THE ORIGIN AND EVOLUTION OF PRODUCED WATERS FROM THE TUSCALOOSA MARINE SHALE IN MISSISSIPPI AND LOUISIANA

Waters that are co-produced during hydrocarbon extraction frequently contain high concentrations of salts, dissolved organics, and metals. It is likely that the chemistry of produced waters can provide important clues about the subsurface environment and the hydrocarbon system. Unfortunately, our understanding of the origin and evolution of these fluids remains incomplete. In this study, 24 samples of produced waters were collected from operating wells in the Tuscaloosa Marine Shale (TMS). The samples were analyzed for field parameters (pH, conductivity), concentrations of major cations and anions, and selected trace elements.

The pH of the produced waters ranged from 5.5-6. The most abundant cation was Na followed by Ca, Mg, and K. Concentrations of Na and Ca ranged from about 3 to 5% and 0.5 to 2%, respectively. Chloride was the dominant anion (about 3% to 6%) with lesser amounts of SO₄ (up to 500 mg/L) and other species. Concentrations of dissolved Fe and Mn ranged from 67 to 275 mg/L and 1.3 to 28.8 mg/L, respectively. Of the trace elements analyzed, the concentrations of Cu were highest (3.8 to 15.4 mg/L), while the concentrations of Zn, Se, Pb, Mo, and Cd were below detection limits (< about 50 µg/L). There was a weak positive correlation in the concentrations of Na, Mg, Ca, As, and V with the total volume of water produced. The concentrations of most elements had no relationship to the amount of hydrocarbon produced or the ratio of oil to gas produced; However, the concentration of Se increased with increasing oil/gas ratio while the concentration of Ba decreased with increasing oil/gas ratio. The ratios of Ca/Mg and Ca/Na increased with increasing salt content, suggesting that Ca was preferentially released to the system during progressive amounts of water-rock interaction.

In conclusion, the compositions of produced waters in the TMS are likely influenced by a combination of factors, including interactions with the hydro-fracking process and fluids, water-rock interaction, and perhaps (for some elements) hydrocarbon generation. Additional chemical analyses are planned, and work is underway to compare the produced water compositions with available mineralogical data.