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, 1994. "Pore Fluid Chemistry of a Pressure Seal Zone, Moore-Sams-Morganza Gas Field, Tuscaloosa Trend, Louisiana", Basin Compartments and Seals, Peter J. Ortoleva
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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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Basin Compartments and Seals
Basins worldwide exhibit an unexpected degree of hydrologic segregation. There can be regions of a sedimentary basin that are isolated from their surroundings by a relatively thin envelope of low-permeability rock with an interior of sufficiently high permeability to maintain a consistent internal hydrostatic fluid pressure gradient. These have been named pressure compartments. Presure compartments have several remarkable features, just one of which is that internal fluid pressures can greatly exceed or be significantly less than any regional topographically controlled hydrologic head or drain. This publication contains 30 chapters that take detailed looks at pressure compartments in general, and detail case studies of these compartments in specific basins, such as the Anadarko and Gulf of Mexico. The volume also looks at other considerations in sedimentary basins such as hydrodynamic and thermal characteristics, and mechanical properties of rock.