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Abstract
Chapter from: M
61: Basin Compartments and Seals
Edited by
Peter J. OrtolevaAuthors:
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 |
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Chapter 10
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Pore Fluid
Chemistry of a Pressure Seal Zone, Moore-Sams-Morganza Gas Field, Tuscaloosa
Trend, LouisianaThomas 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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