RELATIONSHIPS BETWEEN REGIONAL FLUID MIGRATION AND FRACTURES IN THE MARCELLUS SHALE

Natural fractures are found throughout the Marcellus shale in the Appalachian basin. Despite a systematic understanding of the mechanical theory of fracture generation, and many studies on fractures in both core and outcrop, the relationship between fracturing and regional fluid flow is opaque. In some settings, fractures are obviously conduits for fluid flow (e.g. open fractures), while in other settings fractures may not significantly impact fluid migration (e.g. closed or mineralized fractures). Although the generation and long distance migration of fluids to (and/or through) the foreland of fold thrust belts has been studied within the context of mountain building events (e.g. Bethke and Marshak, 1990; Cathles, 1990), little is known about the role of fractures as pathways for fluid migration in foreland basins.

Here, we present data from mineralized veins found within the Marcellus shale, in order to determine the controls on fracturing and fracture fill. Because vein filling minerals (e.g. calcite, quartz) incorporate the chemical composition of pore fluids into their structure, periods of vein growth provide a geochemical archive of the fluids that filled a fracture. For example, previous research (Evans, 1995) has established a correlation between mineralizing temperature and the type of cement mineralization found in joints across the Appalachian plateau. For this study, we sampled vein-filled fractures within the Marcellus shale found in 3 drill-hole cores in northern Pennsylvania, 4 cores in New York, and both the northern and the southern outcrop belts. This transect from the Appalachian structural front to the northern extent of the Appalachian plateau allowed us to examine the effects of burial history, distance from the Appalachian structural front, and proximity to know geologic structures on fracture fill. Preliminary results indicate that high temperature fluids are associated with fractures found near thrust faults. Additionally, we see little difference between veins taken from core and those seen at the northern extent of the outcrop belt. Both sets of veins show methane rich inclusions, pyrobitumen inclusions, and chemical maps of veins indicate relatively high temperatures and reducing conditions.