LITHIUM CONCENTRATIONS AND GEOCHEMISTRY OF PALEOZOIC AND MESOZOIC STRATA OF SOUTH ARKANSAS: IMPLICATIONS FOR SMACKOVER FORMATION BRINES

The Jurassic Smackover Formation is a petroleum producing formation that is part of an extensive Mesozoic sedimentary package within the Gulf of Mexico Basin. Recent attention has focused on the unique geochemistry and high lithium (>100 ppm) concentrations of Smackover brines, particularly in southern Arkansas. To understand the mechanism of lithium enrichment in these brines, we sampled 120 core and cutting samples from nine wells, representing formations deposited during the initial establishment of the basin—from the Eagle Mills Formation, a Late Triassic-aged terrigenous sandstone with abundant red beds, through the Late Cretaceous-aged units of the Nacatoch Formation. Further, we sampled nine archived core and cutting samples from underlying Paleozoic basement. The core locations represent a geographic range through the high lithium brine zone in southern Arkansas and include well sites near the updip limit of the Smackover (depth ~3900 ft) to the center of the zone with highest confirmed brine lithium concentrations (depth ~8500 ft). Here we present the results of whole rock and trace element geochemistry from these samples and provide solid phase geochemical context for historical and recent brine lithium concentration data. Notably, the upper Smackover, a relatively pure oolitic carbonate, is lithium poor (<10 ppm). The highest lithium concentrations are found in the Eagle Mills Formation (150 ppm), ~5× average crustal abundance, and in the Buckner Formation (90 ppm), a red shale with abundant anhydrite that provides a permeability trap for the upper Smackover Formation. Broad regional consistencies between solid phase lithium and whole rock major element concentrations (e.g., Al, K, Si) provide clues to brine lithium sources. Furthermore, these geochemical relationships will be necessary for estimating the balance of lithium supplied from connate brines, with salinity derived from evaporation of paleoseawater, and lithium that was entrained during later stage alteration of siliciclastic sources.