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ABSTRACT: CO2 in Natural Gases of Brazilian Sedimentary Basins

Cristian Leite Sombra, T. Takaki, G. I. Henz, A. S. Barroso

CO2 concentrations have been investigated in natural gases of sandstone reservoirs from three Brazilian sedimentary basins: Campos, Sergipe/Alagoas and Potiguar. Two distinct zones of CO2 concentrations can be differentiated on the basis of temperature.

In the upper zone, from surface temperatures to 60-80°C, CO2 concentrations are extremely variable, from almost 0 to 12 mol %. The origin of this CO2 is probably related to biogenic processes.

In the lower zone, at temperatures higher than 60-80°C, C02 concentrations are initially very low (less than 1.0 mol %) and increase gradually as a function of time-temperature exposure, as revealed by vitrinite reflectance from shales adjacent to the reservoirs. The origin of this CO2 is probably related mainly to thermal decarboxylation of organic acids.

At equivalent thermal maturation stages, CO2 concentrations in the lower zone are greater in marine reservoirs than in lacustrine reservoirs. This difference can be related to the oxydation of organic matter right after deposition in some of the marine shales, which increased their potential for generation of organic acids and CO2. The lacustrine shales commonly have much lower oxygen content than the marine shales. The higher CO2 content of marine reservoirs compared to lacustrine reservoirs has important implications on the evolution of porosity and permeability during burial.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990