Water, natural gas and oil moving through the earth’s crust often encounter barriers or seals that halt their movement. A consequence of the trapping of subsurface fluids by barriers is the elevation of fluid pressures (overpressures) to such levels that the host rock may experience hydraulic fracturing (Lacazette and Engelder, 1992).  Crucial to the entrapment of fluids in the subsurface is the seal that slows or halts (at least temporarily) the migration of fluids under pressure.  Seals typically are low permeability rocks such as salt or shale that are capable of maintaining overpressures for tens of millions of years.  In truth, however, few rocks are impermeable enough to maintain highly pressured conditions for longer than this.  Indeed, eventually overpressured fluids diffuse through whatever intrinsic permeability exists in the rock, or the overpressures exceed the internal strength of the seal resulting in fluid escape through the hydraulically fractured or breached seal (Roberts and Nunn, 1995). We propose that the 12.5-meter-thick Upper Devonian Dunkirk shale was a seal to fluids migrating upward through the immediately underlying more permeable and porous Hanover shale (Figure 1) during the early phases of the Alleghanian orogeny.  We also offer an explanation for the efficacious seal properties of the Dunkirk shale that involves the early generation of biogenic methane.
The most compelling evidence for the early sealing behavior of the Dunkirk shale is very pervasive NNW-trending fractures or joints found only in the upper third of the Hanover shale (Figures 2).  Significantly, very few of these joints extend more than several decameters into the Dunkirk shale (Figure 3).  Individual joints are more than three meters high, can be traced laterally for well over 100 meters and exhibit orthogonal spacings on the order of one to five meters (Figure 4).  The NNW-trending joints are interpreted to be hydraulic fractures that formed as a consequence of abnormally high pore fluid pressures at the top of the Hanover shale. Indeed, the locally great joint height:joint spacing ratio of these joints is most consistent with their formation as a consequence of elevated pore fluid pressure (Ladiera and Price, 1981). Abutting relations suggest that NNW joints are older than NW- and ENE-trending joints, which are found locally in the Hanover shale.  The fact that NNW joints are roughly coaxial with a very modest strain produced during Alleghanian compression (Craddock and van der Pluijm, 1989) suggests that the joints formed as a consequence of Alleghanian deformation. 



«Figure 1: generalized Devonian stratigraphy of western New York.  This study focuses on the Dunkirk shale-Hanover shale contact.


Figure 2: comparative rose plots of joint orientations in the lower and upper Hanover; note abundance of NNW joints in the upper Hanover.
Figure 3: field photographs of NNW terminating at or within the lower 10-15 cm of the Dunkirk shale (the Hanover shale occupies the lower half of each picture.
Figure 4: field photograph of a very continuous NNW trending joint in the Hanover shale approximately 2-3 m beneath the base of the Dunkirk shale.
Restriction of the early formed NNW joints to the top of the Hanover shale suggests that the Dunkirk shale was an effective seal to formation fluids within the Hanover shale.  The robust seal properties of the Dunkirk shale can be attributed to its strongly oriented microfabric of platy clay and organic grains that may have formed as a consequence of shear sorting (Figure 5; Bowen and Stow, 1978).  In contrast, the Hanover shale is characterized by an open microfabric due in part to bioturbation, more abundant quartz silt grains, and perhaps the expansive force of overpressured fluids at the top of the gray shale (Figure 5).  Biogenic methanogenesis may have further reduced the permeability of the organic-rich Dunkirk shale by forming capillary seals at interfaces between siltstone and water-bearing clay (shale) laminae (Figure 6; Revil et al, 1998; Cathles, 2001). Early on, buoyancy caused the methane to move vertically out of the organic-rich clay laminae into the more porous silt laminae.  High interfacial tension between the gas in the silt layers and water in the overlying clay, however, created a high gas-water displacement force (the force required to push the methane through the overlying clay) thereby effectively precluding the movement of both water and gas through the rock. The durability of the gas capillary seal derives from the additive effect of the many small capillary barriers that would have formed at most of the many silt-clay interfaces that comprise the Dunkirk shale (e.g., Revil et al, 1998; Cathles, 2001; Shosa and Cathles, 2001).  


Figure 5: scanning electron micrographs illustrating the strongly planar microfabric of the Dunkirk shale (right) and the relatively open microfabric of the Hanover shale.  The dark, open areas on the Hanover in the upper right and lower left corners of the Hanover micrograph mark the locations of quartz silt grains.


Figure 6: field photograph of the well-laminated Dunkirk shale, which would have enhanced the strength of the capillary seal.
Confinement of this earliest phase of jointing to the top of the Hanover pressure compartment is consistent with drilling results that demonstrate that rock strength is lowest (i.e., effective confining stresses are minimal) at the top of modern overpressure compartments situated beneath seals (Leonard, 1993).  The source of pressures within the top of the Hanover shale high enough to reduce the effective stress (rock stress minus pore fluid pressure) within these rocks to such low levels that low that hydraulic fracturing would occur remains elusive.  Typically the generation of abnormally high fluid pressures reflects the interplay of multiple mechanisms (Swarbrick and Osborne, 1998).  It is possible that accumulation of the increasingly coarse-grained deposits that overlie the Dunkirk shale, those that record progradation of the Catskill Delta across the Acadian foreland basin (Baird and Lash, 1980), resulted in compaction disequilibrium of deeper shales thereby inhibiting expulsion of pore water from these low-permeability rocks.  Indeed, Chapman (1980) notes that compaction disequilibrium is most common to regressive sequences, and Burrus et al. (1993) point out that deeply buried shales within the Mahakan Delta complex, Indonesia, are overpressured whereas shallow sand-rich deposits are hydrostatically (normally) pressured.  But compaction disequilibrium alone appears to be insufficient to generate fluid pressures capable of driving natural hydraulic fractures (Hart et al., 1995; Kooi, 1997).  One possible source of the additional internal pressures required to exceed the tensile strength of the upper part of the Hanover, thereby causing the gray shale to fail by hydraulic fracturing, would be the Alleghanian horizontal loading that controlled the orientation of the NNW joints (e.g., Kooi, 1997). The strong seal properties of the Dunkirk shale, especially its reduced permeability due to gas capillary sealing, would have enabled these rocks to maintain overpressures in the underlying Hanover shale long enough for hydraulic fracturing to occur.   


In sum, then, during the early phases of the Alleghanian orogeny, fluids migrating upward from more deeply buried parts of the Catskill Delta Complex in the northern Appalachian basin were trapped at the base of the Dunkirk shale.  The strong seal capabilities of the Dunkirk shale appear to  reflect a combination of intrinsic shale properties (i.e., fine grain-size, strongly planar microfabric) as well a striking reduction in permeability by the introduction of biogenic methane into the partially water-saturated shale.  The resulting durable capillary seal halted the vertical movement of fluids through the more permeable and porous underlying Hanover shale.  Pore fluid pressure within the overpressured upper part of the sealed Hanover shale continued to elevate as a consequence of compaction disequilibrium as well as Alleghanian NNW-oriented horizontal loading.  These these formation pressures failed to achieve the level of the entry pressure of the Dunkirk shale before they reached the fracture strength of the rocks at the top of the Hanover shale resulting in the hydraulic fracturing of the rock.



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contact Gary Lash, Department of Geosciences, SUNY Fredonia. Fredonia, NY 14063