USING STRUCTURAL DIAGENESIS TO INFER THE TIMING OF NATURAL FRACTURES IN THE MARCELLUS SHALE

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 need not be the same age, and uncertainty in knowledge of fracture timing and origin impedes use of outcrop data for subsurface applications.

Fractures in the subsurface typically share common orientations with those observed in outcrop, but most fractures in outcrop are barren joints whereas some 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 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, NY, with fractures in four cores from a currently producing reservoir in southwest Pennsylvania.

Previous inferences of fracture timing correlated fracture strikes with inferred paleostress directions from past tectonic events. Structural diagenesis data collected do not rule out that some fractures formed in this way. But differences in cement content of outcrop fractures suggest that not all outcrop fractures that share the same strike are of the same age.

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 ‰ δ¹³C PDB and -8.0 to -12.0 ‰ δ¹⁸O PDB. Assuming burial history predicts thermal history, isotopic compositions together with fluid inclusions suggest calcite formed under deep burial conditions.