SPATIAL VARIATION OF DEVONIAN SHALE PALEOFLUID CHEMISTRY: AN INDICATOR OF LOCAL AND REGIONAL FRACTURE CONNECTIVITY

Fluid inclusion microthermometry and carbon and oxygen stable isotope values of vein minerals are used to evaluate the chemistry of fluids trapped in the Devonian shale sequence (including the Marcellus) during the Alleghenian orogeny.

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 T_hA (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.