Rare earth element and yttrium (REY) geochemistry in carbonate reservoirs during deep burial diagenesis: implications for REY mobility during thermochemical sulfate reduction

The impact of burial diagenesis (especially deep burial-related processes such as thermochemical sulfate reduction, TSR) on the distribution of rare earth elements and yttrium (REY) in diagenetic minerals in carbonate reservoir has gained little attention even though they may be unique indicators of the diagenetic system being closed or open to external influx of material. Trace element and REY concentrations, ⁸⁷Sr/⁸⁶Sr, δ¹⁸O and δ¹³C have been determined for limestone, host dolomite, pore-filling calcite (calcite-2), and late stage fracture-filling calcite (calcite-3), barite (barite-2) and anhydrite (anhydrite-3) from Lower Triassic Feixianguan Formation reservoirs in northeast Sichuan Basin, China. Calcite-2, calcite-3, barite-2, and anhydrite-3 precipitated during deep burial (from 110°C to 220°C), demonstrated by petrology and fluid inclusion thermometry. Sr isotope analysis revealed that diagenetic carbonate minerals and anhydrite have largely identical ⁸⁷Sr/⁸⁶Sr ratios to Triassic seawater, indicating negligible input of Sr from terrigenous sources and probably a relatively closed diagenetic environment for precipitation of these minerals. Carbon isotope analysis showed that calcite-2 has relatively low δ¹³C values (down to -18.9‰ V-PDB), suggesting that they are TSR calcites with the carbonate derived from oxidized, isotopically-light hydrocarbons. Unlike the seawater-like REY patterns of the limestone and dolomites, calcite-3 and anhydrite-3 are enriched in rare earth elements (σREE) and show light rare earth element enrichment and heavy rare earth element depletion, exhibiting a chevron-like pattern of shale normalized REY trends. In contrast, TSR calcite (calcite-2) has similar σREE to the host dolomite but a relatively flat REY_SN pattern, suggesting strong variations in elemental and REE compositions of the burial fluids. Significantly, TSR calcite shows a prominent positive Eu anomaly and an unusually high-chondritic Y/Ho ratio. Both yttrium versus holmium fractionation and Eu²⁺ oxidation to Eu³⁺ must have occurred during thermochemical sulfate reduction. Hence, a positive Eu anomaly and an elevated Y/Ho ratio may be used as effective proxies to differentiate calcite resulting from TSR from ordinary calcite cement. This is especially useful when carbon isotope analysis cannot be used to give an unambiguous interpretation of the origin of the calcite.