A NEW GEOCHEMICAL AND ISOTOPIC CHARACTERIZATION OF THE SMACKOVER FORMATION OF SOUTHERN ARKANSAS

The Smackover Field was discovered in 1922, two years after the first productive oil and gas wells were drilled in southern Arkansas and has produced >600 million barrels of crude oil. Additional attention within the region has focused on the potential for co-produced water to contain economic concentrations of important commodities (e.g., Br, Li) and elucidating the geochemical pathways for their enrichment in Smackover Formation brines. Southern Arkansas is also known for the ‘sour belt’, an extensive region where oil and gas wells contain volumetrically significant (>70%) quantities of hydrogen sulfide (H₂S). High H₂S was likely sourced through the high temperature (100-140°C) reaction between hydrocarbons and massive sulfate evaporites locally abundant within the Mesozoic age units. To better understand and characterize the geology and geochemistry of the region, we sampled archived core held by the Arkansas Geological Survey in May 2023, from eight locations spanning a geographic range across the historical oil, gas, and brine producing region of southern Arkansas. A single, centrally located core targeted the lithostratigraphic units with ages from the Early Triassic to Late Cretaceous, including the lower and upper Smackover Formation, with distinct geochemistry and petrographic features. The other core samples, further afield, targeted a NW-SE trend of the contact between the upper Smackover and Buckner Formations, which is a conformable boundary with a depth range spanning 1100-3200 meters through Southern Arkansas. We highlight the bulk (Ca/Mg) and trace element geochemistry (e.g., Sr), of the upper Smackover, a locally dolomitized oolitic limestone, and that of the overlying Buckner Formation, a terrigenous shale interbedded with nodular anhydrite, and present sulfur (d³⁴S) isotopic evidence for the sources of high concentrations of H₂S within the Smackover brines, by comparison between sulfate evaporites and aqueous sulfides. We further present these results with a regional perspective towards understanding the resource potential (e.g., Li) of Smackover Formation brines by elucidating structural and stratigraphic pathways of basin fluid flow and quantifying the timing of fluid-rock interaction that strongly influences the unique geochemistry of these fluids.