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