Top seal development in the shale-dominated Upper Devonian Catskill Delta Complex, western New York State

https://doi.org/10.1016/j.marpetgeo.2006.02.001Get rights and content

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 fine-grained 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.

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.

References (75)

  • S. Schlömer et al.

    Experimental characterization of the hydrocarbon sealing efficiency of cap rocks

    Marine and Petroleum Geology

    (1997)
  • H.F. Shaw et al.

    Diagenesis in shales from a partly overpressured sequence in the Gulf Coast, Texas, USA

    Marine and Petroleum Geology

    (1989)
  • H.F. Shaw et al.

    Diagenesis of mudrocks from the Kimmeridge Clay formation of the Brae area, UK north Sea

    Marine and Petroleum Geology

    (1991)
  • N.L. Watts

    Theoretical aspects of cap-rock and fault seals for single- and two-phase columns

    Marine and Petroleum Geology

    (1987)
  • W.R. Almon et al.

    Pore system aspects of hydrocarbon trapping

  • R.A. Baird

    Maturation and source rock evaluation of the Kimmeridge Clay, Norwegian Sea

    AAPG Bulletin

    (1986)
  • G.C. Baird et al.

    Submarine erosion on the anoxic sea floor: stratinomic, palaeoenvironmental, and temporal significance of reworked pyrite-bone deposits

  • Baird, G.C., Lash, G.G., 1990. Devonian strata and environments: Chautauqua County region: New York State. New York...
  • K. Bjørlykee et al.

    Effects of burial diagenesis on stresses, compaction and fluid flow in sedimentary basins

    Marine and Petroleum Geology

    (1997)
  • C.H. Bruce

    Smectite dehydration—its relation to structural development and hydrocarbon accumulation in northern Gulf of Mexico basin

    AAPG Bulletin

    (1984)
  • G. Caillett

    The caprock of the Snorre field, Norway: a possible leakage by hydraulic fracturing

    Marine and Petroleum Geology

    (1993)
  • Cathles III, L.M., 1996. Gas transport of oil: its impact on sealing and the development of secondary porosity. Gas...
  • L.M. Cathles

    Capillary seals as a cause of pressure compartmentation in sedimentary basins

  • W.C. Dawson et al.

    Top seal character and sequence stratigraphy of selected marine shales in Gulf Coast style basins

    Gulf Coast Association of Geological Societies Transactions

    (1999)
  • W.C. Dawson et al.

    Top seal potential of Tertiary deep-water Gulf of Mexico shales

    Gulf Coast Association of Geological Societies Transactions

    (2002)
  • D. Deming

    Factors necessary to define a pressure seal

    AAPG Bulletin

    (1994)
  • D. Deming et al.

    Self-sealing in sedimentary basins

    Journal of Geophysical Research

    (2002)
  • D.N. Dewhurst et al.

    Compaction-driven evolution of porosity and permeability in natural mudstones: a experimental study

    Journal of Geophysical Research

    (1998)
  • M.W. Downey

    Evaluating seals for hydrocarbon accumulations

    AAPG Bulletin

    (1984)
  • D.D. Eberl et al.

    The kinetics of illite formation

    GSA Bulletin

    (1976)
  • T. Engelder et al.

    Natural hydraulic fracturing

  • T. Engelder et al.

    The correlation between undercompaction and tectonic jointing within the Devonian Catskill Delta

    Geology

    (1985)
  • T. Engelder et al.

    Sorting out the role of organic carbon content in maintaining overpressure. Evidence based on joint development within Devonian black shales in the Catskill Delta

  • F.R. Ettensohn

    The Catskill Delta complex and the Acadian orogeny: a model

  • R.T. Faill

    The Acadian orogeny and the Catskill Delta

  • Gaarenstroom, L., Tromp, R.A.J., de Jong, M.C., Brandenburg, A.M., 1993. Overpressures in the Central North Sea:...
  • S. Geir

    Clay mineral and organic diagenesis of the Lower Oligocene Schöneck Fishshale, western Austrian Molasse Basin

    Clay Minerals

    (2000)
  • Cited by (33)

    • The release of petroleum from Central Africa rift basins over geological time as deduced from petroleum systems modelling

      2021, Journal of African Earth Sciences
      Citation Excerpt :

      Methane seeped from the basin expelled more than 4340 tons per year from the Paleocene to Eocene (Table 4). Formations with potential to develop sealing capacity are more compacted during the Cenozoic than the Cretaceous, but still do not have the low porosity typical of shales with sealing capacity (Soeder, 1988; Lash, 2006; Wang and Reed, 2009; Aguilera, 2016). As was the case during the Upper Cretaceous, continued rifting in the Cenozoic would have generated accommodation space permitting further burial and compaction of sealing formations, but increased faulting and fracturing could also breach seals and lead to methane being lost via fractures (Roberts and Nunn, 1995; Løseth et al., 2009).

    • Bedding-parallel calcite veins as a proxy for shale reservoir quality

      2021, Marine and Petroleum Geology
      Citation Excerpt :

      In the oil-mature Upper Devonian Dunkirk Shale, western New York State (avg. 0.62% Ro; Lash and Engelder, 2005), horizontal micro-fractures are common in the interval with a TOC abundance range of 2.5–4.6 wt%, but they are generally absent in the interval where TOC <2.3 wt% (Lash, 2006). In the Los Molles Formation and Vaca Muerta Formation of Nerquén Basin, abundant bedding-parallel calcite veins are located in organic-rich (TOC up to 12 wt%) layers, but they are absent in adjacent organic-lean layers (Rodrigues et al., 2009; Gale et al., 2014).

    • Major factors affecting the closure of marine carbonate caprock and their quantitative evaluation: A case study of Ordovician rocks on the northern slope of the Tazhong uplift in the Tarim Basin, western China

      2017, Marine and Petroleum Geology
      Citation Excerpt :

      The caprock is one of the key and indispensable elements in the formation of oil and gas reservoirs. Gypsum salt rock (evaporite) and shale (fine clastic rock) are recognized as regional caprocks that control the formation of large oil and gas fields (Lash, 2006; Van Geet et al., 2008; Zhou et al., 2012; Li and Tong, 2012; Amann-Hildenbrand et al., 2013; Snedden, 2014). Statistical data show that in the sedimentary caprocks of the world, oil and gas fields with gypsum salt rock as the caprock account for only 8% of the oil and gas fields that have been evaluated but account for 55% of the total oil and gas reserves (Jin et al., 2010).

    • Geochemistry characteristics and significance of two petroleum systems near top overpressured surface in central Junggar Basin, NW China

      2016, Marine and Petroleum Geology
      Citation Excerpt :

      The deep formations generally contain post-diagenesis high overpressure systems (Hunt, 1990, 1991; Yang et al., 2008; He and Middleton, 2002; 2009). Most of oil and gas reserves discovered are distributed in the favorable hydrocarbon accumulation areas near top overpressured surface (abbreviation for TOS) (300 m above and 100 m below TOS) in most overpressured sedimentary basins worldwide (Tigert and Al-Shaieb, 1990; Leach, 1993a,b,c; Law and Spencer, 1998; Swarbrick and Schneider, 1999; Swarbrick et al., 2000; Thyne, 2001; He and Middleton, 2002; Yardley and Swarbrick, 2002; Lash, 2006; Wilkinson et al., 2006; Yang et al., 2010). Taking the Junggar Basin as an example, the geochemical and distribution characteristics of hydrocarbons in reservoirs near TOS are discussed in this paper.

    View all citing articles on Scopus
    View full text