Using structural diagenesis to infer the timing of natural fractures in the Marcellus Shale
L. Pommer, J. F. W. Gale, P. Eichhubl, A. Fall, and S. E. Laubach
The University of Texas at Austin, Bureau of Economic Geology, Jackson School of Geology Austin, Texas, USA
Economic production of hydrocarbons 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 growth of hydraulic fractures by conducting fluid, opening or slipping during treatment. Knowledge of these attributes 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 share common orientations with those observed in outcrop, but most outcrop fractures are barren joints whereas some subsurface fractures 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 outcrop samples from Union Springs, NY, with fractures in four cores from a 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, fibrous calcite fill, and other mineral cements. Using fluid inclusions from synkinematic fracture cements we can tie fracture growth to burial history. Stable isotopes in these calcite fracture cements 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.
AAPG Search and Discovery Article #90157©2012 AAPG Foundation 2012 Grants-in-Aid Projects