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Abstract

Gas signatures are used to characterize the processes occurring from generation of hydrocarbons in source rocks to their accumulation in reservoirs. Two main geologic parameters to consider are the maturity of the hydrocarbons (primary cracking of kerogen or secondary cracking of oils) and their degree of segregation during transport, linked to a migration distance. Several new diagrams and geochemical parameters are now suggested to characterize the directions and distances of migration of gases more quantitatively Two examples from Brazil include the Ceará Basin and the Recôncavo Basin. In the Ceará Basin, a gas geochemical study shows that normal faulting associated with rifting of the passive margin has locally altered the global direction of hydrocarbon migration. Such estimations of global and local directions of hydrocarbon migration allow a better reconstruction of the filling of geologic traps with hydrocarbons. The Recôncavo Basin case study shows our enhanced ability to distinguish bacterial activity, mainly in the central part of the basin, from segregative transport, more visible in the southern part. We demonstrate the efficiency of a major fault (the Mata–Catu fault) as a conduit along which hydrocarbons migrate to distances of 40 km away from the oil kitchen. These geologic examples using gas geochemical and isotopic signatures provide a platform for further applications of these tools in modeling of hydrocarbon migration and accumulation for both basin exploration and reservoir estimation.

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