Pitzer's method for computing activity coefficients has been applied to the Mississippi and Arkansas Smackover Formation, the Texas Edwards Group, the Texas Frio Formation, and the Louisiana Miocene sequence. The activity quotients from carbonate reservoirs imply a reaction relation between calcite and disordered dolomite, in which ordering increases at higher temperatures. This relationship indicates that an increasing Mg:Ca molality ratio with decreasing temperature does not necessarily imply dolomite dissolution by an updip-moving fluid, it may reflect an approach to equilibrium between calcite and metastable dolomite. Activity quotients in fluids from clastic reservoirs imply a reaction relation between chlorite, illite, potassium feldspar, and quartz at temperatures above 100 degrees C (212 degrees F). Present fluid compositions in the Smackover and the Edwards appear to be related to the updip movement of Louann brines from the Gulf Coast basin, accompanied by dolomitization. Bromide plots for the Edwards fluids indicate simple dilution of Louann brines with interstitial fluids. Similar plots for fluids from the Smackover show the complicating effects of illite formation, sylvite dissolution, and halite recrystallization. Albitization may have occurred during fluid movement through clastics prior to entering the Smackover and Edwards. Fluid compositions in the Frio and Miocene are apparently not related to Louann brines. The compositional data reflect the effects of halite dissolution, the subsequent albitization of plagioclase and potassium feldspar, and the dissolution of carbonates during secondary porosity formation.--Modified journal abstract.