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

The Jurassic Smackover Formation is a prolific petroleum-producing unit within the thick and laterally extensive Mesozoic sedimentary sequence of the Gulf Coast region. Recent attention has focused on the high lithium (>100 mg/L) concentrations of Smackover brines, which in southern Arkansas may contain between 5 and 19 million metric tons of dissolved lithium. To identify potential lithium sources within these brines and understand the mechanism of lithium enrichment, we sampled 120 core and cuttings samples from nine petroleum-producing wells. These samples represent formations deposited during the initial establishment of the basin, including the Eagle Mills Formation, which is a Late Triassic terrigenous sandstone with abundant redbeds, through the Late Cretaceous units of the Nacatoch Formation. We also sampled nine archived core and cuttings samples from the underlying Paleozoic basement. 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 portion of the Smackover, a relatively pure oolitic carbonate, is lithium-poor (<10 mg/kg). The highest lithium concentrations in the region are found in the Eagle Mills Formation (150 mg/kg), which are ~5× average crustal abundance, and in the Buckner Formation (90 mg/kg), a red shale with abundant anhydrite that provides a permeability seal above the 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 and lithium that was entrained during later stage alteration of siliciclastic sources. A better understanding of the lithium balance between brines and associated rocks may enhance lithium recovery mass estimates from the Smackover Formation.