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Abstract


 
 
Chapter from: M 61: Basin Compartments and Seals 
Edited by 
Peter J. Ortoleva

Authors:
Thomas P. Ross, Arthur W. Rose, and Simon R. Poulson

Methodology and Concepts



Published 1994 as part of Memoir 61
Copyright © 1994 The American Association of Petroleum Geologists 
All Rights Reserved
Chapter 10

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

Thomas P. Ross
Arthur W. Rose
Simon R. Poulson
Pennsylvania State University
University Park, Pennsylvania, U.S.A.


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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/l Cl, appears to be modified seawater that is leaking from the overpressured zone into the normally pressured zone. Type 2 has about 33,500 mg/l Cl, was probably derived by moderate evaporation of seawater, 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/l, 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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