Synthetic fluid inclusions XVIII: Experimental determination of the PVTX properties of H2O–CH4 to 500 °C, 3 kbar and XCH4 ⩽4 mol.%

The pressure–volume–temperature–composition (PVTX) properties of H2O–CH4 were determined from the bubble point curve to 500 °C and 3 kbar for compositions ⩽4 mol.% CH4 using the synthetic fluid inclusion technique. H2O–CH4 inclusions were produced by loading known amounts of Al3C4 and H2O into platinum capsules along with pre-fractured and inclusion-free quartz cores. During heating the Al3C4 and H2O react to produce CH4, and the H2O–CH4 homogeneous mixture was trapped as inclusions during fracture healing at elevated temperature and pressure. The composition of the fluid in the inclusion was confirmed using the weight loss technique after the experiment and by Raman spectroscopic analysis of the inclusions.Homogenization temperatures of the inclusions were determined and the results were used to construct iso-Th lines, defined as a line connecting the formation temperature and pressure with the homogenization temperature and pressure. The pressure in the inclusion at the homogenization temperature was calculated from the Duan equation of state (EOS). The slope (ΔP/ΔT) of each iso-Th line was calculated and the results fitted to a polynomial equation using step-wise multiple regression analysis to estimate the slope of the iso-Th line as a function of the homogenization temperature and composition according to: (ΔP/ΔT)=a+b·m+c·m4+d·(Th)2+e·m·Th+f·m·(Th)4,(ΔP/ΔT)=a+b·m+c·m4+d·(Th)2+e·m·Th+f·m·(Th)4, where ΔP/ΔT is the slope of the iso-Th line in bars/°C, m is the CH4 molality, Th is the homogenization temperature in degrees Celsius, and a, b, c, d, e, f are the fitting parameters. The equation is valid from the bubble point curve to 500 °C and 3 kbar for compositions ⩽4 mol.% CH4.