Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Fractures, Veins, Fluid Migration and Hydrocarbon Generation in the Utica Shale, Northern Appalachian Basin, New York
(1) Geology, Colgate University, Hamilton, NY, United States.
Introduction:
The Upper Ordovician Utica Formation is a widespread hydrocarbon source rock in the Appalachian Basin, and development of the Utica as a gas shale reservoir in Quebec demonstrates the potential of this unit in New York and Pennsylvania. Successful well stimulation of gas shale reservoirs is best carried out with an understanding of the orientation and character of natural fractures in the shale (e.g. Engelder, et al, 2009). Natural fractures provide conduits for gas flow to stimulated zones, and the status of natural fractures, whether open or mineralized, is of considerable interest. The Utica Formation is widely exposed in the type area of the Mohawk River Valley of New York State, and adjacent Tug Hill Plateau region of New York State. This generally east-west outcrop belt also allows observation of structurally deformed Utica equivalents in the vicinity of the frontal thrust zone of the Taconic Orogen (Lim, et al 2005) and study of progressively less deformed rocks to the west. The central Mohawk Valley region exposes generally N-S trending faults that were been active during the downwarping of the Appalachian Basin during the onset of the Taconic Orogeny (Bradley and Kusky, 1986). These faults were likely active during deposition of the Utica Shale, and controlled and localized sediment deposition and facies patterns (Jacobi, 1981; Brett and Baird, 1982).
Field sites
In the current report, data is presented from field measurements of natural fractures in the Utica Formation and immediately overlying Frankfort Formation. Field work was focused on exposures of the Flat Creek Member of the Utica (Figure 1), which contains the most organic –rich intervals in the Utica (Nyahay and Martin, 2008). The Flat Creek is present in the eastern and central Mohawk Valley area, and is replaced to the west by interbedded limestone and shale of the Dolgeville Member (Brett and Baird, 1982).
Measurements were taken on near-vertical fractures and veins in stream and road cuts. Fractures were also assessed to determine mode type. Systematic sampling of veins and sand dikes for petrologic and geochemical analyses was also carried out.
Results:
Fracture types and orientations: Significant differences in fracture mode and orientation are present across the outcrop belt, and between fracture systems in the Flat Creek Member compared to overlying units. In the eastern and central area, E-W and N-S fractures are most common in the Flat Creek Member (Figure 2). The E-W fractures in the eastern outcrop belt are of Mode 2 (strike-slip) origin (Figure 3). N-S fractures are of Mode 1 (tensile) origin, and are commonly parallel to sand injectite dikes (Figure 4).
Fluid inclusions and stable isotopes: Dilational jogs in Mode 2 fractures host thick calcite veins with hydrocarbon stains, and methane-dominated and low-salinity aqueous fluid inclusions . Fluid inclusion (Th ˜105-185 C, TMice = -0.5 to -4.5 C) and stable isotope (δ13Ccalcite= -3 to +15 PDB; δ18Ocalcite = -9 to -11 PDB) data (Figure 5) indicate that vein generation occurred during hydrocarbon maturation and that vein-forming fluids were mainly derived from within the Flat Creek Member. Carbon isotope values of calcite from two veins from the Flat Creek Member at Chuctanuda Creek vary systematically from the vein margin to vein center (Figure 6). The more positive carbon isotope values result from hydrocarbon degradation by microbial or thermal processes.
Sand injectites: Sand injectite dikes were sourced from remobilized volcanic ash derived from within the Utica, mixed with siliciclastic sand and dolomite sourced from underlying Paleozoic strata and faulted Proterozoic basement (Figure 4). Horizontal calcite veins in the Flat Creek Member document high fluid pressures and/or relatively low confining pressure during vein formation.
Taken together, these features suggest active seismic pumping of fluid and sediment slurry during fracturing and vein development in the Flat Creek. Strike-slip fracturing was dominant, and provided alternating compression and dilatation along near-vertical fractures. These processes drove the diagenetic fluid systems in the Utica, and promoted influx of fluids that gave rise to hydrothermal dolomite and quartz/bitumen mineralization in units that underlie the Utica. Vertical transport of hydrocarbon-bearing fluids downward from the Utica along vein systems in the Flat Creek Member, and upward transport of fluids with elevated salinity from basement and lower Paleozoic sandstone aquifers were accomplished by seismic pumping. Mode 2 fractures developed within an east-west oriented wrench fault system early in the evolution of the Taconic foreland basin.
The types and orientations of fractures in units overlying the basal Utica are markedly different from the Flat Creek Member, suggesting that fracturing and fluid expulsion in the Flat Creek Member were relatively early burial phenomena. Later burial and fracturing of the Utica occurred after deposition of overlying Silurian strata and permitted up-migration of dry gas into Silurian sandstone reservoirs.
Figure 1. Generalized stratigraphy of the Late Ordovician in the Mohawk Valley study area (after Goldman, et al. 1999).
Figure 2. Rose diagram plots of near-vertical fractures and veins from the Utica Formation in the Mohawk Valley and Tug Hill region of New York. Note dominance of E-W fractures in eastern and central Mohawk Valley localities.
Figure 3. Outcrop photos of Flat Creek Member fractures. (A) Calcite vein developed in dilational jog in Mode 2 fracture; Chuctanuda Creek locality. (B) Coarse calcite vein stained with bitumen; Chuctanuda Creek locality. (C) Lineations and left-lateral motion indicators, Mode 2 fracture at Fultonville locality. (D) Motion indicators, Chuctanuda Creek locality. (C) and (D) indicate left lateral strike slip motion.
Figure 4. (A) Sand injectite dike at point of hammer. Flat Creek locality. Note N-S Mode 1 fracture adjacent and parallel to sand dike. (B) SEM-BSE image of sand injectite showing basement-derive angular quartz grains.
Figure 5. Carbon and oxygen stable isotope data for all calcite vein samples from the Utica Formation. Chuctanuda Creek is the easternmost locality; Delta Reservoir is furthest to the west. Note the positive carbon isotope excursion for the Chuctanuda Creek locality. The highly positive values reflect carbonate derived from microbial or thermogenic degradation of petroleum hydrocarbons during the later stages of vein development at this locality. The negative carbon signature of one sample from Reall Creek may result from carbonate derived from oxidation of methane. Note the relatively wide range of oxygen values, particularly for the Flat Creek and Town of Minden localities, resulting from vein formation over a range of temperatures, or mixing of waters with different oxygen signatures.
Figure 6. Carbon isotope values of calcite from vein margin to vein center for two veins developed in dilational jogs at the Chuctanuda Creek locality. Note that the latest calcite (vein centers) has the most positive carbon values, indicating microbial or themogenic degradation of petroleum hydrocarbon to produce dissolved carbonate at the latest stage of vein development.Summary:
- Fractures and veins in the Flat Creek Member of the Utica Formation in the eastern and central Mohawk Valley of New York include E-W Mode 2 and N-S Mode 1 fractures that formed during hydrocarbon maturation.
- Fracture types and orientations in the overlying Indian Castle Member of the Utica, and succeeding Frankfort Formation, do not include early Mode 2 fractures. Fractures in these units are not commonly mineralized, and lack evidence of fluid hydrocarbon migration.
- Fracturing and hydrocarbon maturation in the Flat Creek may have occurred during relatively early burial, and involved heating by fluids derived from underlying Paleozoic and Proterzoic basement rock. Sand injectites in the Flat Creek document communication between Flat Creek and underlying rocks via seismically-pumped fluids.
- Later (Devonian?) burial and fracturing of the Utica occurred after deposition of overlying Silurian strata and permitted up-migration of dry gas into Silurian sandstone reservoirs.
References:
Brett, C. and Baird, G. 2002, Revised stratigraphy of the Trenton Group in its type area, central New York State: sedimentology and tectonics of a Middle Ordovician shelf-to-basin succession. Physics and Chemistry of the Earth, Parts A/B/C, Volume 27, Issues 1-3, p 231-263
Bradley, D. and Kusky, T. (1986) Geologic evidence for rate of plate convergence during the Taconic arc-continent collision, Journal of Geology, v. 94, p, 667-691
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. 857-889
Goldman, D., Mitchell, C.,Bergstrom, S., Delano, J. W. and Tice, S. (1999) K-bentonites and graptolite biostratigraphy in the Middle Ordovician of New York and Quebec: A new chronostratigraphic model Palaios, v. 9, p. 124-143
Jacobi, R. 1981, Peripheral bulge--a causal mechanism for the Lower/Middle Ordovician unconformity along the western margin of the Northern Appalachians, Earth and Planetary Science Letters, v. 56, p. 245-251
Martin, J. P., Hill, D. and Lombardi, T., , 2004, Fractured Shale Gas Potential in New York, Northeastern Geology and Environmental Science, vol. 26, no. 1 & 2, pp. 57-78.
Nyahay, R.E. and Martin, J. P.., 2008, Delineating the Utica Formation From Outcrop to Subsurface, Geological Society of America Northeastern Section - 43rd Annual Meeting, 27-29 March.