STABLE-ISOTOPE GEOCHEMISTRY OF WATERS AND GASES (CO2, CH4) FROM THE OVERPRESSURED MORGANZA AND MOORE-SAMS FIELDS, LOUISIANA GULF-COAST

Oxygen isotope analyses of water, and carbon isotope analyses of disso lved inorganic carbon (DIG), CO2(g), and CH4(g) have been performed on samples from normal and overpressured horizons in the Louisiana Gulf Coast. delta(18)O(SMOW) values of the waters range from -2.2 to +8.7 p arts per thousand. Consideration of the delta(18)O values, the chemica l composition of the waters, and the water and gas production values i ndicate that mixing with a low salinity, isotopically light water has taken place. Mixing lines suggest that the isotopically light componen t may be water vapor condensed during gas production, with a Rayleigh fractionation process accompanying vapor condensation, rather than sha llow groundwater. Mixing lines suggest that all formation waters have delta(18)O approximate to +8 parts per thousand, which is normal for d eep, basinal waters, although Cl- concentrations indicate there are 2 water populations (with approximately 35,000 mg/l and 15,000 to 20,000 mg/l). There appears to be no systematic difference between the overp ressured and normally pressured samples, or between samples from diffe rent fields. delta(13)C(PDB) values of DIC range from -10.6 to -3.2 pa rts per thousand. delta(13)C(PDB) values of CH4 range from -43.9 to -4 0.8 parts per thousand, and delta(13)C(PDB) values of CO2 range from - 8.8 to -6.3 parts per thousand, indicating that these gases have a the rmogenic origin. Significant isotopic differences are observed between the Moore-Sams and Morganza samples which suggest that the Morganza g ases were produced at higher temperatures and/or by the decomposition of more mature source material. The CO2 and DIC are not in isotopic eq uilibrium with calcite and dolomite cements immediately above and belo w the seal, suggesting that precipitation of calcite and dolomite took place in an earlier event(s). The DIC of most samples is at, or near, isotopic equilibrium with CO2, although it appears that some samples have derived a significant component of DIC from isotopically heavier samples located both higher and lower in the section. The lack of isot opic equilibrium between DIC and CO2 for some samples suggests either that some water samples have little or no contact with coexisting gas samples, or that a dynamic process is taking place that prevents equil ibrium being achieved.