Abstract:

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(T_h)ranges from -95.8 to -88.2 ℃, and the corresponding density values range from 0.254 to 0.290 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.

Key words: methane inclusion,Laser Raman spectroscopy,Wufeng Formation,Longmaxi Formation, shale,Jiaoshiba area