Tripolitic chert is present in many Rocky Mountain basins. In the Green River basin, enhanced porosity in tripolitic chert comprises a significant portion of the porosity in the Ericson Sandstone. Prediction of enhanced reservoir porosity within tripolitic chert is dependent on understanding the mechanism for creation of this porosity. Dissolution experiments were performed on selected chert samples to determine the causal mechanism.

Chert samples used in the dissolution experiments were from the Phosphoria Formation in the Overthrust belt. Paleocurrent directions and the presence of sponge spicules and apatite in the Ericson cherts all indicate a Phosphoria source within the thrust belt for much of the Ericson chert. Both acid-treated and untreated chert fragments were placed in organic acid solutions. Results of the experiments show it is the 25-30% carbonate fraction within the chert that dissolves to create tripolitic chert. Silica solubility was not affected by the organic acids.

The opportunity to dissolve calcite and dolomite within detrital chert fragments exists at several times during progressive burial. Depending on the fluid chemistry, carbonate could be dissolved by an early pulse of organic acids generated prior to hydrocarbon migration. This would be

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the ideal situation in which enhanced porosity is created immediately before migration. Later in the burial history, after thermal decarboxylation of organic fluids produces CO₂, a second opportunity exists to create enhanced porosity. Using these diagenetic concepts, it may be possible to predict enhanced reservoir porosity within tripolitic chert.

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