Marine and Petroleum Geology 23 (2006) 317–335 www.elsevier.com/locate/marpetgeo Top seal development in the shale-dominated Upper Devonian Catskill Delta Complex, western New York State Gary G. Lash * Department of Geosciences, College at Fredonia, State University of New York, Fredonia, NY 14063, USA Received 29 March 2005; received in revised form 8 December 2005; accepted 4 February 2006 Abstract The preferential generation of vertical natural hydraulic fractures at the contact of the Upper Devonian Hanover gray shale and overlying Dunkirk black shale of the Catskill Delta Complex, western New York State, suggests that the latter served as a hydraulic top seal to formation fluids migrating upward from deeper in the sediment pile. Petrophysical properties and small-scale textural characteristics of these siliceous finegrained rocks confirm the crucial role of depositional environment and sequence stratigraphic position of a shale lithotype in determining its sealing capacity. The especially high sealing capacity of the basal interval of the Dunkirk shale, inferred early high-stand systems tract (HST) strata, reflects the anoxic depositional environment of these deposits that favored the preservation of their abundant organic matter and finely laminated depositional texture. The absence of bioturbation enabled the undisrupted sediment, notably carbonaceous clay-rich laminae, to undergo rapid mechanical compaction, platy grain reorientation, and porosity reduction. Compaction-induced squeezing of ductile organic matter into void spaces further reduced pore throat diameters. Immediately underlying heavily bioturbated deposits of the organic-lean Hanover shale, inferred upper HST or low-stand wedge sediments, accumulated in a dysoxic depositional environment. Disruption of layering and homogenization of sediment by burrowing organisms produced a more porous and permeable microfabric through which formation fluids moved only to be arrested by the high capillary entry pressures at the base of the Dunkirk shale. Natural hydraulic fractures, some of which propagated into the Dunkirk shale, formed when fluid pressure at the top of the Hanover shale reached the fracture gradient. The high sealing capacity of the basal Dunkirk shale was probably enhanced by its finely laminated nature and the generation of biogenic methane, both of which contributed to the formation of a near-impermeable gas capillary seal. q 2006 Elsevier Ltd. All rights reserved. Keywords: Top seal; Porosimetry; Upper Devonian; Appalachian basin; Gas capillary seal 1. Introduction The fine grain size, small pore throat diameters, and high capillary entry pressures of shale and mudstone exert a primary control on the transmission of formation fluids, including hydrocarbons, through these deposits (Aplin et al., 1995; Schlömer and Krooss, 1997; Dawson and Almon, 1999, 2002). Indeed, some shale lithotypes are especially efficient top seals to fluid flow, enabling the buildup of overpressure in underlying deposits (e.g. Krushin, 1997; Luo and Vasseur, 1997; Dawson and Almon, 1999, 2002). Locally, however, top seals may be compromised by the generation of natural hydraulic fractures before capillary leakage takes place (e.g. Watts, 1987; Caillett, 1993; Darby et al., 1996). Such occurrences can have important * Tel.: C1 716 673 3842; fax: C1 716 673 3347. E-mail address: "$10#>lash@fredonia.edu 0264-8172/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2006.02.001 implications for hydrocarbon migration and entrapment as well as for exploration and production strategies. The study of wellexposed top seals is crucial to gaining a greater appreciation of the milieu of factors that affect top seal formation and behavior. Indeed, the unique perspective offered by field exposures (e.g. close sample spacing, relative ease of analysis of macro-textural features and stratigraphic relationships) combined with observations gained through investigations of top seals in overpressured producing basins can enhance our understanding of this most essential element of the petroleum system. Vertical joints (mode I cracks), pervasive across the Appalachian Plateau of western New York State and interpreted to be natural hydraulic fractures (Engelder and Oertel, 1985; Lacazette and Engelder, 1992; Lash et al., 2004), provide indirect evidence that the shale-dominated Upper Devonian clastic succession of the Catskill Delta Complex was once overpressured. The especially high density of joints in Upper Devonian organic-rich black shales is due in large part to the generation of hydrocarbons in these very tight rocks 318 G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 during the Carboniferous-Permian Alleghanian orogeny (Loewy, 1995; Engelder et al., 1998; Lash et al., 2004). However, the Upper Devonian shale succession exposed along the Lake Erie shoreline in western New York State carries a joint set that shows no affinity for black shale; instead, fractures of this set, similar in most respects to other vertical natural hydraulic fractures in Devonian rocks of the Appalachian Plateau, are confined to the upper third of the organic-lean Hanover gray shale and the basal few meters of the overlying Dunkirk black shale. This paper makes the case that the Dunkirk shale, by virtue of its depositional and early diagenetic history, served as a top seal to overpressured formation fluids migrating upward from deeper in the sedimentary pile. At some point in time, fluid pressure at the top of the Hanover shale reached the local fracture gradient resulting in the propagation of vertical natural hydraulic fractures, some of which penetrated a short distance into the overlying Dunkirk shale. Downey’s (1984) suggestion that seals be studied at both the macro- and microscopic scale is followed in this analysis. First, field evidence for the existence of a top seal above the Hanover shale is considered; this is followed by discussion of petrophysical and microscopic parameters that may have been vital to vertical fluid flow through the Hanover shale– Dunkirk shale succession. The lack of layer-parallel shortening strain produced during compressional tectonics of the Alleghanian orogeny in these rocks (e.g. Hudak, 1992) allows for detailed analysis of those factors critical to the development of top seals in basinal marine shale- or mud-dominated depositional systems. 2. Stratigraphic framework The Upper Devonian clastic succession of western New York State comprises a thick interval of marine shales and scattered siltstone beds that grades upward into shallow marine or brackish-water deposits (Fig. 1; Baird and Lash, Fig. 1. Location map of the Walnut Creek section and generalized Upper Devonian stratigraphy of the Lake Erie shoreline region of the Appalachian Plateau, western New York State. Note the field location of the stratigraphic interval shown in Fig. 2. G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 1990) thus recording progradation of the Catskill delta across the Acadian foreland basin (Faill, 1985; Ettensohn, 1992). The shale-dominated basinal marine deposits are arranged in several cycles, each one defined by a basal unit of black shale that passes upward through a transition zone of alternating black and gray shale beds into strata dominated by gray shale and occasional siltstone and thin black shale beds (Fig. 1). The basal black shale unit of each cycle has been interpreted as a record of rapid cratonward movement of the Acadian fold and thrust load followed by deposition of gray shale (Ettensohn, 1985, 1992). 319 Specifically, each phase of thrust-sheet imbrication was accompanied by rapid subsidence of the basin and deposition of clastic-starved, organic-rich black shale. Overlying gray shale and siltstone reflects tectonic relaxation, establishment of terrestrial drainage systems and delta progradation (Ettensohn, 1985, 1992). However, the tectonostratigraphic explanation for cyclic deposition of Devonian black and gray shales in the Appalachian Basin has been challenged by models involving eustatic oscillations and/or fluctuations in the productivity of marine organic matter (Johnson et al., 1985; Werne et al., 2002). Fig. 2. Stratigraphic log of the Hanover shale, Dunkirk shale, and lower Gowanda shale along Walnut Creek. 320 G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 This study focuses on the Upper Devonian Hanover shale– Dunkirk shale succession (HDS) most completely exposed along Walnut Creek near the village of Silver Creek, New York (Figs. 1 and 2). As pointed out below, the Hanover and Dunkirk shales contain 40–65% detrital silt and comprise both laminated and non-laminated deposits. According to the shale classification of Lundegard and Samuels (1980), which is based on silt abundance and fabric, the laminated shale is termed mudshale and the non-laminated fine-grained deposits are classified as mudstone. However, in order to minimize terminology and because it is ingrained in the literature, the term shale will be used throughout the balance of this paper. The Hanover shale comprises w30 m of organic-lean (0.09%!total organic carbon [TOC]!0.93%) gray shale, occasional siltstone, thin- to thick-beds of black shale and horizons of carbonate nodules (Fig. 2). The lack of primary structures visible in exposed gray shale is testimony to the heavily bioturbated condition of these deposits (Baird and Lash, 1990) and/or deposition from muddy gravity flows. Laminated black shale beds crop out at intervals throughout the lower half of the Hanover shale along Walnut Creek, yet only one carbonaceous shale bed is observed from w20 m above the base of the unit to within a meter of its contact with the Dunkirk shale (Fig. 2). Thin- to medium calcareous siltstone beds become somewhat more abundant in this interval (Fig. 2). The upper meter of the Hanover shale along Walnut Creek is characterized by interbedded gray shale and finely laminated organic-rich (3.4%!TOC!5.5%) black shale (Fig. 2). The Hanover shale is abruptly overlain by the Dunkirk shale, w17 m of laminated black and grayish-black shale, thin gray shale and siltstone beds, and horizons of large (1.5 m maximum diameter) internally laminated carbonate concretions (Figs. 2 and 3). The Hanover–Dunkirk contact is erosional and marked by a lag deposit of reworked pyrite, Hanover shale Dunkirk shale Fig. 2 (continued) Fig. 3. Hanover shale–Dunkirk shale contact (dashed white line), Walnut Creek section. The contact is a marine (maximum) flooding surface that is interpreted to be the boundary between the Dunkirk high-stand sequence and underlying transgressive system (condensed sequence?) deposits of the upper Hanover shale (see Fig. 16). The white arrow in the Hanover shale points to a NS-trending joint; joints in the Dunkirk shale are younger ENE (065–0728)trending fractures. G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 321 Fig. 4. (A) NS-trending joint cutting shale and medium-bedded calcareous siltstone (tan layers) near the top of the Hanover shale. Photograph taken along the Lake Erie shoreline near Dunkirk, New York. The estimated position of the Hanover shale–Dunkirk shale contact is indicated by the white line (width of the back pack to the right of the jointZ30 cm); (B) close-spaced NS joints at the Hanover shale–Dunkirk shale contact (dashed white line), Walnut Creek section. fish bones, wood debris, and conodonts (Baird and Lash, 1990; Baird and Brett, 1991). The Dunkirk shale grades upward through interbedded black and gray shale into the Gowanda shale (Figs. 1 and 2), w70 m of gray and lesser black shale and bundles of very thin- to medium-bedded siltstone. The Gowanda shale is overlain by more than 560 m of shale and very thin- to thick-bedded siltstone that records the gradual infilling of the Acadian foreland basin in this region of the Appalachian Basin (Baird and Lash, 1990). fluid-driven or natural hydraulic fractures (Lacazette and Engelder, 1992). Further, the locally large height: orthogonal spacing ratio of the joints is at odds with their formation as tensile fractures produced by joint-normal loading or 3. Joints Inferences regarding fluid pressure generation and seal development in the Catskill Delta Complex are based largely on the distribution of several sets of fluid-driven joints (natural hydraulic fractures) in these rocks (Engelder and Oertel, 1985; Engelder and Lacazette, 1990; Lacazette and Engelder, 1992; McConaughy and Engelder, 1999; Lash et al., 2004). Rocks of the HDS carry four of five regional joint sets recognized in the Upper Devonian succession of western New York State (Lash et al., 2004). Of interest here are NS (352–0078)-trending joints, which are confined to the upper third of the Hanover shale and the lower w2.5 m of the Dunkirk shale. NS joints are essentially vertical, very planar and continuous, extending beyond the limits of exposure (Fig. 4A). The planarity and continuity of the joints, as well as their straight overlapping geometries (Fig. 4B), suggest that they formed under conditions of relatively high (for mode I cracks) differential stress (Olson and Pollard, 1989). Occasional arrest lines and plumose structures observed on joint surfaces, especially those that cut calcareous siltstone beds, suggest that the joints are Fig. 5. Box-and-whisker diagrams representing orthogonal joint spacing distribution for NS joints measured at three stratigraphic intervals of the Hanover shale. The box encloses the interquartile range of the data set population; the interquartile range is bounded on the left by the 25th percentile (lower quartile) and on the right by the 75th percentile (upper quartile). The vertical line drawn through the box defines the median value of the data population, and the ‘whiskers’ define the extremes of the sample range. Statistical outliers are represented by data points that fall outside the extremes of the sample range. 322 G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 Fig. 6. (A) Three close-spaced NS joints that propagated various distances into the Dunkirk shale. White arrows denote joint tips (white scale barZ30 cm); (B) two close-spaced NS joints in the Hanover shale that failed to penetrate the Dunkirk shale (white arrows define joint tips). Both images were photographed along the Lake Erie shoreline, Dunkirk, New York. Dashed white lines in both photographs mark the Hanover shale–Dunkirk shale contact. stretching. Rather, the close spacing of the NS joints, especially proximal to the Hanover–Dunkirk contact (Fig. 5), is more consistent with their propagation as fluid-driven fractures (Ladeira and Price, 1981; Fischer et al., 1995; Engelder and Fischer, 1996). All observed abutting relations of NS joints and joints of other sets indicate that the former are the older structures. Especially noteworthy to current considerations is the restriction of NS joints to a narrow stratigraphic interval that encompasses the upper third of the Hanover shale and lower few meters of the Dunkirk shale. The degree of NSjoint saturation, as measured by orthogonal spacing, increases upward through the Hanover to its contact with the Dunkirk shale (Fig. 5), yet fewer than one in five observed joints penetrate more than 20 cm into the black shale (Figs. 4B and 6A,B). Roughly 50% of NS joints examined in this study either were arrested at the base of the Dunkirk shale or failed to reach the black shale (Fig. 6C). Rarely are NS joints found exclusively in the Dunkirk shale; indeed, O80% of studied NS joints in the Dunkirk shale can be traced down into the Hanover shale. These observations suggest that the bulk of the NS joints originated within the Hanover shale. 4. Pressure cell model Confinement of the vertical NS-trending joints to the upper third of the Hanover shale and lower few meters of the Dunkirk shale is consistent with pressure-depth profiles and related in situ stresses documented from modern basins where the interplay of minimum horizontal stress, Sh, and fluid pressure, Pp, through a seal has the potential to induce natural hydraulic fractures (Fig. 7). Industry data, principally in the form of leakoff test and repeat formation test results, from the Central Graben of the North Sea reveals a multi-layered pressure system where normally pressured Upper Cretaceous and overlying deposits serve as a pressure sink for high formation pressures generated deeper in the sedimentary pile (e.g. Holm, 1996, 1998). Approximately hydrostatic pressure conditions exist to the depth of the Lower Cretaceous Comer Knoll group marls and underlying organic-rich upper Jurassic Kimmeridge claystone formation through which Pp increases at more than twice the hydrostatic gradient (Holm, 1998). Formation pressure in overpressured rocks beneath the Kimmeridge Claystone Formation continues to build with increasing depth, but at approximately the hydrostatic gradient, indicating that these deposits are in hydraulic communication (Darby 323 G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 overpressure in the pressure cell will dissipate, at least temporarily. The Dunkirk shale is analogous to the Kimmeridge claystone formation in its role as a top seal, though the depth of fluid retention (i.e. elevation of Pp above hydrostatic pressure at a given depth) may have been well up into the Gowanda shale (Fig. 1). Overlying increasingly siltstone-rich deposits of the Catskill Delta Complex in western New York State are comparable to the upper cretaceous clastic rocks of the Central Graben. Concentration of the early formed vertical natural hydraulic fractures in the upper Hanover shale and lower few meters of the Dunkirk shale suggests that the latter was a hydraulic top seal, a barrier defined by such a high capillary entry pressure that the rock leaked by natural hydraulic fracturing before its capillary entry pressure was exceeded (Watts, 1987). Further, the high height: orthogonal spacing ratio of NS joints, especially at the Hanover–Dunkirk contact, suggests that natural hydraulic fracturing occurred episodically (e.g. Roberts and Nunn, 1995). Elevation of Pp to the fracture gradient resulted in hydraulic fracturing followed by dissipation of Pp and closure of the crack. When Pp again rose, new fractures formed well within the stress reduction shadows of early formed, but closed, fractures and/or older fractures were reopened and lengthened (Roberts and Nunn, 1995). Below, petrophysical, compositional, and small-scale or microscopic textural properties of the Dunkirk shale and Hanover shale that may have contributed to their respective abilities to transmit or inhibit formation fluids are considered. Fig. 7. Idealized pressure–depth plot from normally pressured deposits, through a top seal and into a pressure cell showing changes in pore pressure from (Pp) and resultant changes in effective hydrostatic (Phyd) to overpressured
minimum horizontal stress Sh0 and, therefore, rock strength. Inferred position of the Hanover–Dunkirk–Gowanda succession within the pressure cell-top seal system is also shown. This figure should not be taken to mean that hydrostatic pressure conditions existed down to the top of the Dunkirk shale. Rather, the departure from Phyd probably occurred within the Gowanda shale. SvZ overburden stress; ShZminimum horizontal stress and an approximation of the fracture gradient (modified from Gaarenstroom et al., 1993). 5. Methodology et al., 1996). Thus, the pressure transition zone defined by the Comer Knoll Group and Kimmeridge Claystone Formation is a top seal to overpressured formation fluids in the underlying pressure cell (Baird, 1986; Gaarenstroom et al., 1993; Leonard, 1993; Holm, 1996, 1998). The most critical interval of this system is found at the base of the seal, the Kimmeridge Claystone Formation, and top of the pressure cell where, as predicted by the Terzaghi effective stress relationship, effective minimum horizontal stress, Sh0 , approaches zero placing the rocks in a state of incipient vertical natural hydraulic fracture generation (Fig. 7; e.g. Gaarenstroom et al., 1993; Leonard, 1993; Caillett, 1993; Holm, 1998). The integrity or retention capacity of the top seal under these conditions, then, is predetermined by Sh, which controls the level of maximum sustainable Pp (Gaarenstroom et al., 1993). That is, if Sh0 Z 0, (i.e. ShZPp), the rocks near the top seal–pressure cell contact will fail by fracturing, the seal may be compromised and Shale samples of the HDS were collected for analysis by mercury injection capillary porosimetry (MICP), X-ray diffraction (XRD), thin section and scanning electron microscopy (SEM), and Rock-Eval pyrolysis. All samples were collected from O5 cm into exposures to minimize the effects of weathering. Analytical results are summarized in Table 1. 5.1. Mercury injection capillary pressure The sealing capacity of six shale samples collected over a w35 m interval of the HDS spanning the Hanover–Dunkirk contact, three each of the Dunkirk and Hanover shales (see Fig. 2 for MICP sample locations), was determined by mercury injection capillary pressure measurement (refer to Jennings, 1987; Almon and Thomas, 1991; Vavra et al., 1992; and Krushin, Table 1 Summary table of major characteristics of the Dunkirk shale and Hanover shale Dunkirk shale Hanover shale 10% Hg saturation (psia) Permeability (m2) Porosity (%) Silt (%) Cement (%) TOC (%) Median pore throat diameter (nm) Fabric 10,890–14,200 920–4850 2.6!10K21 2!10K18 3.24 5.97 51 54 1.9 8.9 2.3–4.7 0.1–0.9 7.1 23.9 Strongly oriented Random to moderately oriented 324 G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 1997, for discussion of porosimetry methodology). Samples selected for porosimetry analysis were recovered from polished slabs (typically O50 cm2) in order to assess, as best possible, sample heterogeneity. Each sample collected from a polished slab was studied by optical and electron microscopy to rule out the presence of microfractures. Porosimetry measurements were made perpendicular to layering on epoxy-jacketed samples by PoroTechnology, Stafford, Texas. Capillary pressure curves generated by the step-wise increase in mercury injection pressure were used to evaluate sealing capacity and pore throat diameter sorting (e.g. Jennings, 1987). Schowalter (1979) maintains that compromise of a shale seal by capillary leakage occurs when the level of hydrocarbon saturation of a water-wet seal falls between 5 and 10%. Similarly, Rudd and Pandey (1973) argue that shale transmits fluid at mercury saturation levels of less than 10%. Thus, the mercury intrusion pressure at 10% saturation (the displacement pressure of Schowalter, 1979) serves as a proxy for seal efficiency. In addition, to facilitating the calculation of sealing capacity, porosimetry measurements can be used to estimate the distribution of pore volume accessible by pore throats of a given size, pore throat sorting, porosity, and permeability (Jennings, 1987; Vavra et al., 1992; Boult, 1993; Krushin, 1997). 5.2. Petrographic (thin section and electron microscopic) analysis Thin section analysis provides useful information regarding macro-textural features, including lamination type and bioturbation (O’Brien and Slatt, 1990), mineralogy, and grain shape (i.e. aspect ratios of organic particles) and orientation (e.g. Sutton et al., 2004). The bulk mineralogy of the six shale samples analyzed by MICP was carried out by XRD. Point counts of 300 grains of each sample yielded results within 5% of the correlative XRD results. Having established consistency between both analytical methods, an additional 10 shale samples, seven Hanover and three Dunkirk samples, were point counted for framework grains (quartz, feldspar), clay and diagenetic cement. Carbonate, principally calcite, is the dominant cement, followed in abundance by quartz. Diagenetic quartz forms elongate or lobate grains that appear to have filled void spaces, principally the result of early dissolution of radiolaria by pore water. Detrital quartz grains, on the other hand, are angular and roughly equi-dimensional. Quartz silt grains that could not be positively identified as detrital or authigenic were not counted. Fig. 8. Petrophysical and compositional parameters plotted against stratigraphic position within the Hanover–Dunkirk succession. G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 Each shale sample studied in thin section was analyzed by backscattered and secondary electron imaging techniques. Shale samples, prepared for SEM analysis as per O’Brien and Slatt (1990), were mounted on double-sided adhesive carbon tape such that the viewing direction was normal to bedding. Samples were coated with 20 nm of evaporated carbon to render the surface conductive and analyzed on a Hitachi S-4000 Field Emission SEM operating at 30 keV. Digital secondary electron images were collected with a 4PI digital imaging system. 5.3. Organic carbon content Total organic carbon content of 13 shale samples collected from the HDS was measured in tandem with Rock-Eval pyrolysis measurements. Analytical work was carried out by Geochemical and Environmental Analysis, Université de Neuchâtel, Switzerland. 325 within 3 m of its contact with the Dunkirk shale (Fig. 8). Dunkirk shale samples are defined by markedly higher seal capacities; 10% mercury saturation is achieved at 14,200 psia at the base of the Dunkirk, diminishing to 10,890 psia at the top of the unit (Fig. 8). The difference between the high and low 10% mercury saturation values for both units is great, yet variation among the Hanover samples is proportionately greater than that of the Dunkirk shale samples; i.e. 80 versus 23% of the maximum sealing capacity of the Hanover and Dunkirk, respectively. Dunkirk shale samples possess a lower average porosity (Table 1); median pore throat diameters of the Hanover samples (13.3–44.4 nm) are markedly larger than those of the Dunkirk shale (6.7–7.3 nm; Fig. 8). Moreover, MICP curves (Fig. 9) and pore throat size distribution plots (Fig. 10) illustrate a high degree of pore throat diameter sorting among Dunkirk samples; Hanover shale samples, on the other hand, display a more variable and lower degree of pore throat 6. Results Sealing capacities of the HDS samples vary widely over the limited stratigraphic interval studied (Fig. 8). The 10% mercury saturation level of the Hanover shale samples ranges from a low of 920 psia in the lower third of the unit to 4850 psia Fig. 9. Mercury injection capillary pressure curves for the Hanover shale and Dunkirk shale samples. Note the line denoting the 10% mercury saturation pressure. Fig. 10. Pore throat size distribution curves for the Hanover shale and Dunkirk shale samples. The plots are arranged in stratigraphic order within each unit (refer to Fig. 2 for sample locations). 326 G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 Fig. 11. Plots of sealing capacity versus selected petrophysical and compositional parameters. Gray arrows indicate younging directions of the samples within their respective units. diameter sorting. It is noteworthy that pore throat diameter sorting of the Hanover shale samples decreases upward; a similar but more subtle trend appears to hold for the Dunkirk shale (Fig. 10). Finally, regardless of pore throat diameter sorting, median pore throat diameter correlates negatively with sealing capacity for all six HDS samples (Fig. 11A). Thin section and XRD analysis reveals little difference in total silt content between the Dunkirk and Hanover shales. Indeed, although the mean detrital silt content of studied Hanover shale samples (54%; nZ10) is marginally greater than that of the Dunkirk shale (51%; nZ6), comparison of means by a one-way ANOVA (Analysis of Variance) test failed to reject the null hypothesis that there is no statistically significant difference between the means (rZ0.2089). No consistent relationship between detrital silt content and sealing capacity was observed in the HDS samples (Figs. 8 and 11B). Specifically, the increase in sealing capacity upward through the Hanover shale, which mirrors an increase in the frequency of calcareous siltstone beds (see Fig. 2), appears to be accompanied by increasing sealing capacity (Fig. 11B). On the other hand, the highest sealing capacity Dunkirk shale samples at the base of the unit contain less silt than the lower G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 sealing capacity sample collected from near the top of the unit (Fig. 11B). There is no discernable relationship between the abundance of calcite and quartz cement and sealing capacity over the sampled interval of the HDS (Figs. 8 and 11C). Those samples with the highest sealing capacity, the Dunkirk black shale, contain only minor calcite and quartz cement, whereas Hanover shale samples contain as much as 10% cement (Figs. 8 and 11C). Indeed, that Hanover shale sample defined by the lowest seal capacity (920 psia) contains the highest amount of cement (Figs. 8 and 11C). Clay mineral diagenesis has the potential to affect porosity and fluid migration in compacting sediments (e.g. Bruce, 1984; Shaw and Primmer, 1989; Bjørlykee and Høeg, 1997; Geir, 2000). In particular, the conversion of smectite to mixed-layer illite–smectite and illite can result in the occlusion of pore throats (Selley, 1998). Thus, widespread neoformation of clay minerals in the HDS following NS jointing could have affected petrophysical properties of these rocks, including sealing capacity, porosity, and pore throat sorting. Hosterman (1993) demonstrated that illite comprises the bulk (40–90%) of the clay mineral fraction of Devonian black shales of the Appalachian Basin. XRD analyses of six HDS samples are consistent with Hosterman’s observations; illite is by far the most abundant clay mineral in both the gray and black shale (72–80%), followed by chlorite (11–14%), kaolinite (3–7%), and finally, mixed-layer illite–smectite (2–7%). However, rather than reflecting a high degree of thermal maturation, the plentiful illite may tell of a source terrane rich in illite. Strickler and Ferrell (1989), for example, argued that the great abundance of illite in Louisiana Gulf Coast clays reflects derivation from the Appalachian orogen. Moreover, the location of the HDS along the Lake Erie shoreline places these deposits in a region of inferred very low illite crystallinity (Hosterman, 1993), which is consistent with the low measured vitrinite reflectance values of these deposits (0.55–0.62%; Lash et al., 2004). Finally, in their analysis of clay mineral diagenesis in North Sea deposits, Pearson and Small (1988) demonstrated that illitization occurs at depths of 2.4–3.5 km, within a vitrinite reflectance range of 0.54 and 0.72% and a temperature range of 87–100 8C. The modeled maximum burial depth and temperature of the Dunkirk shale based on a measured vitrinite reflectance of 0.62% and assuming a geothermal gradient of 30 8C/km is 2.3 km and 88 8C, respectively, (Lash et al., 2004), barely at the minimum threshold depth and temperature of illitization in the North Sea. The contention that the Dunkirk shale failed to reach thermal levels high enough for pervasive illitization appears to be confirmed by a lack of textural evidence of widespread clay mineral diagenesis in studied HDS samples. For example, the presence of K-feldspar (0.9–1.5%) in all analyzed HDS shale samples suggests that conversion of smectite to illite–smectite had not resulted in the total loss of feldspar. Moreover, the several K-feldspar grains observed in the backscattered electron mode show no evidence of dissolution, which typically accompanies illitization (Eberl and Hower, 1976; Pearson and Small, 1988; Shaw and Primmer, 1989, 1991). 327 Shaw and Primmer (1991) illustrated that burial-related neoformation of clay minerals in the Kimmeridge Claystone Formation occurred primarily in microfossil tests and/or voids within partially dissolved minerals, notably K-feldspar, rather than within the clay matrix. No microfossil tests have been observed in HDS samples; indeed, the only voids that might have filled with authigenic clay minerals are Tasmanites cysts, many of which were compressed early in the mechanical compaction history of these deposits (e.g. see Fig. 14B). Those cysts that filled during diagenesis contain pyrite framboids and crystallites to the exclusion of clay minerals. In sum, then, clay mineral diagenesis appears to have been minimal in HDS samples, the abundant illite being more a record of source terrane mineralogy than the level of thermal maturity. No doubt, some burial-related neoformation of clay minerals, especially chlorite and illite–smectite, has occurred in these rocks. Indeed, it may be that the very small amount of mixedlayer illite–smectite in analyzed HDS samples reflects the conversion of minimal detrital smectite during burial diagenesis. However, the likely mode of diagenetic mineral growth in HDS samples, conversion of detrital grains such as K-feldspar and smectite, would not have resulted in appreciable post-NS jointing modification of capillary properties of these rocks, assuming that diagenesis occurred after jointing. Microscopic analysis of Hanover gray shale samples confirms the pervasively bioturbated nature of these deposits based on field observations (e.g. Baird and Lash, 1990) and the study of polished slabs. Notably, thin section microscopy reveals detrital silt grains distributed throughout a mottled clay matrix, the result of homogenization of the originally laminated sediment by burrowing organisms (Fig. 12A); rare silt laminae depict the disruptive effects of bioturbation (Fig. 12B). SEM observations show an open microfabric of variably oriented platy grains (Fig. 12C). Angular silt grains dispersed throughout the ‘swirled’ clay matrix appear to have propped open larger voids and pore throats during compaction precluding complete reorientation of the disrupted fabric (Fig. 12D). In addition to a more or less even distribution of silt grains throughout the clay matrix, detrital silt grains also comprise mottles, likely filled burrows (Fig. 12E). The general lack of primary structures and discrete burrow traces in field exposures and thin sections suggests a level of bioturbation equal to ichnofabric index 5 or 6 of Pemberton et al. (1992). Petrographic and SEM examination of shale samples collected from the base to the top of the Dunkirk shale along the Walnut Creek section reveals the unit to be dominated by two shale lithotypes: laminated pyritic black shale and moderately bioturbated black shale (Lash and Engelder, 2005). The former, which dominates the lower third to half of the Dunkirk shale, is characterized by generally continuous thin to thick (O0.1 mm) quartz silt laminae that alternate with dark, silt-poor carbonaceous clay layers (Fig. 13A). Scanning electron microscopy shows the organic-rich clay layers to be defined by a tight, bedding-parallel arrangement of clay grains and flattened organic particles, locally disrupted by angular silt grains and pyrite framboids (Fig. 13B and C). These rocks show no evidence of bioturbation and thus, are classified as 328 G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 Fig. 12. Microfabric of the Hanover shale: (A) thin section image showing mottled texture and evenly dispersed silt grains (scaleZ0.1 mm); (B) thin section image of disrupted (bioturbated?) silt laminae (scaleZ0.1 mm); (C) secondary electron image of the open, random microfabric typical of the gray shale; (D) secondary electron image of angular detrital silt grains (s) and moderately planar microfabric in gray shale; (E) thin section image of a silt-filled mottle in gray shale (scaleZ0.1 mm). ichnofabric index 1 of Pemberton et al. (1992). The moderately bioturbated black shale, most common to the upper part of the Dunkirk shale, lacks the finely laminated structure described above; instead, angular silt grains are distributed throughout the organic-rich clay matrix (Fig. 13D). SEM observations reveal an open to moderately planar clay grain microfabric (Fig. 13E). Disrupted silt laminae and/or flattened silt-filled burrows (Fig. 13F) indicate that the sediment was partially reworked by burrowing organisms (ichnofabric index 2 or 3, Pemberton et al., 1992). G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 329 Fig. 13. Microfabric of the Dunkirk shale: (A) thin section image of interlaminated silt and organic-rich clay (scaleZ0.5 mm); (B) secondary electron micrograph of the planar microfabric of a clay laminae sample; (C) secondary electron micrograph showing compacted clay grains wrapping a pyrite framboid in a clay laminae sample; (D) thin section image of bioturbated silt laminae and dispersed silt grains in clay laminae (scaleZ0.5 mm); (E) secondary electron image showing a large, angular detrital quartz silt grain supported by a matrix of randomly oriented clay grains. The open clay microfabric in this sample is more likely a consequence of bioturbation rather than the shielding effect of this single large quartz grain; (F) thin section image of bioturbated silt laminae and/or flattened silt-filled burrows and abundant dispersed silt grains (scaleZ0.5 mm). Hanover gray shale samples contain uniformly low amounts of organic matter, typically !0.75% TOC (Fig. 8). The organic carbon content of the Dunkirk shale is highest at its base (TOCZ4.67%), diminishing upward through the unit (Fig. 8). Thin section and SEM analysis shows that much of the organic matter in the Dunkirk samples has been flattened by mechanical compaction (Fig. 14A), especially in the high sealing capacity basal interval of the unit where most analyzed organic particles have aspect ratios O10 (Fig. 15). The moderately bioturbated, less organic-rich (TOC!2.3%) deposits higher in the Dunkirk shale are defined by a sealing capacity higher than that of the most resistant Hanover sample but w25% lower than the sealing capacity of those samples 330 G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 Fig. 14. (A) secondary electron image of a clay laminae sample collected from the lower 2 m of the Dunkirk shale. Note planar microfabric and flattened organic particles, mostly Tasmanites cysts (white arrows); (B) thin section image of a silty, moderately bioturbated black shale sample collected from the upper 3 m of the Dunkirk shale. Note that the dark organic matter has been compressed around quartz grains (scaleZ0.1 mm). recovered from the carbonaceous basal interval of the Dunkirk shale (Figs. 8 and 11D). Although organic matter is not as abundant in samples recovered from the upper part of the Dunkirk shale, microscopic observations show that those organic particles present have been molded around inorganic matrix grains and into pore throats thereby enhancing the sealing capacity of these deposits (Fig. 14B). determinants of sealing capacity in the HDS include the nature of distribution of detrital silt grains (confined to discrete laminae versus randomly distributed throughout the clay matrix and/or concentrated in silt mottles), the degree of bioturbation, and TOC content. The highest sealing capacity deposits, the finely laminated organic-rich lower Dunkirk shale, are not bioturbated. The undisrupted condition of these deposits enabled the sediment, especially the clay layers, to undergo relatively rapid mechanical compaction and consequent reorientation of originally flocculated clay flakes into a bedding-parallel planar microfabric (e.g. O’Brien, 1995; Lash and Blood, 2004a). The loss of porosity early in the diagenetic history of these deposits probably accounts for their low porosity and minimal cement. Mechanical compaction of clay laminae was accompanied by the flattening of abundant ductile organic particles into void spaces thereby further reducing pore throat diameters. Recently, Worden et al. (2005) argued that the planar microfabric of illite-rich mudstones can be produced as a consequence of the replacement of smectite by illite. Several lines of evidence argue against the development of the anisotropic microfabric observed in Dunkirk shale samples as a result of illitization. As noted earlier, the inferred low level of thermal stress and relatively shallow depth of burial of the Dunkirk shale is not consistent with widespread illite neoformation. Further, Lash and Blood (2004a) described very open microfabrics typical of flocculated clay preserved in strain shadows adjacent to early formed carbonate concretions in Upper Devonian black shale of western New York State. Laterally equivalent shale samples collected 0.2–0.3 m from the strain shadows, however, are defined by a strongly anisotropic bedding-parallel microfabric suggesting that the microfabric was produced by mechanical compaction of flocculated clay. Finally, illite is equally abundant in those Hanover samples analyzed by XRD as in Dunkirk shale samples, yet the former shows only minimal to moderate planar grain alignment. 7. Discussion Shale units are important barriers to fluid flow in sedimentary basins and serve as effective top seals to the majority of known petroleum reservoirs (Dawson and Almon, 1999, 2002). Nevertheless, these deposits have yet to receive a level of study commensurate with their crucial role in the petroleum system. The well-exposed HDS provides an excellent opportunity to further our understanding of shale top seals over a range of scales, from that of their microfabric to their position within a sequence stratigraphic framework. Those factors that appear to have been the most crucial Fig. 15. Frequency plot of aspect ratios of organic particles in samples collected from the lower 3 m of the Dunkirk shale. Aspect ratios were estimated first by measuring the long and short dimensions of organic particles and then by calculating the ratio of the former to the latter. G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 The comparatively low sealing capacities of the Hanover shale samples reflect the high-degree of bioturbation and related sediment disruption visited on these deposits. Moreover, rigid detrital silt grains, redistributed throughout the clay matrix by burrowing organisms, resisted platy grain reorientation during mechanical compaction by shielding or propping open larger pore throats thereby imparting a higher porosity and permeability to these deposits (e.g. Krushin, 1997; Katsube and Williamson, 1998; Dewhurst et al., 1998). The wide variation in sealing capacity and port throat size and sorting of analyzed Hanover samples is probably more a function of the nature of silt distribution— i.e. mottled versus evenly disseminated—than any other factor. The relatively open microfabric of black shale higher in the Dunkirk shale, a consequence of the moderate level of bioturbation sustained by these deposits, is manifested by a lower sealing capacity than that of the finely laminated carbonaceous strata of the lower Dunkirk shale. The reduction in sealing capacity upward through the Dunkirk shale probably reflects the complex interrelationship of bioturbation and detrital silt and TOC content. The apparent increase in silt toward the top of the unit and, perhaps more importantly, the moderate level of bioturbation and related redistribution of detrital silt grains displayed by these deposits likely contributed to their reduced sealing capacity. However, although the organic carbon content of these deposits is half that of the basal strata, enough disseminated ductile organic matter was 331 forced into pore throats during mechanical compaction to maintain a sealing capacity in excess of the tightest Hanover sample. Dawson and Almon (1999, 2002) and Sutton et al. (2004) make a compelling case for the strong relationship between the depositional environment of a shale lithotype, as expressed in its sequence stratigraphic position, and its sealing capacity. Upper transgressive systems tract (TST) and condensed sequence (CS) shales form the best seals, they suggest; coarser-grained high-stand systems tract (HST) and lowstand systems tract (LST) shales have somewhat lower sealing capacities. The general lack of primary structures in rocks of the shale-dominated HDS precludes easy environmental interpretation; still, the sequence stratigraphic interpretation offered here (Fig. 16) accords in general terms with Smith and Jacobi’s (2001) sequence stratigraphic framework of the Upper Devonian stratigraphic interval w80 km to the east in a more shallow region of the basin. The Hanover shale likely accumulated under dysoxic conditions, which would have favored the extensive bioturbation and, ultimately, the relatively low sealing capacity of these deposits. The strongly bioturbated nature of the Hanover shale suggests that bioturbation kept pace with sediment accumulation, perhaps as hemiturbidites (e.g. Stow and Wetzel, 1990; Stow and Tabrez, 1998). The Hanover shale, by virtue its moderate increase in silt content and decrease in the frequency of black shale beds upward from its contact with the Pipe Creek shale is Fig. 16. Sequence stratigraphic interpretation of the Hanover–Dunkirk sequence. Refer to text for details. 332 G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 ascribed to a HST sequence (Fig. 16). The scarcity of black shale and the subtle increase in the frequency of calcareous siltstone beds near the top of the Hanover shale (see Fig. 2) may reflect accumulation of these deposits as part of a lowstand systems wedge (Posamentier and Vail, 1988), though limited data makes this interpretation difficult at best. Baird and Brett (1991) interpret the Hanover–Dunkirk contact, a submarine erosional disconformity, as a marine flooding surface that formed following accumulation of CS/TST deposits (Fig. 16). The erosional episode reflected in the disconformity may have resulted in the loss of CS/TST deposits leaving only the w1-m-thick interval of interbedded black and gray shale immediately beneath the contact (see Fig. 2). It is noteworthy that the depth of erosion diminishes to the east (Baird and Brett, 1991). Accumulation of the Dunkirk shale above the marine flooding surface probably signaled the onset of a high-stand interval (Fig. 16; Posamentier and Vail, 1988). The lack of bioturbation and high TOC of the lower Dunkirk shale, the most effective seal deposits in the HDS, is reflective of an anoxic depositional environment that favored the preservation of the finely laminated character of these deposits and the abundant organic matter. Gravitational compaction of clay laminae into a tight, beddingparallel microfabric and the molding of abundant ductile organic matter into pore throats occurred soon after deposition. Higher in the Dunkirk HST, a moderate level of bioturbation, likely the result of increased dissolved O2, disrupted the sediment fabric thereby diminishing the sealing capacity of these deposits (Fig. 16). However, compaction-induced squeezing of disseminated organic matter into void spaces of the sediment framework contributed to a sealing capacity manifestly higher than that of the most resistant Hanover shale sample. The 17-m-thick Dunkirk shale is not an especially thick seal by comparison with top seals documented from modern basins. Fluid retention in the post-Dunkirk sedimentary column may have occurred well up into the Gowanda shale, yet the most effective part of the seal was the Dunkirk shale, especially its lower interval. Deming (1994) demonstrated that the time over, which a seal may persist before failing by capillary leakage is proportional to the square of the seal thickness and inversely proportional to the permeability of the seal as per the following expression t Z ðz2 =kÞ !ð2:4 !10K27 Þ; (1) where t is the maximum duration of the seal in million years, z is the seal thickness in meters, and k is permeability. Assuming that the Dunkirk shale had been compacted to its present 17-m thickness by the time it began to prevent vertical migration of fluids and using a mean permeability of 2.6!10K21 m2 for the Dunkirk shale based on MICP measurements indicates that the black shale seal would have been compromised after the unrealistic time of only 270 years. Doubling the thickness of the Dunkirk shale extends the duration of seal integrity only to w1000 years. The estimates of seal duration of the Dunkirk shale cited above are well shy of what would be considered by most to be a geologic interval of time (e.g. Deming, 1994), especially in light of the fact that natural hydraulic fractures were generated before capillary leakage occurred. Confinement of overpressured fluids by the 17-m-thick Dunkirk shale for 1 MY would have required a permeability of less than 7!10K25 m2, two orders of magnitude lower than the lowest measured shale permeabilities (Neuzil, 1994, 1995). The extraordinarily high sealing capacity of the Dunkirk shale, beyond that reflected in its petrophysical and textural/microfabric characteristics, may have been a response to the generation of biogenic methane within these organic-rich deposits. Gas capillary seals, especially durable barriers to vertical fluid flow, form in layered sequences of variable grain size and in the presence of free methane (Revil et al., 1998, 1999; Shosa and Cathles, 2001). Such seals can form very early in the burial history of sediment and at relatively shallow depths. Indeed, Revil et al. (1998, 1999) demonstrate that Pp at ODP Site 975 in the Western Mediterranean at a sub-seafloor depth of only 170– 240 m, immediately below an inferred gas capillary seal, equals lithostatic pressure thereby placing the sediment in a condition of incipient open mode fracture. The accumulation of sediment tends to drive pore water vertically through interconnected pores. When methane is produced by decomposition of organic matter, upward flow involves both water and methane. The methane accumulates preferentially in coarser grained (silt, sand) layers while the water is confined to clay layers (Revil et al., 1998, 1999; Deming et al., 2002). After enough methane has accumulated in a coarse-grained layer to form an interconnected phase, the flow of both methane and water is halted and a nearimpermeable gas capillary seal is formed at the layer interface. A pressure differential (the capillary entry pressure) that is partly a function of the respective pore throat radii of the fine and coarse grained sediment must be overcome (Revil et al., 1998) for the gas to invade the overlying fine-grained layer. However, the great strength of the gas capillary seal comes not from the pressure differential across individual layer interfaces, but rather from the sum of the capillary pressure drops across each interface (Revil et al., 1998; Cathles, 2001; Shosa and Cathles, 2001). Indeed, Cathles (1996) maintains that gas capillary seals can only be compromised by natural hydraulic fracturing. The concentration of NS-trending joints in the upper part of the Hanover gray shale suggests that natural hydraulic fracturing was not linked to the thermal generation of hydrocarbons in the Dunkirk shale, which is the case for subsequent joints preferentially formed in black shale units of the Catskill Delta Complex during the Alleghanian orogeny (Lash et al., 2004). Indeed, the NS joints probably formed before the Dunkirk shale was buried deep enough to produce thermally generated hydrocarbons (Lash et al., 2004). Several aspects of the Dunkirk shale favor the relatively early and shallow formation of gas capillary seals within this organicrich unit. Most notably, the interlaminated siltstone and claystone, especially in the lower part of the Dunkirk shale, would have provided a framework within which to segregate biogenic methane and water. Still, the key ingredient in the development of a gas capillary seal is methane, which, upon sequestration in coarse-grained layers, causes a dramatic G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 reduction in permeability across sedimentary interfaces (Revil et al., 1998; Shosa and Cathles, 2001; Deming et al., 2002). Results of stable carbon isotope analysis of carbonate concretions in the Upper Devonian Rhinestreet black shale of western New York State (see Fig. 1) suggest that the carbon necessary to sustain the growth of the large concretions originated within the zone of biogenic methanogenesis (Lash and Blood, 2004b). Similarly, moderately depleted carbon isotope values of carbonate samples collected from Dunkirk shale concretions (d13CZK11.2‰ PDBG2.4‰; nZ4) probably reflects the generation of biogenic methane in the organicrich host sediment. 8. Conclusions The restriction of vertical natural hydraulic fractures to the contact of the Upper Devonian Hanover gray shale and overlying Dunkirk black shale of the western New York Appalachian Plateau indicates that the latter was a top seal to overpressured fluids early in the Alleghanian orogeny. The generation of open mode joints in this relatively narrow stratigraphic interval comports with known pressure/depth gradients and in situ stresses documented from modern basins, notably the Central Graben of the North Sea where the Kimmeridge claystone formation serves as a top seal inhibiting the movement of overpressured fluids from the underlying pressure cell. Episodic natural hydraulic fracturing at the Hanover–Dunkirk contact suggests that the organic-rich shale was a hydraulic seal that halted the vertical migration of formation fluids. At length, Pp at the top of the Hanover shale reached the fracture gradient resulting in the propagation of natural hydraulic fractures before the capillary entry pressure of the Dunkirk shale was exceeded. The inferred high sealing capacity of the Dunkirk shale based on the preferential development of the NS-trending natural hydraulic fractures is confirmed by consistently high (O10,000 psia) 10% mercury saturation pressures for samples collected from the base and top of the Dunkirk. The high sealing capacity of the Dunkirk shale, especially its basal interval, reflects its finely laminated nature, the confinement of most detrital silt to discrete laminae, the lack of bioturbation, and high organic carbon content. The somewhat lower sealing capacity of moderately bioturbated, less carbonaceous deposits that comprise much of the upper half of the Dunkirk shale confirms the importance of both bioturbation and TOC content as determinants of sealing capacity. Ultimately, these factors are linked inextricably to the depositional environment and sequence stratigraphic position of each shale lithotype. The anoxic depositional environment of the lower part of the Dunkirk shale, inferred early HST deposits, favored the preservation of abundant organic matter and the finely laminated depositional structure of the sediment. The lack of bioturbation enabled these deposits, especially the clay-rich laminae, to undergo rapid mechanical compaction. Compaction-induced squeezing of abundant ductile organic matter into void spaces further reduced pore throat diameters. Immediately underlying heavily bioturbated HST deposits of 333 the organic-lean Hanover shale accumulated in a dysoxic depositional environment. Disruption of layering and homogenization of the sediment by burrowing organisms yielded a more porous and permeable microfabric through which formation fluids ascended the sediment column only to be arrested by the high capillary entry pressure of the basal Dunkirk shale. Petrophysical properties of the organic-rich shale, especially the laminated high TOC basal interval of the Dunkirk shale, were enhanced by the generation of biogenic methane, which resulted in the formation of a near impermeable gas capillary seal that could be compromised only by natural hydraulic fracturing. Results of this study suggest that while organic-rich HST and CS/TST deposits may serve as strong top seals, they are also prone to compromise by natural hydraulic fracturing. Indeed, episodic hydraulic fracturing of these deposits and resultant leakage may manifest itself in the form of gas chimneys, especially over positive structural elements such as uplifted blocks and salt diapers (Holm, 1996). Acknowledgements Randy Blood is thanked for his help in the field and in the microscopic analysis of shale samples. Peter Bush and his staff at the University of Buffalo, South Campus Instrumentation Center, School of Dental Medicine, are acknowledged for their help with the scanning electron microscopy. This paper benefited from the comments of the anonymous reviewers. References Almon, W.R., Thomas, J.B., 1991. Pore system aspects of hydrocarbon trapping. In: Gluskoter, H.J., Rice, D.D., Taylor, R.B. (Eds.), Economic Geology. US Geological Society of America, P-2. The Geology of North America, pp. 241–254. Aplin, A.C., Yang, Y., Hansen, S., 1995. Assessment of b, the compression coefficient of mudstones and its relationship with detailed lithology. Marine and Petroleum Geology 12, 955–963. Baird, R.A., 1986. Maturation and source rock evaluation of the Kimmeridge Clay, Norwegian Sea. AAPG Bulletin 70, 1–11. Baird, G.C., Brett, C.E., 1991. Submarine erosion on the anoxic sea floor: stratinomic, palaeoenvironmental, and temporal significance of reworked pyrite-bone deposits. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Continental Shelf Anoxia. Geological Society, London. Special Publication 58, pp. 233–257. Baird, G.C., Lash, G.G., 1990. Devonian strata and environments: Chautauqua County region: New York State. New York State Geological Association, 62nd Annual Meeting Guidebook, pp. Sat. A1–A46. Bjørlykee, K., Høeg, K., 1997. Effects of burial diagenesis on stresses, compaction and fluid flow in sedimentary basins. Marine and Petroleum Geology 14, 267–276. Boult, P.J., 1993. Membrane seal and tertiary migration pathways in the Bodalla south oilfield, Eromanga basin, Australia. Marine and Petroleum Geology 10, 3–13. Bruce, C.H., 1984. Smectite dehydration—its relation to structural development and hydrocarbon accumulation in northern Gulf of Mexico basin. AAPG Bulletin 68, 673–683. Caillett, G., 1993. The caprock of the Snorre field, Norway: a possible leakage by hydraulic fracturing. Marine and Petroleum Geology 10, 42–50. Cathles III, L.M., 1996. Gas transport of oil: its impact on sealing and the development of secondary porosity. Gas Research Institute, Contract No. 5093-260-2689, Annual Report; July 1994–June 1995, 35 pp. 334 G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 Cathles III, L.M., 2001. Capillary seals as a cause of pressure compartmentation in sedimentary basins. In: Filton, R.H. et al. (Eds.), Petroleum Systems of Deep-Water Basins: Global and Gulf of Mexico Experience, 21st Annual Bob F. Perkins Research Conference [CD-ROM]. Gulf Coast Section. Society of Economic Paleontologists and Mineralogists, Houston, TX, pp. 561–571. Darby, D., Hazeldine, R.S., Couples, G.D., 1996. Pressure cells and pressure seals in the UK Central Graben. Marine and Petroleum Geology 13, 865–878. Dawson, W.C., Almon, W.R., 1999. Top seal character and sequence stratigraphy of selected marine shales in Gulf Coast style basins. Gulf Coast Association of Geological Societies Transactions 49, 190–197. Dawson, W.C., Almon, W.R., 2002. Top seal potential of Tertiary deep-water Gulf of Mexico shales. Gulf Coast Association of Geological Societies Transactions 52, 167–176. Deming, D., 1994. Factors necessary to define a pressure seal. AAPG Bulletin 78, 1005–1009. Deming, D., Cranganu, C., Lee, Y., 2002. Self-sealing in sedimentary basins. Journal of Geophysical Research 107 (B12), 2329 (10.1029/2001JB000504). Dewhurst, D.N., Aplin, A.A., Sarda, J.-P., Yang, Y., 1998. Compaction-driven evolution of porosity and permeability in natural mudstones: a experimental study. Journal of Geophysical Research 103, 651–661. Downey, M.W., 1984. Evaluating seals for hydrocarbon accumulations. AAPG Bulletin 68, 1752–1763. Eberl, D.D., Hower, J., 1976. The kinetics of illite formation. GSA Bulletin 87, 1326–1330. Engelder, T., Fischer, M.P., 1996. Loading configurations and driving mechanisms for joints based on the Griffith energy-balance concept. Tectonophysics 256, 253–277. Engelder, T., Lacazette, A., 1990. Natural hydraulic fracturing. In: Barton, N., Stephansson, O. (Eds.), Rock Joints: Proceedings of the International Symposium on Rock Joints, Balkema, Brookfield, Rotterdam, pp. 35–43. Engelder, T., Oertel, G., 1985. The correlation between undercompaction and tectonic jointing within the Devonian Catskill Delta. Geology 13, 863–866. Engelder, T., Loewy, S.L., Hagin, P., 1998. Sorting out the role of organic carbon content in maintaining overpressure. Evidence based on joint development within Devonian black shales in the Catskill Delta. In: Mitchell, A., Grauls, D. (Eds.), Overpressures in Petroleum Exploration Bulletin Centre Rech. Elf Explor. Prod., Memoir vol. 22, pp. 33–35. Ettensohn, F.R., 1985. The Catskill Delta complex and the Acadian orogeny: a model. In: Woodrow, D.L., Sevon, W.D. (Eds.), The Catskill Delta. Geological Society of America Special Paper 201, pp. 39–49. Ettensohn, F.R., 1992. Controls on the origin of the Devonian–Mississippian oil and gas shales, east-central United States. Fuel 71, 1487–1492. Faill, R.T., 1985. The Acadian orogeny and the Catskill Delta. In: Woodrow, D.L., Sevon, W.D. (Eds.), The Catskill Delta. Geological Society of America Special Paper 201, pp. 15–37. Fischer, M., Gross, M.R., Engelder, T., Greenfield, R.J., 1995. Finite element analysis of the stress distribution around a pressurized crack in a layered elastic medium: implications for the spacing of fluid-driven joints in bedded sedimentary rock. Tectonophysics 247, 49–64. Gaarenstroom, L., Tromp, R.A.J., de Jong, M.C., Brandenburg, A.M., 1993. Overpressures in the Central North Sea: implications for trap integrity and drilling safety. In: Parker, J.R. (Ed.), Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Geological Society, London, pp. 1305–1313. Geir, S., 2000. Clay mineral and organic diagenesis of the Lower Oligocene Schöneck Fishshale, western Austrian Molasse Basin. Clay Minerals 35, 709–717. Holm, G.M., 1996. The central graben: a dynamic overpressure system. In: Glennie, K., Hurst, A. (Eds.), AD1995: NW Europe: Hydrocarbon Industry. Geological Society, London, pp. 107–122. Holm, G.M., 1998. Distribution and origin of overpressures in the central graben of the north Sea. In: Law, B.E., Ulmishek, G.F., Slavin, V.I. (Eds.), Abnormal Pressures in Hydrocarbon Environments AAPG Memoir vol. 70, pp. 123–144. Hosterman, J.W., 1993. Illite crystallinity as an indicator of the thermal maturity of Devonian black shales in the Appalachian basin. USGS Bulletin 1909, G1–G9. Hudak, P.F., 1992. Terminal decollement tectonics in the Appalachian Plateau of northwestern Pennsylvania. Northeastern Geology 14, 108–112. Jennings, J.J., 1987. Capillary pressure techniques: application to exploration and development geology. AAPG Bulletin 71, 1196–1209. Johnson, J.G., Klapper, G., Sandberg, C.A., 1985. Devonian eustatic fluctuations in Euramerica. GSA Bulletin 96, 567–587. Katsube, T.J., Williamson, M.A., 1998. Shale petrophysical characteristics: permeability history of subsiding shales. In: Scheiber, J., Zimmerle, W., Sethi, P. (Eds.), Shales and Mudstones. I.E. Schwiezerbart’sche, Stuttgart, pp. 69–91. Krushin, J.T., 1997. Seal capacity of non-smectite shale. In: Surdam, R.C. (Ed.), Seals, Traps, and the Petroleum System AAPG Memoir 67, pp. 31–67. Lacazette, A., Engelder, T., 1992. Fluid-driven cyclic propagation of a joint in the Ithaca siltstone. In: Evans, B., Wong, T.-F. (Eds.), Fault Mechanics and Transport Properties of Rocks. Academic Press, London, pp. 297–324. Ladeira, F.L., Price, N.J., 1981. Relationship between fracture spacing and bed thickness. Journal of Structural Geology 3, 179–183. Lash, G.G., Blood, D.R., 2004a. Depositional clay fabric preserved in early diagenetic carbonate concretion pressure shadows, upper Devonian (Frasnian) Rhinestreet shale, western New York. Journal of Sedimentary Research 74, 110–116. Lash, G.G., Blood, D.R., 2004b. Geochemical and textural evidence for early diagenetic growth of stratigraphically confined carbonate concretions, upper Devonian Rhinestreet black shale, western New York. Chemical Geology 206, 407–424. Lash, G.G., Engelder, T., 2005. An analysis of horizontal microcracking during catagenesis: an example from the Catskill delta complex. AAPG Bulletin 89, 1433–1449. Lash, G.G., Loewy, S., Engelder, T., 2004. Preferential jointing of upper Devonian black shale, Appalachian Plateau, USA: evidence supporting hydrocarbon generation as a joint-driving mechanism. In: Cosgrove, J., Engelder, T. (Eds.), The Initiation, Propagation, and Arrest of Joints and Other Fractures. Geological Society, London. Special Publications, 231, pp. 129–151. Leonard, R.C., 1993. Distribution of sub-surface pressure in the Norwegian Central Graben and applications for exploration. In: Parker, J.R. (Ed.), Petroleum Geology of Northwest Europe: Proceedings of the Fourth Conference. Geological Society, London, pp. 1295–1303. Loewy, S., 1995. The post-Alleghanian tectonic history of the Appalachian Basin based on joint patterns in Devonian black shales. MS thesis, Pennsylvania State University, University Park, Pennsylvania, 179 pp. Lundegard, P.D., Samuels, N.O., 1980. Field classification of fine-grained sedimentary rocks. Journal of Sedimentary Research 50, 781–786. Luo, X., Vasseur, G., 1997. Sealing efficiency of shales. Terra Nova 9, 71–74. McConaughy, D.T., Engelder, T., 1999. Joint interaction with embedded concretions: joint loading configurations inferred from propagation paths. Journal of Structural Geology 21, 1637–1652. Neuzil, C.E., 1994. How permeable are clays and shales? Water Resources Research 30, 145–150. Neuzil, C.E., 1995. Abnormal pressures as hydrodynamic phenomena. American Journal of Science 295, 742–786. O’Brien, N.R., 1995. Origin of shale fabric—clues from framboids. Northeastern Geology and Environmental Sciences 17, 146–150. O’Brien, N.R., Slatt, R.M., 1990. Argillaceous Rock Atlas. Springer, New York. 141 pp.. Olson, J., Pollard, D.D., 1989. Inferring paleostress from natural fracture patterns: a new method. Geology 17, 345–348. Pearson, M.J., Small, J.S., 1988. Illite-smectite diagenesis and palaeotemperatures in northern north Sea Quaternary to Mesozoic shale sequences. Clay Minerals 23, 111–132. G.G. Lash / Marine and Petroleum Geology 23 (2006) 317–335 Pemberton, S.G., Frey, W.R., Ranger, M.J., MacEachern, J., 1992. The conceptual framework of ichnology. In: Pemberton, S.G., et al. (Eds.). Applications of Ichnology to Petroleum Exploration. SEPM Core Workshop No. 17, pp. 1–32. Posamentier, H.W., Vail, P.R., 1988. Eustatic controls on clastic deposition II—sequence and sequence tract models. In: Wilgus, C.K., Hastings, B.S., Posamentier, H.W., van Wagoner, J.C., Ross, C.A., Kendall, C.G.S.J.C. (Eds.), Sea Level Changes: an Integrated Approach. SEPM, Special Publication 42, pp. 125–154. Revil, A., Cathles III., L.M., Shosa, J.D., Pezard, P.A., de Larouzière, F.D., 1998. Capillary sealing in sedimentary basins: a clear field example. Geophysical Research Letters 25, 389–392. Revil, A., Pezard, P.A., de Larouzière, F.D., 1999. Fluid overpressures in western Mediterranean sediments, sites 974–979. In: Zahn, R., Comas, M.C., Klaus, A. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, 161. Ocean Drilling Program, College Station, TX, pp. 117–128. Roberts, S.J., Nunn, J.A., 1995. Episodic fluid expulsion from goepressured sediments. Marine and Petroleum Geology 12, 195–204. Rudd, M., Pandey, G.N., 1973. Threshold pressure profiling by continuous injection. American Institute of Mining, Metallurgy and Petroleum— Society of Petroleum Engineers, Paper 4597, 7 pp. Schlömer, S., Krooss, B.M., 1997. Experimental characterization of the hydrocarbon sealing efficiency of cap rocks. Marine and Petroleum Geology 14, 565–580. Schowalter, T.T., 1979. Mechanics of secondary hydrocarbon migration and entrapment. AAPG Bulletin 63, 723–760. Selley, R.C., 1998. Elements of Petroleum Geology, second ed. Academic Press, New York, 470 pp. Shaw, H.F., Primmer, T.J., 1989. Diagenesis in shales from a partly overpressured sequence in the Gulf Coast, Texas, USA. Marine and Petroleum Geology 6, 121–128. Shaw, H.F., Primmer, T.J., 1991. Diagenesis of mudrocks from the Kimmeridge Clay formation of the Brae area, UK north Sea. Marine and Petroleum Geology 8, 270–277. 335 Shosa, J.D., Cathles III, L.M., 2001. Experimental investigation of capillary blockage of two phase flow in layered porous media. In: Filton, R.H., et al. (Eds.), Petroleum Systems of Deep-Water Basins. Global and Gulf of Mexico Experience, 21st Annual Bob F. Perkins Research Conference [CDROM]. Gulf Coast Section, Society of Economic Paleontologists and Mineralogists, Houston, TX, pp. 725–739. Smith, G.J., Jacobi, R.D., 2001. Tectonic and eustatic signals in the sequence stratigraphy of the upper Devonian Canadaway group, New York state. AAPG Bulletin 85, 325–357. Stow, D.A.V., Tabrez, A.R., 1998. Hemipelagites: processes, facies and model. In: Stocker, M.S., Evans, D., Cramp, A. (Eds.), Geological Processes on Continental Margins: Sedimentation, Mass-wasting and Stability. Geological Society, London. Special Publications No. 129, pp. 317–337. Stow, D.A.V., Wetzel, A., 1990. Hemiturbidite: a new type of deep water sediment. In: Cochran, J.R., Stow, D.A.V., et al. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, 116. Ocean Drilling Program, College Station, TX, pp. 25–34. Strickler, M.E., Ferrell, R.E., 1989. Provenance and diagenesis of Upper Wilcox Formation clay minerals. Ninth International Clay Conference, Strasbourg, p. 379. Sutton, S.J., Ethridge, F.G., Almon, W.B., Dawson, W.C., Edwards, K.K., 2004. Textural and sequence-stratigraphic controls of lower and upper Cretaceous shales, Denver basin, Colorado. AAPG Bulletin 88, 1185–1206. Vavra, C.L., Kaldi, J.G., Sneider, R.M., 1992. Geological applications of capillary pressure: a review. AAPG Bulletin 76, 840–850. Watts, N.L., 1987. Theoretical aspects of cap-rock and fault seals for singleand two-phase columns. Marine and Petroleum Geology 4, 274–307. Werne, J.P., Sageman, B.B., Lyons, T.W., Hollander, D.J., 2002. An integrated assessment of a ‘type euxinic’ deposit: evidence for multiple controls on black shale deposition in the middle Devonian Oatka Creek formation. American Journal of Science 302, 110–143. Worden, R.H., Charpentier, D., Fisher, Q., Aplin, A.C., 2005. Porosity loss, fabric development and the smectite to illite transition in Upper Cretaceous mudstones from the North Sea: an image analysis approach. AAPG Annual Convention, Calgary, p. A155.