THE UPPER DEVONIAN DUNKIRK SHALE - AN ANCIENT HYDRAULIC SEAL IN THE CATSKILL DELTA COMPLEX: MICROFABRIC AND POROSIMETRY EVIDENCE

Fluid-driven joints (natural hydraulic fractures) within and proximal to the Upper Devonian Dunkirk black shale of western New York provide indirect evidence that this unit served as a hydraulic seal to formation fluids migrating upward from deeper within the sedimentary pile. The highest capacity seal deposits are the finely laminated organic-rich (2.5% <TOC <4.8%) basal strata of the Dunkirk shale. These deposits are defined by a strong bedding-parallel arrangement of clay grains and flattened organic particles squeezed into void spaces during mechanical compaction. The seal capacity of these upper transgressive systems tract or condensed section deposits as measured by mercury injection capillary pressure (MICP) measurements >10,400 psi (median pore throat aperture=7.3 nm). Examination of the underlying, organic lean (TOC<0.5%) Hanover gray shale, which comprises an upper highstand systems tract, reveals an open microfabric of variably oriented clay flakes apparently enhanced by irregularly distributed rigid silt grains that worked to prevent clay grain reorientation during compaction. MICP analysis of gray shale samples collected from near the contact with the Dunkirk shale indicate a seal capacity of <5,000 psi and a median pore throat aperture of 13.3 nm. These results point to the important control of depositional environment on seal quality in shale-dominated sequences. Anoxic bottom conditions precluded bioturbation of the organic-rich sediment thereby preserving its laminated depositional fabric. Rapid mechanical compaction and related platy grain reorientation of the flocculated clay created an especially efficient barrier to fluids migrating up from the underlying more permeable, heavily bioturbated Hanover gray shale. Building pore pressure at the top of the Hanover shale resulted in the propagation of natural hydraulic fractures, some of which penetrated the basal Dunkirk shale before its capillary entry pressure was reached.