Top seal development in the shale-dominated Upper Devonian Catskill Delta Complex, western New York State
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, Dawson and Almon, 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, Dawson and Almon, 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 implications for hydrocarbon migration and entrapment as well as for exploration and production strategies. The study of well-exposed 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 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.
Section snippets
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, 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
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.,
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 leak-off test and repeat formation test results, from the Central
Methodology
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 >5 cm into exposures to minimize the effects of weathering. Analytical results are summarized in Table 1.
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 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 (
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, Dawson and Almon, 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
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
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.
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