Burial History Simulations of the Wilcox Group of the Texas Gulf Coast: The Effects of pCO₂, Organic Acids, and Organic Acid Anions on Clastic Diagenesis

Regina N. Tempel, Wendy J. Harrison

The separate effects of pCO₂, organic acids, and organic acid anions on clastic diagenesis have been investigated using burial history simulations of the Wilcox Group of the Texas Gulf Coast (Tempel and Harrison, 1991, Abstr. with Programs, GSA Ann. Meeting, p. A436). Results reveal that increased levels of pCO₂ and high levels of organic acid anions in pore fluids increase the amount of carbonate cement in a sandstone and reduce porosity. Conversely, dissociating organic acids destabilize carbonate cements and inhibit precipitation of illites thus enhancing porosity development.

Increased levels of pCO₂ cause precipitation of carbonate cements in sandstones because the pH in carbonate-silicate systems is buffered by the precipitation of alumino-silicate mineral phases. In particular, kaolinite is a common result of conditions of increased pCO₂ in burial history simulations. The presence of both kaolinite and carbonate in abundance in Wilcox sandstone paragenetic sequences thus suggests that increased levels of pCO₂ influenced diagenesis.

High levels of organic acid anions (OAA) in solution also promote stability of carbonate phases by buffering the pH of the system. OAA achieve the buffering effect by acting as sinks for H⁺ ions. High levels of OAA do not appear to have a significant influence on the amounts of illite precipitated so that a predominance of carbonate cement in Wilcox sandstones suggests that diagenesis was affected by OAA.

Rapidly dissociating organic acids (OA) destabilize carbonate cements and inhibit precipitation of illite cements by acting as a source for H⁺ ions. The resultant significant reduction in pH enhances porosity and also increases quartz stability. An influence on Wilcox sandstone diagenesis by OA is therefore indicated by secondary porosity in carbonate cements, diminished amounts of illite cement, and a predominance of quartz cement.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995