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 CO 2 in the gas phase of these high-CO 2 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 SiO 2 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. P co2 decreases from overpressured horizons to normally pressured horizons. Exsolution of CO 2 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.

The Devonian Catskill Formation has been divided into four magnafacies (Mf) which have been correlated throughout Pennsylvania. Lithologies in 28 sections and wells were assigned to 10 facies on the basis of grain size, color, bed forms, fossils, and other sedimentary features. Repeating patterns of facies were used to identify magnafacies. Mf-A, the basal magnafacies, is composed dominantly of interbedded marine and non-marine shale and fine sandstone and is inferred to have been deposited mainly in a mud-rich tidal-flat environment. Mf-B is dominated by thick red shales accompanied by thin, fine-grained sandstones and is interpreted as a low-energy fluvial deposit on an inactive arid coastal plain. Mf-B is interrupted occasionally by thin, transgressive sandstones of tidal origin. Mf-C is composed of thick sandstones deposited by braided rivers and localized in three major northwest-trending zones of sediment input. Mf-D is composed of thick, fining-upward cycles with sub-equal amounts of sandstone and red shale. It is inferred to have been deposited by meandering rivers and is thickest in the same areas that Mf-C is thick. Mf-A and Mf-B are richer in sand near the sediment-input centers. In most sections, the Mf are present in the order A, B, C, and D upward, with C missing in many areas and more complex relations in extreme northeastern Pennsylvania. All Mf tend to thin northwestward. This pattern of distribution presumably results from northwestward progradation of the fluvial environment. Numerous small non-economic Cu-U occurrences are concentrated in areas of thick, shale-rich Mf-B lying between the major sediment-input areas. Most are closely associated with marine transgressions. A few occurrences are in the upper part of Mf-A and in Mf-D, but none is in Mf-C. Most of the Cu-U occurrences are localized by small accumulations of plant trash in shallow fluvial or tidal channels. In contrast, Wyoming-type, roll-front uranium occurrences are localized near Jim Thorpe in large channel sandstones of thick Mf-D in the sediment-input areas. The Cu and U occurrences appear to have formed during diagenesis by migration of metalliferous pore fluids.