黔北地区牛蹄塘组高—过成熟页岩气富氮特征及机理探讨

The characteristics and mechanism of high-over matured nitrogen-rich shale gas of Niutitang Formation,northern Guizhou area

黔北是我国页岩气勘探开发试点地区之一,区内页岩气组分及成因尚不清楚。基于黔北地区普遍发育循环型地热系统这一典型地质特征,紧密结合区内牛蹄塘组页岩气富含氮气问题,通过页岩气组分及同位素特征、温泉及其伴生气地球化学特征,讨论了页岩气成因、富氮气机理及富集条件。结果表明:黔北地区牛蹄塘组富氮页岩气中氮气主要来自大气,烃类气体及二氧化碳来自有机质热裂解作用,是有机质过成熟阶段的产物;循环型地热系统是导致页岩气富含氮气的主因,造成页岩气与温泉伴生气发生组分交换,使得储层中剩余页岩气甲烷δ^(13)C值明显低于有机质热裂解成因甲烷;断层、循环型地热系统、底板岩性是影响黔北地区牛蹄塘组页岩气保存条件的关键因素。通过气体组分、同位素特征,结合地质环境,尝试寻找评价页岩气保存条件的有效方法,有助于完善页岩气成因理论,能为黔北地区牛蹄塘组页岩气勘探方案部署提供理论参考。

The northern Guizhou area was selected as a precursor of shale gas,in which the composition and genesis of shale gas are still unclear.The genesis,nitrogen-rich mechanism and preserve condition of Niutitang shale gas were discussed by its molecular and isotopic compositions,and the following results can be drawn from this study.（1） The nitrogen in shale gas originated from atmosphere,and the carbon dioxide and oxygen are the products of thermal cracking.（2） Circular geothermal system is the main reason of nitrogen enrichment in shale gas,component exchange between shale gas and thermal spring-associated gas,and changing the δ^13C1 value of shale gas.（3） Fault,circular geothermal system and the lithology of seam floor are key factors affecting the preserve condition of shale gas.This study is trying to find an effective evaluating method of shale gas preservation condition integrating with molecular composition,isotope and geological environment,and providing theoretical reference for deploying exploration scheme.