有机质石墨化及其对页岩气储层的影响——以四川盆地南部海相页岩为例

Effect of graphitization of organic matter on shale gas reservoirs:Take the marine shales in the southern Sichuan Basin as examples

页岩气是中国实现能源转型和碳中和的重要过渡能源,已有的勘探开发实践表明有机质石墨化会给页岩气勘探带来风险。为了明确有机质石墨化对页岩气储层的影响,以四川盆地南部下志留统龙马溪组和下寒武统筇竹寺组页岩为例,利用激光拉曼光谱、高分辨率透射电镜、气体吸附、高压压汞及核磁共振等技术开展了页岩有机质石墨化系统的测试与分析,对比分析了有机质石墨化样品与未石墨化样品的储层特征,探讨了有机质石墨化对储层影响的机理。研究结果表明:(1)威远、泸州地区W202、L205井龙马溪组页岩和W201、W207井筇竹寺组页岩尚未进入石墨化阶段,长宁地区N206井筇竹寺组页岩激光拉曼谱出现明显的G’峰,已进入有机质石墨化阶段;(2)有机质石墨化页岩样品孔隙度普遍低于3%,微孔、介孔、宏孔体积均大幅度减少,比表面积大幅度下降,介孔成为有机质石墨化样品比表面积的绝对贡献者,占总比表面积的89.25%;(3)页岩有机质石墨化后,有机质孔发育程度大幅度降低,无机质孔隙成为有机质石墨化页岩孔隙的主体,占比高达91.8%;(4)页岩有机质石墨化导致生烃能力衰竭,侧链、杂原子脱落,芳香核缩聚与进一步定向排列,破坏了孔隙保存的机械和化学机制。结论认为,有机质石墨化对页岩气储层物性具有破坏性作用,因此在页岩气勘探中要明确有机质石墨化在优质页岩段的空间分布范围,进而规避勘探风险。

Shale gas is an important transitional energy source for China to achieve carbon neutralization and energy transformation in the future.However,previous exploration and development practices show that graphitization of organic matter can bring risks to shale gas exploration.In order to clarity the effect of graphitization of organic matter on shale gas reservoirs,we take the Lower Silurian Longmaxi Formation and the Lower Cambrian Qiongzhusi Formation in the Southern Sichuan Basin as examples.The Laser Raman Spectroscopy,high-resolution transmission electron microscopy(HRTEM),gas adsorption,high-pressure mercury injection(HPMI) and nuclear magnetic resonance(NMR) were used to systematically test and analyze the graphitization of organic matter in shale.The reservoir characteristics of graphitized and non-graphitized samples were compared and analyzed,and the mechanism of graphitization of organic matter on reservoir was discussed.The following research results are obtained.(1) Longmaxi shale samples from Well W202 in Weiyuan and Well L205 in Luzhou,and Qiongzhusi shale samples from Wells W201 and W207 in Weiyuan have not yet entered the graphitization stage,while there is an obvious G’ peak in Raman spectrum of Qiongzhusi shale samples from Well N206 in Changning,which demonstrates that organic matter of N206 samples has been graphitized.(2) The porosity of the graphitized shale samples is generally lower than 3%,which is manifested in the significant decrease of the micro,meso and macro pore volumes.And the specific surface area of the graphitized samples also significantly decreases with mesopores contributing the most to the total specific surface area,accounting for 89.25%.(3) After the graphitization of organic matter,the development degree of organic pores is greatly reduced,and the inorganic pores become the main component of the graphitized samples,accounting for up to 91.8% of the total shale porosity.(4) The graphitization of organic matter leads to the depletion of hydrocarbon generation capacity,the shedding of side chains and heteroatoms,the condensation of aromatic nuclei and further directional arrangement,which destroys the mechanical and chemical mechanisms of pore preservation.It’s concluded that graphitization of organic matter has a destructive effect on shale gas reservoirs.In future exploration,it’s necessary to further clarify the spatial distribution range of graphitization in favorable shale intervals to avoid exploration risks.