流体包裹体拉曼定量技术在致密砂岩气藏研究中的应用——以四川盆地中部侏罗系沙溪庙组为例

Application of fluid inclusion Raman quantitative technique to the study of tight sandstone gas reservoirs:Case study of Jurassic Shaximiao Formation in central Sichuan Basin

致密气勘探是中国非常规能源战略的重要组成部分,四川盆地中部金秋气田侏罗系沙溪庙组致密气多期河道立体勘探连续取得上产突破,致密气藏烃类流体演化过程分析是致密气成藏机理研究的关键。以四川盆地中部秋林、金华气田侏罗系沙溪庙组储层砂岩岩心为研究对象,基于岩石学观察和原位显微拉曼光谱观测,发现储层成岩矿物赋存有纯CH_(4)气包裹体、纯CO_(2)气包裹体、混合CH_(4)—CO_(2)气包裹体和富N2气包裹体4类。通过流体包裹体定量拉曼分析和热力学模型得到了含CH_(4)和CO_(2)包裹体的压力—温度—时间—组成(PVT—x)性质,利用CO_(2)费米双峰的间距计算出纯CO_(2)包裹体的密度,并发现2期CO_(2)流体成藏阶段:原生CO_(2)成藏期(高密度:0.874~1.020 g/cm^(3);高均一温度:>210 ℃)和次生CO_(2)成藏期(高密度:0.514~0.715 g/cm^(3);低均一温度:约180~200 ℃)。异常高均一温度的CO_(2)流体推测深部热液流体活动产生,并对储层产生侵位作用。通过甲烷拉曼v1伸缩振动峰计算的甲烷包裹体捕获时古流体压力为44.0~58.5 MPa,平均古压力系数为1.29,提供了恢复压力演化的重要证据。油气成藏于白垩纪晚期(约75~65 Ma),此时期接近于早喜马拉雅隆升早期(地层埋藏最深阶段),晚期有机气驱替早期无机CO_(2)成藏。流体包裹体古压力恢复表明储层是从弱超压到常压状态的演化过程,抬升早期弱超压指示气藏具有较好的保存条件。

Tight gas exploration is an important part of China's unconventional energy strategy.The multi-stage channel three-dimensional exploration of Jurassic Shaximiao Formation tight gas in Jinqiu Gas Field in central Sichuan Basin has made continuous production breakthroughs.The evolution process analysis of hydrocarbon fluids in tight gas reservoirs is the key to study the accumulation mechanism of tight gas.Taking the sandstone core of Jurassic Shaximiao Formation in Qiulin and Jinhua gas fields in central Sichuan Basin as the research objects,based on petrological observation and in-situ micro-Raman spectroscopy observation,it is found that there are four types of reservoir diagenetic minerals:pure CH_(4) inclusions,pure CO_(2) inclusions,mixed CH_(4)-CO_(2) gas inclusions and N2 rich gas inclusions.The pressure-temperature-time-composition(PVT-x)properties of CH_(4) and CO_(2) inclusions are obtained by Raman analysis and thermodynamic model of fluid inclusions.The density of pure CO_(2) inclusions is calculated by using the distance between Fermi peaks of carbon dioxide,two stages of CO_(2) fluid accumulation were found:primary CO_(2) accumulation stage(high density:0.874-1.020 g/cm^(3);high homogenization temperature:>210℃)and secondary carbon dioxide accumulation period(high density:0.514-0.715 g/cm^(3);low homogenization temperature:about 180-200℃).CO_(2) fluids with abnormal high and uniform temperatures speculate that deep hydrothermal fluid activity and emplacement of reservoirs.The paleo-fluid pressure(44.0-58.5 MPa,mean paleo-pressure coefficient of 1.29)calculated by the methane Raman stretching vibration peak provides important evidence to recover the pressure evolution.The hydrocarbon accumulation was in the Late Cretaceous(about 75-65 Ma),close to the early Himalayan uplift period(the deepest stage of stratigraphic burial),and the late organic gas displacement replaced the early inorganic carbon dioxide accumulation.The paleo-pressure recovery of fluid inclusions indicates that the reservoir evolves from weak overpressure to atmospheric pressure,and the weak overpressure indicates that the reservoir has better preservation conditions in the early uplift.