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Na-Ca-Cl brines in Jurassic and Cretaceous reservoirs in the Gulf Coast have been attributed to the diagenesis of concentrated Jurassic seawater related to Louann Salt deposition and alternatively to the diagenesis of brines produced by halite dissolution. These brines contain up to 35,000 mg/L Ca, up to 4,000 mg/L Mg, from 400 to 2,400 mg/L Br, and up to 13,000 mg/L K. Mutual relationships of Na, Cl, total divalent cations minus sulfate and bicarbonate, K, and Br are similar to those in seawater that has been evaporated past the initial stage of halite deposition, particularly when the K content of the brine exceeds 5,000 mg/L. The concentrations of divalent cations and K increase, and the mutual relationships of all the dissolved salts become increasingly similar to tho e in seawater with increasing proximity to bedded salt. The abundance of authigenic K-feldspar in rocks above the salt beds explains the relatively rapid decrease in the K content of the brines upsection. The Ca and K contents of Jurassic Gulf Coast brines are similar to those in Na-Ca-Cl brines in feldspar-poor carbonate sequences in other basins.
C. S. Land and D. R. Prezbindowski suggested in 1980 that the Na-Ca-Cl brines in the Edwards formation of Texas originated from halite dissolution and gained Br from halite recrystallization, Ca from the albitization of plagioclase, and K from the alteration of K-feldspar. Since the Br content of the brines is high and the Br content of halite is low (generally < 100 ppm), Br would have to be stripped from an enormous volume of (impermeable) salt and transferred to a relatively small volume of fluid. Mass-balance calculations indicate that Br would have to be stripped from more than 7.5 km of salt to account for the bromide in the brines of the Mississippi salt basin. If Ca and K in Na-Ca-Cl brines are derived from feldspars, these elements should increase in concentration relative to chloride with increasing distance from the source of NaCl. This is the reverse of the field relations in Mississippi, where unaltered authigenic K-feldspar is present in rocks above the salt, and the K content of the brines decreases relative to Cl with increasing distance from the halite. Finally, it is not clear how 3 completely independent processes can operate to produce such an excellent match to the dissolved constituents of evaporated seawater over such a wide geographic area and in strata with varying amounts of feldspar. The simplest genetic model is that Gulf Coast Na-Ca-Cl brines formed from evaporated seawater and evolved to their present composition accompanying the processes of dolomitization and loss of sulfate.
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