GEOCHEMISTRY AND ORIGIN OF PRODUCED WATERS FROM THE PERMIAN BASIN, TEXAS AND NEW MEXICO

The Permian Basin is the largest tight oil producer in the United States (~1.6 million barrels per day), but study of the associated brines is lacking. The source and geochemistry of Permian Basin brines across the major tight oil producing plays were investigated (Ordovician- to Guadalupian-age reservoirs) by combining previously published data with results for 40 new produced water samples.

Salinity of these Ca-Cl-type brines generally increases with depth reaching a maximum in Devonian (med.=154 g/L) reservoirs, followed by decreases in salinity in the Silurian (med.=77 g/L) and Ordovician (med.=70 g/L) reservoirs. Ion chemistry, isotopic data for O and H, and Na-Cl-Br systematics suggest three major sources of water. Lower salinity fluids (<70 g/L) in Guadalupian and Leonardian reservoirs of meteoric origin likely represent waters that infiltrated through and dissolved halite and anhydrite in the overlying evaporite layer. Saline (>100 g/L), isotopically (O and H) heavy, Br-rich fluids in Guadalupian to Pennsylvanian reservoirs exhibit a composition similar to fluid inclusions in overlying Ochoan-age halite samples. Nearly uniform ⁸⁷Sr/⁸⁶Sr values for this group of brine samples (0.7089) matches the initial ⁸⁷Sr/⁸⁶Sr values for halite and polyhalite (0.7093), suggesting that these fluids represent evaporated late Permian seawater that formed the Ochoan evaporites and later infiltrated into deeper reservoirs. Ca/SO₄, Ca/Mg, and δ¹⁸O data for this group of brine samples suggest that SO₄-reduction, dolomitization, and oxygen isotope exchange with calcite further altered the composition of the infiltrating paleoseawater. Lower salinity waters (<100 g/L) in the Devonian and deeper reservoirs, which plot near the modern local meteoric water line, are distinct from the water in overlying reservoirs. Initial interpretation suggests these fluids were emplaced prior to the Pennsylvanian and kept isolated by low-permeability Devonian shale units.

Our results indicate that at least two separate aquifer systems exist in the Paleozoic strata of the Permian Basin. Despite eastward tilting of the basin and uplift-associated underpressuring starting in the Miocene, meteoric infiltration into the Permian Basin appears limited to the uppermost Paleozoic reservoirs and the western basin margins.