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PSHydrogeologic Analysis of the Oriskany Sandstone of the Appalachian Basin: Implication for Large-Scale Geologic Storage of CO2*
Jamie C. Skeen1 and Timothy R. Carr1
Search and Discovery Article #80056 (2009)
Posted September 25, 2009
* Adapted from poster presentation at AAPG Annual Convention and Exhibition, Denver, Colorado, USA, June 7-10, 2009.
1Geology, West Virginia University, Morgantown, WV. ([email protected])
The Oriskany Sandstone of the Appalachian basin is a widely
distributed saline aquifer which has produced large quantities of hydrocarbons.
Currently the Oriskany is host to numerous gas storage fields and is a
potential
target for large-scale geologic storage of CO2. Published
and unpublished data of rock characteristics, pressure, temperature, and
formation water geochemistry along with new brine samples were integrated
within a geographical information system to better understand the regional-scale
hydrogeological regime and its relation to the migration of hydrocarbons and
geologic CO2 sequestration
potential
. The topographically driven
up-dip flow of the Oriskany Sandstone formation waters is generally controlled
by outcrops at high elevation to the east and at low elevation to the west. The
up-dip flow is opposed by increased salinity induced buoyancy forces down-dip.
The flow pattern is substantiated by the salinity distributions, with
relatively lower salinity at recharge to the east and discharge to the west due
to mixing with fresh meteoric water and higher salinity between the recharge
and discharge zones. This flow pattern is also substantiated by the
distribution of Oriskany gas fields that occur in the Central Appalachian
basin; the major productive gas fields occur at the boundary between lower
salinity and are typically absent in areas of higher salinity. It is believed
that hydrocarbon distribution is influenced by basinal variations in buoyancy
and entrainment by the formation water flow. Improved containment of
large-scale CO2 injection appears to be associated in the Oriskany
with convergent flow located in the eastern Appalachian basin.
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The Appalachian basin is approximately 500 kilometers wide and
1,000 km long. It encompasses a broad area between the Canadian Shield to the
north, the Allegheny front to the east and the Cincinnati arch to the west
(UTBEG, 2008). It represents part of an ancient foreland basin in the eastern
United States that contains complex geology formed by a series of continental
plate collisions. This deformation resulted in the formation of the Appalachian Mountains and large areas of stretched, faulted, and deformed ridges
and valleys (USGS, 2008; UTBEG, 2008). The axis of the Appalachian basin is
underlain by a succession of strata greater than 3000 meters thick (UTBEG,
2008). (Figure 1) Overlying a major interregional unconformity in the
Appalachian basin is the Oriskany Sandstone, a widespread gas reservoir and
saline formation The Oriskany Sandstone of the Appalachian basin represents
the Deerpark stage of the Early Devonian (Diecchio, 1985). The sandstone is
typically a fossiliferous quartz arenite cemented with quartz or calcite. It
can be traced continuously through New York, Pennsylvania, Ohio, Maryland, West Virginia, Virginia, and Kentucky (Diecchio, 1985; Bruner and Smosna,
2008). The Oriskany typically overlies strata of Helderberg-age limestone or
equivalents, and is overlain by Onondaga-age strata, which vary from
limestone to chert to shale and are locally sandy (Diecchio, 1985). Since the
Oriskany is a major deep gas producer within the basin data such as pressure,
temperature, porosity, permeability, and brine composition are available
(Diecchio et al., 1984). The data indicate that there exists intergranular
and fracture porosity within the Oriskany, and thick low-permeability zones
within the Appalachian basin with the
As part of the effort by the National Energy Technology
Laboratory (NETL) of the US Department of Energy and the Regional Carbon
Sequestration Partnerships (RCSPs), data was assembled to address questions
of carbon capture and storage (CCS). Data on sources and
Saline formations are natural salt-water bearing intervals of
porous and permeable rocks that occur beneath the level of potable
groundwater (<10,000 mg/l by SDWA standards). A number of the saline
formations in the Appalachian basin are
While not all saline formations in the U.S have been examined,
the RCSPs have documented the locations of such formations with an estimated
sequestration
There are two types of carbon dioxide (CO2) emission sources: stationary sources and non-stationary sources. Non-stationary source emissions include CO2 emissions from the transportation sector. Stationary source emissions come from a particular, identifiable, localized source, such as a power plant. CO2 from stationary sources can be separated from stack gas emissions and subsequently transported to a geologic storage injection site. The “North American CO2 Sources” map displays the location and relative magnitude of a variety of CO2 stationary sources. (Figure 5)
According to the EPA, in 2006, total U.S. GHG emissions were
estimated at 7,054.2 million metric tons CO2 equivalent. This
estimate included CO2 emissions as well as other GHGs such as
methane (CH4), nitrous oxide (N2O), and
hydrofluorocarbons (HFCs). Annual GHG emissions from fossil fuel combustion
primarily CO2 were estimated at 5,637.9 million metric tons with
3,781.9 million metric tons from stationary sources. While not all
The northern Appalachian basin is an elongate, asymmetric foreland basin with a preserved northeast-southwest trending central axis that extends through Pennsylvania, western Maryland, and West Virginia. The eastern margin of the basin is concealed beneath thrust sheets in the Blue Ridge Province of the Appalachian Mountains. The western margin of the basin occurs in east-central Kentucky and central Ohio. The Cincinnati and Findlay arches separate the Appalachian basin from the Illinois and Michigan basins. Following Cambrian (Iapetan) rifting, the basin was enlarged by periodically reactivation of geologic structures that developed in response to collisional tectonics along the eastern margin of North America during the Taconic (Upper Ordovician), Acadian (Middle to Upper Devonian), and Alleghany (Upper Carboniferous) orogenies of the Paleozoic Era (Tankard, 1986; Quinlan and Beaumont, 1984; Thomas, 1995; Shumaker, 1996). The structure on the Oriskany Sandstone forms a surface that dips toward the center of the Appalachian basin forming a northeast-southwest trend. At the center of the study area the Oriskany Sandstone reaches a subsea depth in excess of 2000 meters. The surface shallows towards the outcrop area to the east along the Allegheny Front and Eastern Overthrust Belt. The surface shallows toward the Cincinnati Arch to the west where the Oriskany Sandstone subcrops or outcrops at ground level. (Figure 6)
The Oriskany Sandstone reaches depths of over 2,500 meters along a northeast-southwest trend in the center of the Appalachian basin. It shallows toward the Eastern Overthrust Belt which coincides with the know outcrop area at the east of the basin. The Oriskany gradually shallows toward the Cincinnati Arch to the west in Ohio where it subcrops or outcrops at ground level. (Figure 7)
Bottom-hole temperatures (BHTs) are recorded during logging of
the borehole and commonly are not at equilibrium with formation temperature
and require correction. In general, BHTs from shallow boreholes are too high,
and BHTs from deep boreholes are too low. One method to correct the BHT
values is to plot them versus depth (Forster et al., 1999). This same type of
crossplot was constructed for the Oriskany Sandstone. The average surface temperature
for the Appalachian basin (12 °C) provides an intercept, and a correction
factor can be generated by comparing the regression
Oriskany temperature = 0.0195 x (depth in meters) + 12°C
Corrected BHTs were
The thickness of the Oriskany Sandstone varies across the Appalachian basin from zero in the “no sand” area to a thickness of over 75 meters. It is typically thickest in the High Plateau and Eastern Overthrust Belt regions. The Oriskany thins to the west, northwest and to the south, where it is generally less than 12 meters thick. (Figure 9)
Under hydrostatic groundwater conditions, pressure increases with depth at the rate of 9.74 kPa/m for freshwater and roughly 10.71 kPa/m for brine with 145,000 mg/L TDS and is related to both depth and burial history. Final shut-in pressure recorded from gas wells of the Oriskany Sandstone (based on n = 258 wells) is significantly below the expected hydrostatic gradient for fresh groundwater. Pressure plotted against depth indicates that maximum recorded formation pressure follows a gradient of 6.8 kPa/m. This underpressuring may be due to drawdown from nearby wells and lack of sufficient shut-in time to reach pressure equilibrium. The Low Plateau and High Plateau of the central Appalachian basin are the deepest and higher pressured areas. (Figure 10)
Previously analyzed core data were
Existing brine geochemical data were gathered from published and unpublished state and federal geological surveys, as well as local oil and gas companies. Additional brine samples were collected from existing oil and gas wells distributed throughout the Appalachian basin, with an attempt made to locate the sample sites where know data was lacking. Across the extent of the Oriskany Sandstone the TDS ranges from freshwater (TDS < 10,000 mg/L) to dense brine (TDS > 300,000 mg/L). The brine samples were characterized by large differences between the reported TDS concentrations from neighboring wells within the same gas field. The cause of these differences may be related to areal, vertical, and temporal variability, errors introduced from sampling procedures, or to varying methods of chemical analysis (Jorgensen, et al., 1993). The dense brines are concentrated in the Oriskany structural lows at the center of the basin. The relatively lower TDS concentrations are associated with the recharge area along the outcrop area to the east and the discharge area to the west. (Figure 12)
Formation waters associated with microbial gas have low Ca/Mg ratios (<2) associated with high alkalinity values from calcite precipitation, induced by microbial methanogenesis (Martini et al., 1998). The ratio of calcium concentration to magnesium concentration for the Oriskany Sandstone across the Appalachian basin is generally less than ten. The ratio is typically lower at the recharge area to the east and the discharge area to the west, with values less than five. The ratio is generally higher in the northeast area of the basin, reaching values of up to 30. (Figure 13)
Temperature and pressure conditions must be adequate to keep injected CO2 in the dense supercritical or liquid phase (at its critical point for CO2, its temperature Tc is 31.1°C and Pc is 7.83 megapascals, or MPa), this is usually interpreted as a depth greater than 800 meters. An Oriskany Sandstone depth map was constructed to determine areas suitable for CO2 sequestration suing the 800 meter determining factor. (Figure 14) Within the study site only the areas to the west and northwest were deemed too shallow for sequestration, as well as the outcrop belt to the east. This excluded area consists of the central and western counties of Ohio, as well as the northwest section of Erie and Crawford counties of Pennsylvania.
Map of contours reflecting levels of
Formation pressure was
Ho = z + p / ρog
where z is elevation, p is pressure, ρo = 1000
kg/m3, and g is the gravitational constant (Bachu and Undershultz, 1993,
1995; Anfort et al., 2001). The density of formation waters in sedimentary
basins, a function of temperature and salinity, is not equal to the density
of freshwater and has the
DFR = Δρ|ΔE|/|ρo|ΔHo|h
where |ΔE| is the magnitude of the aquifer slope, |ΔHo|h
is the magnitude of the horizontal component of the freshwater hydraulic-head
gradient, Δρ is the difference between formation-water and
freshwater densities. If the DFR value is greater than 0.5, neglecting
buoyancy effects will introduce significant errors in flow analysis (Davies,
1987). Within the Oriskany Sandstone the values were generally much less than
0.5, with the exception of two areas located in south-central Pennsylvania . The distribution of freshwater hydraulic head shows the expected trends
of northwestward and southeastward flow from the basin center toward the western
subcrop area and the eastern outcrop area. (Figure 16)
Use of freshwater hydraulic heads in the flow analysis of variable density
formation waters may introduce significant errors, depending on interaction
between the
The Oriskany Sandstone of the Appalachian basin is a widely distributed saline aquifer which has produced large quantities of hydrocarbons. Using published and unpublished data of rock characteristics, pressure, temperature, and formation water geochemistry along with new brine samples were
GCO2 = A hg фtot ρ E
GCO2 = estimate of total saline formation storage capacity in grams A = area of basin greater than 800 meters in depth (151,072,049,688.5 m ), hg = average thickness of formation at depths greater than 800 meters (17.01 meters), фtot = average formation-scale porosity for thickness hg (8.08 %) ρ = density of CO2 at pressure and temperature that represents storage conditions for saline formation averaged over hg (800 kg/m at P = 18.01 mPa and T = 43.29 °C), and E = storage efficiency factor that reflects a fraction of total pore volume filled by CO2 (USDOE estimations of E are a low of 0.01 to high of 0.04). The result is a storage resource estimate of 1.6665 to 6.666 gigatonnes of CO2.
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