Secondary porosity in Miocene Stevens sandstones of the North Coles Levee Field results from dissolution of calcite, ferroan dolomite, and calcic plagioclase (An30). Kaolinite is a leach product of plagioclase, and mass balance calculations indicate that alumina is conserved on a thin-section scale. Iron released from dissolution of Fe-carbonate is possibly conserved in late-stage pyrite.

Detrital K-feldspar and early formed albite fracture filling in plagioclase are unaffected by the leach fluids, suggesting that these components are stable with respect to the leach fluids, whereas the anorthite component is unstable.

Thermodynamic considerations indicate present-day pore waters at 100°C, 260 bars fluid pressures, are nearly at equilibrium for the reaction:

[EQUATION]

and for a similar reaction involving dolomite.

Compaction following or perhaps contemporaneous with leaching has resulted in at least two diagenetic events. One involves albitization of plagioclase at stressed grain contacts with quartz, possibly as a result of higher silica activities stabilizing albite at these contacts. The other involves crushing of detrital biotite resulting in crystallization of carbonate adjacent to it. This phenomenon is due to depletion of H⁺ in the pore water adjacent to newly formed mica surfaces as K⁺^glH⁺ exchange occurs between pore water and mica.