焦石坝页岩气田中高密度甲烷包裹体的发现及其意义

Discovery of high density methane inclusions in Jiaoshiba shale gas field and its significance

根据流体包裹体激光拉曼光谱和显微测温分析,在焦石坝页岩气田五峰组-龙马溪组页岩石英和方解石脉体样品中发现高密度甲烷包裹体。利用甲烷包裹体的甲烷拉曼散射峰v1以及均一温度分别计算了甲烷包裹体的密度,其甲烷拉曼散射峰v1分布在2 910.57~2 911.27 cm-1,甲烷包裹体均一温度(Th)分布在-95.8^-88.2℃,相对应的甲烷包裹体密度为0.254~0.290 g/cm3,具有高密度特征。结合页岩脉体样品中与高密度甲烷包裹体共生的气-液两相盐水包裹体的均一温度最小值,利用CH4体系的状态方程计算了高密度纯甲烷包裹体的捕获压力为102.6~137.3 MPa,相应的压力系数达到1.63~2.18,具有中等-强超压特征。该区高密度甲烷包裹体形成时的异常高压和较高的温度可能指示了燕山运动抬升初期含气页岩的地温条件和超压状态。焦石坝页岩气田中高密度甲烷包裹体的发现可为焦石坝页岩超压形成和演化以及页岩气富集与页岩超压的关系研究提供重要的地质依据。

High density methane inclusions are discovered in the samples of quartz and calcite veins from Wufeng-Longmaxi Formation of Jiaoshiba shale gas field through micro-thermometric and Raman spectral analysis of fluid inclusions.The Raman scatter peak v1 and the homogenization temperatures of methane inclusions are applied to calculate the density of pure methane inclusions. The Raman scatter peak v1 is in the range from 2 910. 57 to 2 911. 27 cm- 1,and the homogenization temperatures（ Th） ranges from- 95. 8 to- 88. 2 ℃,and the corresponding density values range from 0. 254 to0. 290 g / cm3,which signifies methane inclusions of high density. Taking the minimum homogenization temperatures of two-phase fluid inclusions paragenetic with high density pure methane inclusions as the trapping temperature of the latter,the state equation of CH4 system is employed to calculate the trapping pressure for the high density pure methane inclusions in geologic history,and the calculated trapping pressure is 102. 6- 137. 3 MPa and the corresponding pressure coefficients is 1. 63- 2. 18,indicating medium-to-high overpressure characteristics. The abnormal high pressure and relatively high temperature during the formation of the high density methane inclusions may represent the temperature and overpressure of gas shale at the early stage of the Yanshanian uplifting. The discovery of high density pure methane inclusions can provide a key evidence for further studies on the formation and evolution of overpressure and the relationships between the enrichment of shale gas and the overpressure in the Wufeng-Longmaxi Formation of Jiaoshiba shale gas field.