Structural Anomalies in Black Shales of the Northern Appalachian Basin in New York State
The general framework for regional fracture sets in the northern Appalachian Basin has been well-established (e.g., Engelder, Geiser, Lash). However, over 100,000 fractures and other structural features measured in outcrop and well logs in NYS demonstrate significant local variations in fracture spacing and orientation (for a given lithology, bed/interbed thickness, TOC content/height above the base of the black shale). Many of these structural anomalies were influenced by local stress deviations associated with through-going fault systems that were episodically reactivated, such as reactivated intra-Grenvillian sutures and Iapetan-opening faults.
Examples of the local variability include duplexing in Devonian Marcellus black shale outcrops in Otsego County (eastern NYS) that indicate multiply oriented SH directions over short distances, consistent with local fault control and/or multiple phases. FMI logs show that the Marcellus at depth has open fractures both parallel and, locally, highly oblique to contemporary SH; the latter are parallel to local faults and folds. Cores of the Marcellus in central NYS show worked surfaces indicative of low angle faulting, and seismic/well log data in western NYS indicate that thrust ramps through the Onondaga deform the lower Marcellus (Union Springs) in duplexes. In the Finger Lakes region, Fracture Intensification Domains (FIDs) occur in the Devonian Geneseo black shale that are not related to the high fracture frequencies typical of basal black shales; rather, they are coincident with faults proposed on the basis of stratigraphic offsets and seismic data. In Ordovician Utica black shale outcrops in eastern NYS, WNW-striking fracture frequency increases toward Taconic NNE-striking faults, related to a later compressional event across the formerly extensional faults. Several NNE-striking Taconic faults have very narrow damage zones, and drag folds of the Utica against the Taconic faults also have a narrow footprint. In western NYS, a curving-parallel intersection of fracture sets on the scale of 100m demonstrates the possible intermediate scale of structural anomalies.
Fracture systems in black shale units are one of the critical elements in planning an optimum shale gas drilling program. Based on the above considerations, localized open fracture trends and spacings may be successfully predicted from fault/fold trends inferred from seismic/well logs.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009