Grain-size control on organo-clay complexation and REE fractionation in the Paleozoic strata of the Permian Basin (West Texas, U.S.A.)

Abstract

Abstract As a major component of mudstone, clay minerals are known to conserve organic matter (OM) as well as a range of trace elements through the mechanisms of adsorption, encapsulation, and/or intercalation. The associations of the clay minerals on one hand and trace elements and OM on the other hand impact the diagenetic evolution of such rocks and is of substantial importance for their characterization and assessment. Subsurface core samples collected from late Paleozoic mudstone in the Permian Basin of Texas were separated into four grain-size fractions (> 2 µm, 2 to 1 µm, 1 to 0.6 µm, and < 0.6 µm) to determine the clay mineralogy, OM abundances, and rock geochemistry using a suite of diffraction, spectroscopic, and chemical analyses. All separates largely consisted of illite–smectite (I-S), illite–tobelite–smectite (I-T-S), mica/illite and chlorite coupled with some minor quartz and feldspar. The smectite component of I-S was shown to increase with decreasing grain-size fractions. Additionally, the rise in the content of smectite interlayers correlated with an increase in the total-organic-carbon (TOC) content towards the finest grain-size separates in all samples. This suggested that a significant portion of the TOC content resided in the fraction below 2 µm and that smectite interlayers promoted the preservation of OM. Concentrations of rare earth elements (REE) were found to be the highest in the finest grain-size separates and align with an increased content of smectite interlayers, denoting a plausible interaction between the two. Further on, REE and TOC concentrations display a significant positive correlation in all size fractions and increase with respect to smectite-interlayer content. This three-component relation suggests the REE adsorption to illite–smectite was likely promoted by OM. Understanding the close relation between the clay minerals, OM, and trace-element content is indicative of polyvalent cationic bridging, ligand exchange, and organo-metallic complexation, which eventually leads to the enrichment of OM and fractionation of REE in mudstone.