Print this pageComposition of Gases in Individual Fluid Inclusions from Calcite Cements Associated with the Deep Smackover Formation, Mississippi Salt Basin
BARKER, COLIN, and E. GEORGE SULLIVAN
Deep reservoirs in the Smackover Formation of the Mississippi Salt Basin have gas compositions that cover a wide range, and extend from essentially pure methane, to methane with very high concentrations of H[2]S, N[2], and CO[2]. The methane appears to be the product of thermal cracking of crude oil, with later thermochemical sulfate reduction producing H[2]S, CO[2], and indirectly elemental sulfur. Fluid inclusions in overgrowths and fracture- filling cements provide samples of gases that were present in the system at various times in the past. The composition of gases in individual inclusions has been determined mass spectrometrically. Inclusions in calcite cements were ruptured by heating in vacuum, where each inclusion produces a 25 msec burst of gas that is analyzed by a high-speed, computer-controlled mass spectrometer to give the gas composition for the individual inclusion. Typically several hundred individual inclusions were analyzed for 10 mg samples. Inclusions in calcites from three wells -- Mary Higgins (19,177 - 19,460 ft), Josephine (19,194 - 20,009 ft), and USA No. 1 (20,211 - 20,359 ft) -- have been analyzed, and show very variable CH[4]/CO[2]/H[2]S, even locally. Supporting information for calcite growth temperatures was obtained from fluid inclusion homogenization temperatures, and free-energy minimization thermodynamic calculations were used to estimate equilibrium gas composition.