Geochemical characterization of natural gas: A physical multivariable approach and its applications in maturity and migration estimates

Gas geochemistry has recently been shown to enhance information regarding t he geological history of hydrocarbons. In this paper, graphical representat ions of physico-chemical processes affecting the chemical and isotopic sign atures of natural gases are exemplified. These diagrams are based on experi mental studies and the use of basic statistics to extract significant and s ynthetic parameters from the geochemical data. From 11 chemical and isotopi c ratios, a statistical analysis (PCA) yields two very important parameters . The first parameter, using mainly the C2+ fraction of the gas, relates to maturity and the second parameter, involving the proportions and delta(13) C values of methane, indicates mainly segregative migration. Positive value s of the second parameter indicate that gases accumulated far from their so urce, whereas negative values correspond to residual gas pools after leakag e of a part of the fluids. A tentative reconstruction of the gas history ha s been performed in two Brazilian basins: the Espirito Santo basin and part of the Reconcavo basin. The Espirito Santo basin is located on the passive continental margin of the Atlantic Ocean, and the Reconcavo basin correspo nds to an intracontinental aborted rift. In both cases, the source rocks ar e mainly lacustrine, with thermal maturities ranging between the oil window and the beginning of the gas window. Results show that in the Reconcavo ba sin, a major fault (the Mata Catu fault) acts as a drain for hydrocarbon mi gration at the basin scale, associating a major isotopic fractionation to t he gas migration with a clear correlation between isotope fractionation and the distance of migration. fh the Espirito Santo basin, this segregation a ppears in the platform sediments to a lesser extent and is absent in the ga s pools located in the paleocanyons filled with turbidites. This long-dista nce migration in the platform sediments suggests that a hydrocarbon kitchen is located offshore.