Fluorine in formation waters,Alberta Basin,Canada

Fluorine is a common minor constituent of formation waters throughout the Alberta Basin, and it was detected in each of the 469 samples analyzed for it. Contents range up to 22 mg/l, with an arithmetic mean of 1.83 mg/l and a median value of 1.20 mg/I. There is a trend toward increased mean and median contents of F from the shallower, cooler, less saline formation waters to the deeper, hotter, more saline formation waters. This trend parallels a trend of increasing saturation with respect to fluorite, with most of the less saline formation waters being undersaturated with respect to fluorite. At the same time, the major portion of F occurs as F⁻ in the less saline formation waters, with increasing amounts held as MgF⁺ as the Ca content, ionic strength and temperature increase. Complexes between Al and F, and VO and F account for only a relatively minor portion of the complexed F. A significant portion of the F in high-salinity formation waters from marine strata could well have originated from the initial seawater. However, it is speculated that water-rock reactions with bentonite or bentonitic shales may have to be invoked to account for the very high contents of F in the less saline formation waters from some of the shallow, cooler, non-marine aquifers of the Upper Cretaceous post-Colorado aquifer-aquitard system. Finally, it is possible to use knowledge of the distribution of F in formation waters from individual aquifers to check on the “correctness” of F determinations (primarily when formation waters of similar composition show saturation with respect to fluorite), and in the same case to predict the F content of formation waters in the Alberta Basin for which only the appropriate major ions have been determined.