四川盆地志留系龙马溪组页岩气碳同位素倒转机理及其地质意义

The reversal patterns of carbon isotopic composition with carbon number are found as a typical identifying characters of core areas of shale gases in the world shale gas play, and are related to high yields of shale gas. The reversal patterns of carbon isotopic composition with carbon number are also discovered in the Longmaxi formation organic-rich shale, Sichuan basin, China. The mechanism and its geological significance, especially, relationship with shale gas yield, remains unclear. The produced gas after hydraulic fracturing and retained gas in core samples of Longmaxi shales in Changning and Weiyuan areas, Sichuan basin, China will be collected at systematically produced date and analyzed for chemical composition, C, H and He, Ne and Ar isotopic composition. The pyrolysis simulation experiments using low mature marine shale will be conducted to investigate the gas geochemical features of gas products generated at different pyrolysis stages of organic matters by controlling the temperature. These gas geochemical data will be used together with organic geochemical data to establish the gas yield, chemical and C, H, He, Ne and Ar isotopic composition variation of single well with produced date to reveal the effects of gas diffusion during shale gas production. The gas geochemical parameters of produced gas will be compared with retained gas for the relationship between the carbon isotope characteristics and gas content to discuss mechanism of shale gas generation and isotope fractionation. Then, we can confirm the mechanism and geological significance of carbon isotopic reversals of Longmaxi shale gas, Sichuan basin. The results will be helpful for understanding accumulation theory and the exploration and development of Paleozoic shale gas, Sichuan basin, China.

碳同位素随碳数配分模式倒转是世界众多页岩气富集区的标志之一，四川盆地志留系龙马溪组页岩气普遍存在碳同位素倒转特征，其机理及其与含气性及产量的关系等地质意义仍不清楚。本申请项目以四川盆地志留系龙马溪组页岩气碳同位素倒转机理及地质意义为研究目标，以四川盆地长宁和威远地区志留系龙马溪组富有机质页岩产出气和残余气为研究对象，在有机地球化学研究的基础上，开展页岩气井产出气和页岩岩心中残余气的化学组成、C-H和He-Ne-Ar同位素组成研究，配合低成熟度海相页岩热模拟实验不同阶段的热解产物中气体组成特征，建立产出气组成随时间与单井产量的变化关系，探讨气体扩散对页岩气组成、碳同位素与剩余气量的关系，对比产出气与残余气的气体地球化学参数的相关性、与不同演化阶段气体的同位素组成差异，探讨四川盆地龙马溪组页岩气碳同位素倒转机理及与含气性的关系，相关认识对于南方古生界页岩气成藏理论和勘探开发具有重要的意义。

页岩气是全球新兴开发的一种重要的非常规天然气资源。中国页岩气资源潜力巨大，其中四川盆地下古生界志留系龙马溪组海相页岩是我国页岩气勘探开发的最成功的靶区，威远、长宁等地区志留系龙马溪组页岩气实现了商业生产。本项目对四川盆地威远和长宁两地志留系龙马溪组页岩气产出过程中的气体化学组成、碳-氢同位素和稀有气体同位素组成进行了分析，发现四川盆地威远与长宁地区页岩气在产出过程中持续存在化学组、碳-氢同位素组成以及稀有气体丰度及同位素组成差异。通过对页岩气气体地球化学特征研究，揭示了志留系龙马溪组页岩气的成因及来源，阐述了初次裂解气和二次裂解气的不同比例混合，构造运动以及水-有机质相互作用在不同程度上导致威远、长宁两地页岩气在气体地球化学方面的差异。通过长期(3.5年)观察页岩气产出过程中气体化学组成和C-He-Ar同位素组成变化，认为龙马溪组页岩气产出过程中气体地球化学特征的规律性变化特征可能揭示气源供给的充足程度，甲烷碳同位素组成变化是判识页岩气气源供给的一个有力指标。稀有气体(特别是放射性成因的4He和40Ar)含量的变化，结合威远、长宁页岩气井口压力，可以指示页岩气的气源供给的丰富程度。