Abstract

Gas geochemistry has recently been shown to enhance information regarding the geological history of hydrocarbons. In this paper, graphical representations of physico-chemical processes affecting the chemical and isotopic signatures of natural gases are exemplified. These diagrams are based on experimental studies and the use of basic statistics to extract significant and synthetic parameters from the geochemical data. From 11 chemical and isotopic 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 δ13C values of methane, indicates mainly segregative migration. Positive values of the second parameter indicate that gases accumulated far from their source, whereas negative values correspond to residual gas pools after leakage of a part of the fluids. A tentative reconstruction of the gas history has been performed in two Brazilian basins: the Espirito Santo basin and part of the Recôncavo basin. The Espirito Santo basin is located on the passive continental margin of the Atlantic Ocean, and the Recôncavo basin corresponds to an intracontinental aborted rift. In both cases, the source rocks are mainly lacustrine, with thermal maturities ranging between the oil window and the beginning of the gas window. Results show that in the Recôncavo basin, a major fault (the Mata Catu fault) acts as a drain for hydrocarbon migration at the basin scale, associating a major isotopic fractionation to the gas migration with a clear correlation between isotope fractionation and the distance of migration. In the Espirito Santo basin, this segregation appears in the platform sediments to a lesser extent and is absent in the gas pools located in the paleocanyons filled with turbidites. This long-distance migration in the platform sediments suggests that a hydrocarbon kitchen is located offshore.

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