Geochemistry of post-uplift calcite in the Permian Basin of Texas and New Mexico

Abstract

Integration whole-oil gas chromatography of produced oil and oil inclusions, formation-water chemistry, and stable isotopes has identified environment-diagnostic differences in calcite cements between oil field and outcrop environments in the Permian Basin of Texas and New Mexico. Calcite-δ13C and fluid-inclusion composition are the most diagnostic of pore-fluid evolution and can help interpret rock-fluid reactions. Late-stage calcite cement in the northwestern part of the basin formed in a meteoric aquifer that was emplaced by Neogene-age uplift and tilting of the Guadalupe Mountains. Where the confined aquifer intersects the Henderson oil field, the water, which is less saline than sea water, has 900-1,400 ppm bicarbonate alkalinity because of oil oxidation and contains 750 ppm H2S as a result of anhydrite calcitization and sulfate reduction. The oil field has been severely damaged by biodegradation. Modeling of δ13C in pore-filling calcite from the field (mean δ13C = -17‰ PDB) suggests oxidation of oil provided nearly 100% of the carbon in the cement. Comparison of gas chromatograms of produced oil and oil liberated from fluid inclusions in calcite shows that inclusion oil is older and more severely biodegraded (paraffin-free) than produced oil. This implies that oil in the reservoir was remobilized soon after Neogene-age meteoric invasion and carbonate cementation. The Algerita Escarpment in the Guadalupe Mountains is the site of active meteoric water recharge and growth of phreatic calcite cement. The phreatic cement contains single-phase, aqueous fluid inclusions. The cement is depleted in 13C to an extent that is diagnostic of a 1:1 mixture of soil-CO2 from decay of C4-type plants (desert grasses) and carbon derived from dolomite matrix by ground-water dissolution. This phreatic zone calcite displays a trend of 13C enrichment with depth due to increased rock-water interaction along the flow path.