Introduction: Stratigraphic framework:

Size: px

Start display at page:

Download "Introduction: Stratigraphic framework:"

Clare Taylor
5 years ago
Views:

987565 Deformation in the Appalachian Foreland: Detachment Structures in the Basal Marcellus Shale, Central New York Bruce Selleck 1 (1) Geology, Colgate University, Hamilton, NY, United States.

1 Deformation in the Appalachian Foreland: Detachment Structures in the Basal Marcellus Shale, Central New York Bruce Selleck 1 (1) Geology, Colgate University, Hamilton, NY, United States. Introduction: Décollement zones within the organic-rich black shale members of the basal Marcellus Formation in New York and Pennsylvania have been recognized for some time (Nickelsen, 1986; Evans, 1994, among others). Outcrop study of the upper Onondaga Formation-Marcellus Formation contact at three localities in the central New York outcrop belt (Figure 1) demonstrates that thrust fault structures are present. Given recent interest in the Marcellus as a gas shale reservoir, the relationships among faulting, fracturing, fluid migration, vein mineralization and hydrocarbon maturation in these zones may provide additional insights as resource development proceeds. In addition, the décollement zones are intensely fractured and may serve as attractive targets for horizontal well development. The thrust fault systems described here were active during hydrocarbon maturation in the Marcellus, and are most likely the result of regional deformation in the Appalachian Foreland during the Alleghanian Orogeny. Stratigraphic framework: Ver Straeten and Brett (2006) have revised the long-used formation-level status of the Marcellus Formation, which was traditionally considered one of four formations in the Hamilton Group. This new nomenclature raises the Marcellus to subgroup status within the Hamilton Group, and provides revised formation- and member-level subdivisions that are useful in the thicker, proximal portion of the sequence in the Hudson Valley region. These subdivisions of the Marcellus are difficult to define in the central New York outcrop belt, and boundaries are likewise are not clear in most well logs (Lash and Engelder, 2011). Hence, the formation-level status of the Marcellus, and member-level subdivisions of Rickard (1984), shown in Figure 2, are used in this report. Figure 1. Regional map showing location of exposures of thrust fault structures in the central New York outcrop belt of the Marcellus Formation.

2 The basal Marcellus Formation in the central New York region consists of the Union Springs Member, which conformably overlies the Seneca Member of the Onondaga Formation. The sudden clastic and carbonate sediment starvation, and accumulation of organic-rich muds was driven by the onset of foreland basin subsidence during Acadian Orogeny Tectophase II (Ettensohn, 1985, Werne, et al, 2002). The Union Springs Member, in thickness in the central New York area, contains the highest total organic carbon (TOC) facies, and has the most elevated gamma ray signature in the Devonian section in the region (Figure 3). The Union Springs consists of decimeter-scale interbeds of dark lime mudstone and sooty black shale. The Union Springs carbonate beds contain abundant pelagic (?) biogenic carbonate mudstone and wackestone with interlaminated organic-rich mudstone (Figure 4). Silt-size quartz grains form thin lamina; mm-scale pyrite/sphalerite lenses and laminae are common. Organic matter includes collapsed and bituminized algal cysts (Figure 4). The succeeding Cherry Valley Member (8-20 ) consists of brown to black organic-rich limestone with a modest benthic fauna; calcispheres and styliolinid shell hash (Figure 4) is interbedded with burrowed, bioclast-rich intervals. Nodular bedding and discrete limestone concretions surrounded by black shale are common. The Oatka Creek Member consists of high TOC basal black shale that grades upward into dark gray shale and siltstone. The upper Onondaga to basal Marcellus interval has a diagnostic gamma ray log pattern; a sharp positive gamma spike is commonly associated with the Tioga Bentonite, a volcanic ash layer in the upper Onondaga. A sharp gamma increase marks the basal Union Springs Member followed by lower gamma counts in the Cherry Valley. The succeeding basal Oatka Creek Member is marked by a gamma increase which characteristically declines to background shale levels. The Oatka Creek is overlain by gray shale of the Bridgewater Member. This contact may be defined in logs by a consistently lower gamma signature (Figure 3) Figure 2. Schematic time-stratigraphic relationships of the Marcellus Formation and other Devonian strata in the central New York region. Nomenclature for Hamilton Group in Chenango River Valley and Central Mohawk Valley follows Rickard (1984)

3 Figure 3: Stratigraphy of the lower Marcellus Formation, central New York, with typical gamma ray log. Note characteristic elevated gamma in Union Springs, elevated gamma followed by decline in Oatka Creek. Upper Oatka Creek Member is overlain by organic-lean, lower gamma Bridgewater Member. Thrust Fault Locality Descriptions: Cherry Valley - Bosworth (1984) has described exposures in the vicinity of Route 20 and Otsego Route 34A, although the best features are no longer observable due to slumping of soft shale at road cutting outcrops. At this locality the upper 5-8 feet of the Union Springs, the entire Cherry Valley and overlying Oatka Creek Members are exposed. As described by Bosworth (1984), the Union Springs hosts a cm. subhorizontal shear zone consisting of phacoidally-cleaved calcite-veined shale. This feature is not visible in the outcrop at the present time, however, small-displacement thrust faults occur within the Cherry Valley Member, and mineralized fractures and veins are common. The transport direction within the shear zone described by Bosworth (1984) is top to the north; the small thrust faults in Cherry Valley dip S10E with top N10W. The magnitude of structural transport was estimated to be 10 s to 100 s of meters by Bosworth (1984). Mineralization in fault-related fractures at the Cherry Valley locality includes mm-scale terminated nailhead calcite crystals, rare saddle dolomite and sharply terminated quartz crystals that are clouded by hydrocarbon and bitumen inclusions. Sooty bitumen coats calcite crystals within the veins. Hanson Quarry, Oriskany Falls,NY - Deformation at this quarry is apparently confined to a decollement zone confined within the Union Springs Member of the Marcellus. Horizontal motion in the Union Springs was accommodated within carbonaceous shale, with striated and polished surfaces developed on stiffer, bounding carbonate units (Figure 5). Carbonate beds are cut by ramp faults with cm- to m-scale displacement. Shale layers are thickened to form imbricate, cleaved and polished shale wads. Structural transport was top to N10E, as documented by orientation of fold axes within the Union Springs, lineations on carbonate beds, and orientation of thrusts within the shear zone. Stacking and repetition of carbonate beds within the zone indicate a minimum of 150 meters of structural transport. Black shale of the

Figure 4. Thin-section images showing typical basal Marcellus high-organic carbon microfacies.( A) Brachiopod+ crinoid bioclast lamina; Union Springs Member.

4 Figure 4. Thin-section images showing typical basal Marcellus high-organic carbon microfacies.( A) Brachiopod+ crinoid bioclast lamina; Union Springs Member. (B) Quartz silt lamina (top) with TOC mudstone and bioclast lamina. Brachiopods and crinoids in the Union Springs shell hash laminae may record an epiplanktic Sargassum-like community. (C) Styliolinid/calcisphere mudstone, carbonate bed in upper Union Springs member. (D) Calcisphere (E) Styliolinid. Styliolinids may have comprised a zooplanktic fauna grazing on planktic algae (F) Collapsed organic-walled cysts. Note spar filling. Black cyst wall is solid high-reflectance bitumen. These are likely algal cysts that were an important organic matter delivery mechanism to the Union Springs basin floor.

$Veins formed in extension fractures associated with fault motion document two major episodes of mineralization; early calcite and quartz crystal growth occurred during evolution of fluid$

5 overlying basal Oatka Creek Member lacks evidence of thrust-related deformation at the Oriskany Falls locality. However, vertical jointing is intensified and may provide enhanced permeability. Veins formed in extension fractures associated with fault motion document two major episodes of mineralization; early calcite and quartz crystal growth occurred during evolution of fluid hydrocarbons; a second phase of calcite-dolomite mineralization was accompanied by emplacement of high-reflectance bitumen in vein pore space. Veins preserve significant vuggy porosity. Mineralization is also present in dilatent spaces within bedding-plan thrusts (Figure 6). Veins with spectacular bi-layer mineralization are common (Figure 6), with the bottom portion of the vein fill consisting of lacey calcite and bitumen; the upper layer of vein fill consists of clear nailhead calcite with late saddle dolomite crystals, which are often vertically aligned along minor fractures that intersect veins (Figure 6). Terminated quartz crystals are found on the boundary between the upper and lower vein lining layers. The upper portions of the quartz crystals contain water + gas fluid inclusion; the lower portions of the crystals contain solid hydrocarbon inclusions. The bilayer veins are interpreted as forming from a two phase fluid within the vein during the later phase of fluid evolution. The lower part of the vein was occupied by a water + heavy hydrocarbon fluid; the upper part of the vein contained a water+gaseous hydrocarbon fluid. Silica was apparently soluble in the denser fluid phase, and quartz crystals preferentially nucleated and grew at the interface. Figure 5. Field photos of fault zones at Hanson and Seneca Stone Quarry localities. (A) Décollement zone confined to Union Springs Member at Hanson Quarry, Oriskany Falls, NY. Note stacking and folding of carbonate beds. (B) Ramp and flat portion of fault at Seneca Stone Quarry, Fayette, NY. Fault originates within quarry wall to left (south) as flat within bentonite layer in Onondaga Formation. Seneca Stone Quarry, Fayette, NY - The western and eastern quarry walls in the southern portion of the Seneca Stone Quarry expose a ramp-flat duplex structure (Figure 5). The thrust originates in outcrop within a bentonite unit (Tioga Bentonite) within the Onondaga and climbs through the upper Onondaga Formation (Seneca Member) and flattens into the Union Springs Member of the Marcellus. Lineations are well-developed on carbonate beds in the Union Springs, and demonstrate structural transport was top to N10W. Carbonate beds within the Union Springs are boundinaged within strongly fissile black shale. Bilayer veins resembling those at Hanson Quarry at Oriskany Falls are developed in extension fractures in carbonate beds. Significant vuggy porosity is present in extension fractures in carbonate beds of he Union Springs Member and in the upper Seneca Member of the Onondaga Formation. The basal Oatka Creek Member is intensely fractured by near-vertical joints, but lacks evidence of direct involvement in thrust-related deformation

Fracture mineralization and hydrocarbon maturation: Fractures and mineralized veins in the décollement zones at Cherry Valley, Oriskany Falls and Seneca Stone localities provide important constraints

6 Fracture mineralization and hydrocarbon maturation: Fractures and mineralized veins in the décollement zones at Cherry Valley, Oriskany Falls and Seneca Stone localities provide important constraints on the timing of motion relative to hydrocarbon maturation. Veins associated with fault motion record two major episodes of mineralization; early calcite and quartz crystal growth occurred during evolution of fluid hydrocarbons (Figure 7a) as documented by abundant hydrocarbon inclusions in brownish early calcite cement. A second phase of calcite-dolomite mineralization was accompanied by emplacement of high-reflectance bitumen in vein pore space (Figure 7b). Notably, early calcite in extensional veins is twinned, and is often separated from un-twinned later calcite by dissolution surfaces decorated with bitumen (Figure 7c). These features demonstrate the synchronicity of vein development and hydrocarbon maturation in extension fractures formed during active motion in the thrust fault system. The early hydrocarbon maturation gave rise to fluids which migrated into veins and stained early phase calcite; during later evolution of the fluid system, elevated temperatures converted trapped fluid hydrocarbon to solid bitumen and gas phase hydrocarbons. Faultrelated deformation had ceased by this time, as late stage calcite is un-twinned. Figure 6. Mineralized veins at Hanson Quarry and Seneca Stone Quarry. (A) Extension fractures in carbonate beds, Union Springs Member. Scale divisions in cm. View looking northwest. Structural transport is top to right. (B) Calcite and dolomite mineralization in dilatent voids developed in beddingparallel thrust. (C). Bilayer vein with oriented saddle dolomite crystals aligned along minor fractures that intersect vein. Note quartz crystals on boundary between lower dark (calcite + bitumen) layer and upper light (calcite + dolomite) layer. (D) Bilayer vein from Union Springs Member carbonate bed, Seneca Stone Quarry Fluid inclusions in calcite and dolomite from fault-related veins at all three localities are dominated by solid dark hydrocarbon (in early phase calcite) or gas phase (methane). Quartz crystals in bilayer veins contain solid hydrocarbon in the lower part of the crystal and hydrocarbon gas plus water inclusions in the

upper parts of the crystals. These inclusions have only a thin outer layer of water trapped against the inclusion wall by a large hydrocarbon gas bubble.

7 upper parts of the crystals. These inclusions have only a thin outer layer of water trapped against the inclusion wall by a large hydrocarbon gas bubble. Homogenization is difficult to determine because of this geometry, but preliminary data indicate temperatures of mineralization from C in the eastern site (Cherry Valley) and C in the western (Seneca Stone Quarry) site. These reflect the minimum temperatures achieved during burial, and are consistent with the overmature character of the hydrocarbon system at these sites. Notably, the extreme paucity of aqueous phase inclusions in the vein carbonate and quartz is consistent with the dry nature of the organic-rich intervals in the Marcellus, as reflected by significant fluid uptake during hydrofracturing, and the elevated salinity of flowback water. Figure 7. Thin section images of vein fill. (A) Veins in extension fractures in Union Springs carbonate layer, Hanson Quarry. Note that the early-formed calcite adjacent to the vein wall is darker than the later, vein center material. (B) Dark, bitumen-rich layer intersects early formed calcite at vein wall. Note staining of early phase calcite. Bitumen layer represents overmature petroleum liquid trapped during outmigration into fracture. Note spar-filled shrinkage void associated with bitumen layer. (C) Close-up of vein wall showing dark, hydrocarbon-stained, twinned early calcite and later, untwinned, clear calcite spar. Note dissolution surface decorated with bitumen that separates the two calcite generations. (D) Hydrocarbon gas (mostly methane) and water fluid inclusion, upper portion of quartz crystal from bilayer vein, Hanson Quarry. Inclusion is approximately 120 microns in long dimension. Note that the water forms a thin layer wetting the quartz wall. This inclusion homogenized at approximately 140 C; the precise homogenization temperature is difficult to observe.

8 Stable Isotopes: Carbonate mineral stable isotope data (Figure 8a) for calcite and dolomite at the three localities suggest that vein mineralization was accomplished by hot, evolved waters that were in equilibrium with the biogenic carbonate sedimentary materials within the Marcellus Formation. Since precipitation of vein carbonate occurred at elevated temperatures, more negative oxygen values, as shown by the data, are expected. During active structural deformation within the detachments, significant pressure solution of carbonate occurred, and thus dissolved carbonate was freely available. Vein carbonate that developed in association with detachment faulting is distinct from meteoric calcite veins developed in open fractures during Holocene oxidation of methane in the upper Marcellus (Selleck and Clayton, 2011). A B Figure 8 Stable isotope plots for vein minerals. (A) Carbon and oxygen for calcite and dolomite in veins at Cherry Valley, Hanson and Seneca Stone localities (blue). Hamilton Group brachiopod data from Koff and Selleck, 2008; meteoric vein calcite data from Selleck and Clayton, (B) 8 Ο qtz-cc for quartz and calcite plotted vs. geographic position for the three localities. Note that relatively lower values of 18 Ο qtz-cc suggest relatively higher temperatures of precipitation of quartz and calcite in equilibrium with the same water.

9 The difference in oxygen stable isotopes of quartz and calcite from crystals in veins at the three localities is shown in Figure 8. While these data cannot be used for direct calculation of equilibrium temperature of precipitation, slightly greater differences ( 18 Ο qtz-cc ) exists between the stable isotope values at the Seneca Stone Quarry locality, Fayette compared to the Hanson Quarry and Cherry Valley locality. This suggests that temperature during mineral precipitation at Cherry Valley was higher than the other two localities, consistent with the fluid inclusion observations. The relatively higher temperature at Cherry Valley likely reflects greater depth of burial during vein development. Summary: 1. Detachment structures in the basal Marcellus Formation extend deep into the foreland of the Appalachian Orogen. 2. Faulting occurred during hydrocarbon maturation. Liquid hydrocarbon was generated during early phases of structural deformation, as extensional veins formed in carbonate layers. Later heating resulted in high-reflectance bitumen and hydrocarbon gas. 3. Vein mineralization involved locally-derived fluids that were surprisingly water-poor. 4. Fluid inclusion and stable isotope data are consistent with mineralization occurring at temperatures beyond the oil window during the latest stages of vein-filling. 5. Significant vuggy porosity and tectonic fracturing are present in the detachment zones in the Union Springs and Cherry Valley Members, and fracturing is intensified in the overlying basal Oatka Creek Member. References: Bosworth, W., 1984, Foreland deformation in the Appalachian Plateau, central New York: the role of small-scale detachment structures in regional overthrusting; Journal of Structural Geology, v. 6, p Ettensohn, F. R., 1985, The Catskill delta complex and the Acadian orogeny: A model, in D. L. Woodrow and W. D. Sevon, eds., The Catskill delta: Geological Society of America Special Paper 201, p Engelder, T., Lash, G. and Uzcategui, 2009, that enhance production from the Middle and Upper Devonian Gas Shales of the Appalachian Basin; AAPG Bulletin, vol. 93, # 7, p Evans, M.A., 1994, Joints and décollement zones in Middle Devonian shales: Evidence for multiple deformation events in the eentral Appalachian Plateau. GSA Bulletin, Vol. 106, pp Faill, R. A., 1997, A Geologic History of the North-central Appalachians, Part 2, Silurian through Carboniferous; American Journal of Science, vol. 297, p Koff, A and Selleck, B., 2008, Stable isotope signature of Middle Devonian Seawater from Hamilton Group brachiopods, Central New York State; Northeastern Geology and Environmental Sciences, v. 30, no. 4, p Lash, G. and Engelder, T., 2011, Thickness trends and sequence stratigraphy of the Middle Devonian Marcellus Formation, Appalachian Basin: Implications for Acadian foreland basin evolution, AAPG Bulletin, V. 95, no. 1, p Nickelsen, N., 1986, Cleavage duplexes in the Marcellus Shale of the Appalachian foreland, Journal of Structural Geology, v. 8, no. 3-4, p

10 Selleck, B. and Clayton, P. 2011, Fracture sidewall cementation and vein carbonate: Tracking vertical migration and oxidation of natural gas in the Marcellus Formation, central New York, Geological Society of America,Abstracts with Programs, Vol. 43, No. 1, p. 94 Rickard, L.V., 1984 Correlation of the subsurface low and mid-devonian of New York, Pennsylvania, Ohio and Ontario: New York State Museum and Science Service Map and Chart Series 39, 59 p. Ver Straeten, C and Brett, C., 2006, Pragian to Eifelian strata (middle Lower to lower Middle Devonian), northern Appalachian Basin-stratigraphic nomenclatural changes: Northeastern Geology and Environmental Sciences, v. 28, p Werne, J.P, Sageman, B. B., Lyons, T.W. and 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, v. 302, p ,

Calcite Cements in Middle Devonian Dunedin Formation:

Geochemistry of the Fracture-Filling Dolomite and Calcite Cements in Middle Devonian Dunedin Formation: Implication for the Strata Dolomitization Model Sze-Shan Yip 1, Hairuo Qing 1 and Osman Salad Hersi