SPATIAL VARIATION OF DEVONIAN SHALE PALEOFLUID CHEMISTRY: AN INDICATOR OF LOCAL AND REGIONAL FRACTURE CONNECTIVITY
Samples from the Pennsylvania and West Virginia Valley & Ridge and Plateau provinces show that regionally, the early vein mineral assemblage is dolomite ± calcite ± barite. These minerals contain bitumin, liquid hydrocarbon, and condensate-type fluid inclusions that are interpreted to have been trapped during pre-folding oil generation.
In three deep cores from the Plateau, barite also contains high salinity (25-26 wt.% NaCl eq.), low ThA (69-88ºC) aqueous inclusions. The calcite and dolomite in the cores have a narrow range of stable isotope values (-0.8 to +2.5 ‰ δ13C PDB, 17.2 to 18.6 ‰ δ18O V-SMOW) indicating a relatively closed fluid system. However, late quartz contains mature CH4±CO2inclusions. The quartz also contains fragments of corroded calcite, indicating dissolution due to a fluid influx and mixing event related in increased fracture connectivity.
In the Valley & Ridge, veins may contain multiple mineral stages and fluid trapping events that reflect a dynamic vein opening history related to changes in fluid connectivity associated with syn-folding fracturing that allows for increased fluid mobility. The fluid trapping events are characterized by diferent fluid salinities (10 to >20 wt. % equiv.) and CH4:CO2 ratios. Stable isotope values reflect this fluid mobility with a wide range of values (-11.8 to +5.5 ‰ δ13C PDB, 16.2 to 25.0 ‰ δ18O V-SMOW).
In the transition between the Plateau and Valley and Ridge, the shale veins indicate a variation of fluids along strike. In central Pennsylvania, the lack of quartz and the presence of only liquid hydrocarbon and condensate-type inclusions suggests little fluid mobility. Carbon isotope values from calcite and dolomite are particularly high (+1.8 to +11.1 ‰ δ13C PDB), while oxygen values are 19.6 to 21.3 ‰ δ18O V-SMOW). To the south, late quartz contains mature CH4±CO2inclusions, suggesting a fluid influx and possible regional fluid migration from the Valley & Ridge into the Plateau.
In summary, fluid chemistry within a rock sequence does not remain static during orogenesis, but varies temporally and between structural settings.