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Pore Fluid Chemistry of a Pressure Seal Zone, Moore-Sams-Morganza Gas Field, Tuscaloosa Trend, Louisiana

By
Thomas P. Ross
Thomas P. Ross
Pennsylvania State University University Park, Pennsylvania, U.S.A.
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Arthur W. Rose
Arthur W. Rose
Pennsylvania State University University Park, Pennsylvania, U.S.A.
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Simon R. Poulson
Simon R. Poulson
Pennsylvania State University University Park, Pennsylvania, U.S.A.
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Published:
January 01, 1994

Abstract

A set of water and gas samples from 17 wells in the Moore-Sams and Morganza gas fields, producing from 17,800 to 19,100 ft (5400 to 5800 m) depths on the deep Tuscaloosa trend, have been chemically analyzed in order to investigate possible mechanisms for forming the pressure seal separating overpressured from normally pressured fluids in these fields. Calculated corrections for condensation of water from the gas phase for these wells indicate that hydration of CO2 in the gas phase of these high-CO2 gases is significant.

Two main types of water are present in the reservoirs. Type 1, with about 20,000 mg/1 CI, appears to be modified sea water that is leaking from the overpressured zone into the normally pressured zone. Type 2 has about 33,500 mg/1 CI, was probably derived by moderate evaporation of sea water, and occurs mainly near the northwest corner and in the upper reservoirs of the Morganza field. In general, pore waters in these fields and the lower Tuscaloosa are heterogeneous, indicating complex hydrology. Median concentrations of dissolved SiO2 are 340 mg/1, greatly supersaturated relative to quartz at reservoir temperatures of 160-175°C. The high supersaturation suggests active silicate breakdown, combined with inhibition of precipitation by chlorite coats on quartz grains. Pco2 decreases from overpressured horizons to normally pressured horizons. Exsolution of CO2 into the gas phase on leakage of overpressured fluid to normally pressured conditions should cause carbonate precipitation and also act to seal the overpressured zone.

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Contents

AAPG Memoir

Basin Compartments and Seals

Peter J. Ortoleva
Peter J. Ortoleva
Department of Chemistry Indiana University Bloomington, Indiana
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American Association of Petroleum Geologists
Volume
61
ISBN electronic:
9781629810935
Publication date:
January 01, 1994

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