Abstract: Burial History and Porosity Evolution

Cristiano Leite Sombra

A new parameter, the time-depth index, is defined and applied to a quantitative investigation of the influence of burial history on the evolution of sandstone porosity. This index represents the area enclosed by the curve and the axes of a burial history diagram of the reservoir. Paleobathymetry is disregarded in calculations.

Sandstone reservoirs from several sedimentary basins along the Brazilian continental margin were analyzed. These Jurassic to Tertiary sandstones lie at depths ranging from 700 to 4,900 m and are hydrocarbon-saturated (oil or gas).

The sandstones have been grouped into there main reservoir types, based on texture and composition:

Type I: fine- to coarse-grained, moderately to poorly sorted arkoses. Quartz content ranges from 50 percent to 70 percent.

Type II: Very coarse to conglomeratic, poorly to very poorly sorted lithic arkoses. Rock fragments are mainly granitic.

Type III: fine to coarse, moderately to well sorted quartzarenites and sub-arkoses. Quartz content greater than 85 percent.

Porosity-depth plots reveal a great variation in porosity values, even within each specific reservoir type, mostly due to differences in burial histories. When porosity values are plotted against time-depth index for individual reservoir types, well-defined trends can be seen. The decrease in porosity is slower in Type III reservoirs, intermediate in Type I, and faster in Type II. These plots suggest that it is possible to make good porosity predictions based on the reservoir time-depth index, texture and composition, within the constraints of the reservoir depth/age and basin tectonics analyzed in this study.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994