--> Abstract: Gas Geochemistry as a New Tool for Hydrocarbon Exploration: the Recõncavo Basin, Brazil, by A. Prinzhofer, T. Takaki, and M. R. Mello; #90933 (1998).

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Abstract: Gas Geochemistry as a New Tool for Hydrocarbon Exploration: the Recõncavo Basin, Brazil

Prinzhofer, A. - IFP; Takaki, T. and Mello, M. R. - Petrobras/Cenpes

Gas geochemistry is becoming a powerful indicator of the geological history of hydrocarbons. From eleven chemical and isotopic ratios, a statistical analysis (PCA) allows the gathering of two very important sets of information. The first one, using mainly the C2+ fraction of the gas, relates to maturity, and the second one, involving the proportions and d13C values of methane, indicates segregative migration: Positive values are indicative of gases accumulated far from their source, where negative values correspond to residual gas pools, after leakage of a part of the fluids (Prinzhofer et al., submitted).

The Recõncavo basin is an intra-continental North-South aborted rift basin, immediately preceding the opening of the Atlantic ocean (Figueiredo et al., 1994). The source rocks are mainly lacustrine, with thermal maturities between the oil window and the beginning of the gas window. Chemical and carbon isotope analyses have shown that at the basin scale, major gas segregation is visible, using the C1-C2 diagram (Prinzhofer & Pernaton, 1997, Fig. 1). Looking only at the samples collected in reservoirs in the vicinity of the Mata Catu fault, a major North-West/South-East accident, crosscutting the whole basin, the hydrocarbon fields localized just along the fault have a very different behavior than those set on a perpendicular line, centered in the deepest part of the basin, close to the kitchens. The data are presented on figure 2 in the PCA diagram V1-V2 (distance of migration versus maturity), with a distinction for the gases collected along the fault, and the gases collected in a transept perpendicular to the fault. It is seen that the gases located at the apex of the maximum of maturity of the source rocks, and perpendicular to the Mata Catu fault, do not show any fractionation due to migration, but a clear maturity gradient. On the contrary, gases along the fault show an important trend of segregative migration, with a very consistent evolution with regards to their position along the fault. This has direct implications on the non- existence of a second kitchen at the northwestern part of the basin, and gives clues to the relation between gas compositions and geological reconstruction in terms of maturity and migration.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil