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Abstract

Chemical reaction path models of mineral-fluid-gas equilibria can be constrained by data from hydrologic reconstructions of sedimentary basins to make predictions of diagenetic pathways. This approach allows estimates to be made of volumes of diagenetic products, their temporal order, and their consequent modifications of sediment porosity through geologic time. Diagenetic pathways for Wilcox Group and Frio Formation strata are presented as examples. A time- dependent diagenetic pathway for onshore Wilcox Group strata shows that episodic cementation is superimposed on a background of continuous chemical reactions and that significant changes in diagenetic style are related to changes in hydrologic regime during sediment burial. Calculations of the diagenetic consequences of fluid degassing during flow from the geopressured zone to the nearhydrostatic zone show that carbonates, clays, and quartz may precipitate alone or in combination, depending on the permitted physical conditions. These results can be used to estimate the attendant porosity loss in strata lying in the transition zone between geopressured and near-hydrostatically pressured pore fluids: maximum porosity decrease is between 3 and 4% while more likely porosity decreases are in the range 0.5 to 1%. Results from this study indicate that computer simulation is a critical step in the effective prediction of diagenetic pathways in sedimentary basins. The diagenetic makeup of a sedimentary rock is the result of combinations of processes and does not appear to be dominated by any one particular mechanism.

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