POST-DEPOSITIONAL FAULT EFFECTS IN BLACK SHALES OF THE APPALACHIAN BASIN OF NEW YORK STATE: FRACTURE/FAULT HETEROGENEITY AND THERMAL MATURITY

Post-depositional reactivations of numerous fault systems have influenced localized fracturing, faulting and thermal maturity of Devonian black shales. These fault systems include (among others) reactivated intra-Grenvillian sutures and Iapetan-opening faults. The reactivated and newly developed fault systems resulted in local stress deviations of a regularly varying far-field SH. Over 100,000 measured fractures and other structural features in outcrop and FMI logs demonstrate local spatial complexity of fracturing, folding and mesoscale faulting that developed in response to the local stress deviations. These local complexities are not the familiar regularly varying fracture patterns. Changes in stress orientations through time can add an even greater complication to the fracture patterns.

Examples of the structural complexity include duplexing in Devonian Marcellus black shale outcrops in eastern NYS that indicate multiply oriented SH directions over relatively short distances, consistent with local fault control and/or multiple phases. Unoriented cores of the Marcellus in western NYS and outcrop in central/western NYS show downdip slickenfibers on worked surfaces indicative of low angle faulting, and in outcrop are orthogonal to Alleghanian folds. Also in eastern NYS, some Marcellus outcrops exhibit anomalous fracture systems, related to coincident fault systems, and not the typical J1/J2. In the Devonian Geneseo black shale in the Finger Lakes, N- and ENE-striking Fracture Intensification Domains (FIDs) are coincident with faults proposed on the basis of stratigraphic offsets and seismic data. In the Devonian Middlesex black shale in the same region, anomalous fracture patterns are also common.

Steep gradients in thermal maturity (indicated by CAI contours, Weary et al., 2001) in the Utica have been shown to coincide with fault systems such as the Keuka Lake Fault System (Jacobi, 2007). Although less compelling in the Devonian shales, observed steep gradients between CAI of 2 to 3.5 would be an equivalent of ~5,000 m offset, significantly more than is possible along faults in central NYS. We therefore suggest that the steep gradients are influenced by relatively hot fluid migration along fault systems. Thus, the local thermal maturity index may not be simply measuring a simple burial history.