Economic production of oil and gas from mudrocks such as the Devonian Marcellus Shale relies on hydraulic fracture stimulation. The orientation, size, porosity, and strength of subsurface natural fracture systems can influence the growth of hydraulic fractures by conducting fluid, opening, or slipping during treatment. Knowledge of the orientation, size, porosity, and other attributes of natural fractures in the Marcellus Shale is based on core and outcrop data. Fractures in outcrop and core may not be the same age, however, and uncertainty in knowledge of fracture timing and origin impedes use of outcrop data for subsurface applications.
Previous studies of fracture timing correlated fracture strikes in outcrop with inferred paleostress directions from past tectonic events. Fractures in the subsurface typically share common orientations with those observed in outcrop, but most fractures in outcrop are barren joints, whereas most of those in the subsurface are lined or sealed with cement. We compare rare fracture cements in outcrop with subsurface examples to test the hypothesis that some fractures in outcrops are equivalent to subsurface fracture systems. We compare fracture cement morphology, texture, mineralogy, and geochemistry from a suite of outcrop samples from Union Springs, New York, with fractures in four cores from a currently producing reservoir in southwest Pennsylvania.
Light-microscope petrography and cold cathodoluminescence of calcite in outcrop and some core samples reveal crack-seal and blocky textures that record fracture opening and sealing. Other core samples have fibrous calcite fill and other mineral phases. Using aqueous and hydrocarbon fluid inclusions from synkinematic fracture cements, we can tie fracture growth to burial history. Stable isotopes in calcite fracture cements from different fracture types in cores and outcrop range from −21.5‰ to +4.4‰ d13C PDB and −8.0‰ to −12.0‰ d18O PDB. Assuming burial history predicts thermal history, isotopic composition together with fluid inclusions suggest calcite formed at 50–100°C, and that fracture timing was Acadian or early Alleghanian, forming during burial. The outcrop fractures tested in this study appear analogous to subsurface fractures, although other fracture types are present in the cores that are not observed in outcrop; additionally, the orientations in outcrops and the subsurface do not always match.