GEOCHEMICAL PATHWAYS FROM DISSOLVED NaCl TO BASINAL BRINE, INSIGHTS FROM GREEN CANYON 65, DEEPWATER GULF OF MEXICO

Many of the saline waters of the northern Gulf of Mexico sedimentary basin have acquired their elevated salinities through the subsurface dissolution of halite (NaCl). However, the major cation compositions of most of these waters reflects significant water-rock interaction following salt dissolution. These diagenetic modifications include the introduction of increasing proportions of the dissolved divalent cations Mg, Ca, and Sr with increasing salinity. A detailed study of waters produced at the Green Canyon 65 (GC65) field, outer continental shelf, Gulf of Mexico, provides insight into the early stages of diagenesis of saline waters produced by dissolution of salt.

The GC65 field lies along the northwest margin of a Plio-Pleistocene salt withdrawal basin. Hydrocarbon production has been from a late Pliocene channel and sheet sand turbidite complex which is truncated updip by a salt stock. Extensive sampling of produced waters from 21 wells in this field from the onset of water breakthrough to watering-out presents a unique opportunity to reconstruct the pre-production composition of waters within the reservoir sands and to assess the effects of advection and dispersion on the salinities and chemical composition of the produced waters. Salinities of the original oil-leg waters, i.e., pre-production immobile waters located above the original oil-water contact (oowc), systematically decreased over a distance of 1.5 km from nearly 350 g/L near salt to 150 g/L at the oowc. The waters are dominated by Na and Cl. The concentrations of K, Mg, and HCO₃ are similar to those in normal seawater and can be explained by marine porewaters having dissolved halite, but Ca, Sr, and Ba are strongly enriched. The introduction of Ca and Sr can be accounted for by dissolution of anhydrite (CaSO₄), but sulfate is strongly depleted. Depending on what solid phase assemblage is influencing water compositions, mass balance calculations show that significant amounts of albite, and Na-smectite would have to be precipitated and anorthite, illite, and quartz dissolved for the diagenetically-immature waters at GC65 to acquire compositions more typical of basinal brines. Lack of mobility of the waters may account for their reduced modification in composition.