Spatial variation of major element content (Na, Ca, and Cl) in formation waters in the Oligocene Frio Formation, Texas, show that mixing occurs vertically and that the chemistry of Ca-rich fluids varies from northern coastal to southern Texas. Relative amounts of major and minor elements (Sr, Mg, and K) in Ca-rich fluids from south Texas differ from those in other areas of Texas and also are different from updip Mesozoic fluids. Such differences seem to preclude the known Mesozoic fluids as a source of Ca in the south Texas Frio Formation fluids, barring complicated, nonuniform water-rock alteration changing the Mesozoic-fluid composition.

Approximately coast parallel (and strike parallel) cross sections show that Ca-rich fluids in the Frio are usually found at depth. Some anomalies occur where fluids are present near piercement structures and in a relatively large part of south Texas. In the latter region, Ca-rich fluids exist above the top of geopressure, suggesting that they may have been emplaced before geopressure became widespread (2-3 m.y. ago). Because there are no shallow Ca-rich fluids in the vicinity of major river systems in south Texas, meteoric fluids may have flushed Ca-rich fluids out of the areas near the rivers during past pluvial periods. This process does not preclude the dilution of formation water by clay-dehydration reactions, another process that has been proposed to explain deep, dilute formatio waters in Gulf basin reservoirs.