Interpretation of seismic data over the Cottageville gas field in West Virginia reveals the presence of numerous changes in the amplitude and shape of reflections within the lower and middle Huron Shale. Production from the lower Huron is fracture controlled, and some of the more pronounced changes in amplitude and shape occur in the more productive areas of the field. These changes are related to the development of low-impedance intervals that extend into the middle Huron Shale. Gamma-ray logs and a velocity log from the field indicate that the basal part of the middle Huron is homogeneous in character. Changes in reflection character from this interval arise from changes in the bulk properties of the rock over an area the size of a Fresnel zone. The Fresnel zone radius at the lower Huron depth is approximately 400 ft, so changes in bulk properties of the rock over this scale may not be observable in the well bore. Reduction of the shear and bulk moduli caused by increased fracture intensity and the dependance of P-wave velocity on these properties could produce the observed changes in impedance. The organic portions of the lower Huron have been intensely fractured; however, these fractures are often closed or are partially mineral filled, so fracture related changes in P-wave velocity are less likely.
Our research indicates that the fractured Devonian shale reservoir is detectable on conventional seismic data.