main pathway of gas dissipation and water intru- sion.The three main destruction factors were poor bottom preservation condition,deep-large fault develop- ment and frequent fluid activities,which were the main reasons of high nitrogen content and low hydrocar bon content of shale gas from the Lower Cambrian Niutitang Formation in southeast Chongqing.