FLUID INCLUSION STUDY OF VEINS IN THE MARCELLUS SHALE ALONG THE APPALACHIAN STRUCTURAL FRONT IN CENTRAL PENNSYLVANIA: PALEOFLUIDS AND THERMAL MATURITY

The Marcellus shale in the Pennsylvania Plateau province exhibits distinct zones of high thermal maturation based on vitrinite reflectance data. The origin of these zones is unclear, but may be due to passage of ‘warm’ fluids. In order to test this hypothesis, fluid inclusion microthermometry of calcite and quartz vein minerals was used to investigate the paleofluid history in the Marcellus shale along the Appalachian Structural Front in central Pennsylvania. Samples of vein material were obtained from eight sites, from Buffalo Mills, PA, northeast through Bellefonte, PA. The southernmost sites are adjacent to a high thermal maturity zone in the plateau, while the northernmost sites were adjacent to a low maturity zone.

The vein material from the south is mostly calcite followed by quartz whereas in the northern lower thermal maturity area, the veins consist of calcite only. Primary and pseudosecondary fluid inclusions from the veins in the north show homogenization temperatures (T_h) of 14.7° to 119.3°C, and they fluoresce light blue and yellow-gold indicative of liquid hydrocarbons and condensate-like fluids. In contrast, fluid inclusions from the calcite in the south have T_h values of -93.7 to -78.2°C, while quartz T_h values are -93.7° to -77.9°C, indicating nearly pure methane. The quartz inclusions were trapped at a minimum of 100°C at 55 MPa pressure. Fluid inclusions in quartz exhibit solid melting at -110°C, indicating the presence of 2% CO₂ in the methane. The source of the CO₂ may be from the dissolution of calcite during influx of fluids resulting in quartz precipitation. Quartz with CH₄±CO₂ inclusions is also found in a deep well core from the Marcellus shale in the eastern plateau in the high maturity zone.

In summary, in the low maturity north, veins contain in-situ petroleum fluids, whereas in the high maturity south, the presence of methane and late quartz suggests that ‘warm’ exotic fluids may be responsible for the increased thermal maturation.