GEOCHEMISTRY OF POST-UPLIFT CALCITE IN THE PERMIAN BASIN OF TEXAS ANDNEW-MEXICO

Integration whole-oil gas chromatography of produced oil and oil inclu sions, formation-water chemistry, and stable isotopes has identified e nvironment-diagnostic differences in calcite cements between oil field and outcrop environments in the Permian Basin of Texas and New Mexico . Calcite-deltaC-13 and fluid-inclusion composition are the most diagn ostic of pore-fluid evolution and can help interpret rock-fluid reacti ons. Late-stage calcite cement in the northwestern part of the basin f ormed in a meteoric aquifer that was emplaced by Neogene-age uplift an d tilting of the Guadalupe Mountains. Where the confined aquifer inter sects the Henderson oil field, the water, which is less saline than se a water, has 900-1,400 ppm bicarbonate alkalinity because of oil oxida tion and contains 750 ppm H2S as a result of anhydrite calcitization a nd sulfate reduction. The oil field has been severely damaged by biode gradation. Modeling of deltaC-13 in pore-filling calcite from the fiel d (mean deltaC-13 = -17 parts per thousand PDB) suggests oxidation of oil provided nearly 100% of the carbon in the cement. Comparison of ga s chromatograms of produced oil and oil liberated from fluid inclusion s in calcite shows that inclusion oil is older and more severely biode graded (paraffin-free) than produced oil. This implies that oil in the reservoir was remobilized soon after Neogene-age meteoric invasion an d carbonate cementation. The Algerita Escarpment in the Guadalupe Moun tains is the site of active meteoric water recharge and growth of phre atic calcite cement. The phreatic cement contains single-phase, aqueou s fluid inclusions. The cement is depleted in C-13 to an extent that i s 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-wa ter dissolution. This phreatic zone calcite displays a trend of C-13 e nrichment with depth due to increased rock-water interaction along the flow path.