稀有气体追踪水溶气成藏及定量化研究方法

Water-dissolved gases are not only an important new area for future unconventional natural gas exploartion, but also play a critical role in the formation of conventional gas reserves. The Himalayan movement caused structural uplift. As a consequence, gases dissolved in groudwater can release from the water, and migrate into gas trapping structures, or proving additional gas resources. Water-dissolved gases contribute more or less to all types of gas reservoirs in the Sichuan basin. However, qutifying the amount of water-dissolved gases in gas reservoirs remains a challenging research quesiton. It has made natural gas accumulation study, resrouces assessment and exploration difficult. Based on the unique properties of noble gases, which are low abundances, inertness and distinct endmember istotopic signatues, this study aims to apply noble gas isotope geochemistry in quantifying the contribution of water-dissolved gases in natural gas reservoirs. A full range of noble gases, including all isotopes of He, Ne, Ar, Kr and Xe, will be analysed for natural gas samples from Sichuan Basin. Particularly, natural gases in Dengying formation, Longwangmao formation, Changxing-feixianguan formation and Xujiahe formation, where there are abundant water-dissolved gases, will be studied. By using concentrations and elemental fractionations of water-derived noble gas isotopes, such as 20Ne, 36Ar, 84Kr and 132Xe, models that describe groundwater and natural gas interaction will be established. These models will facilitate the qutificaiton of water-dissolved gases in natural gas reservoirs in the study fields. In addition, radiogenic noble gas isotopes will be used in dating the groundwater systems associated with natural gas reservoirs in the Sichuan basin. The noble gas geochemical techniques developed in this study will greatly improve our understanding in the geochemistry, origin, migration and accumulation mechanisms of natural gases. There will be huge applications of these techniques for natural gas resources assessment and exploration in both conventional and unconventional gases.

水溶气资源量巨大，不仅是未来重要的非常规天然气勘探新领域，其对常规气藏的形成也起重要作用。喜山运动造成的地层抬升促使地层水减压脱溶，释放出的天然气可以给气藏提供额外的气源，因此现今的气藏中或多、或少都有水溶气的贡献。但水溶气对气藏的贡献比例计算确是个难题，给天然气成藏研究、资源评价和勘探带来很大的困扰。本项目根据稀有气体不同的来源和特殊的地球化学性质，采用稀有气体全系列组分和全同位素分析技术，选择四川盆地水溶气资源十分丰富的灯影组、龙王庙组、长兴－飞仙关组和须家河组气藏为研究对象，用稀有气体20Ne、36Ar、84Kr、132Xe等一系列同位素参数和指标，确定来自水中稀有气体含量和同位素分馏模型，建立水溶气对游离气藏贡献的定量化评价指标和方法，同时用稀有气体参数对古老地层水的年龄进行探索性研究。该研究对天然气地球化学、天然气成因、成藏机理、常规与非常规天然气资源评价以勘探思路有重要意义。

水溶气资源量巨大，其对天然气藏的形成起重要作用；氦气是重要的稀缺资源，中国主要依赖进口，富氦气藏的形成与水溶气有关。本项目根据稀有气体不同的来源和特殊的地球化学性质，采用稀有气体全系列组分和全同位素分析技术，选择四川盆地为研究对象，用稀有气体20Ne、36Ar、84Kr、132Xe等一系列同位素参数和指标，确定来自水中稀有气体含量和同位素分馏模型。该研究对天然气地球化学、天然气成因、成藏机理、常规与非常规天然气资源评价以勘探思路有重要意义。.项目取得主要成果有：.1.揭示了乐山－龙女寺古隆起氦气分布规律，水溶气对氦气富集起重要作用。随储集层年代变老氦气含量增加，相同储集层中氦气往往富集在构造高部位。古隆起上的威远气田氦气富集与花岗岩基底和水溶气有关，基底花岗岩中铀和钍的裂变的氦气溶解于水，因喜山期构造大幅度抬升，溶解于水中的氦气发生脱溶，在圈闭中与天然气一起聚集成藏，形成富氦天然气。.2.提出威远气田水溶脱气有利地质条件为储集层大面积含水，水量充沛；古老的原生水有利于水溶气的保存；H、O同位素分析表明地层水保存条件较好；大幅度的构造抬升有利于水溶气脱气形成游离气藏；.3. 初步提出氦气富集机理和主要富集模式，氦气主要来自古老基底，利用地下水作为载体，沿断裂在喜山构造运动作用下进行运移。.4 水溶气对川中须家河组煤成气藏有显著的贡献，揭示了须家河组煤成气iC4/nC4和iC5/nC5与成熟度具有良好的关系，关系二者随成熟度增加而变大。.5.揭示了安岳气田地下水在天然气保存和聚集中的作用，安岳气田龙王庙储层目前只有一小部分天然气与较年轻的地下水接触过，地下水中气体的脱气是开放系统的存在。.6. 通过稀有气体研究发现，大火成岩省引发古油藏裂解的高温甲烷排放可能是晚二叠世生物大灭绝的重要因素。