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| Publications— Scientific Investigations Report |
U.S. Geological Survey Scientific Investigations Report 2007-5085
By Kevin J. Breen, Kinga Révész, Fred J. Baldassare, and Steven D. McAuley
This report is available online in Portable Document Format (PDF). If you do not have the Adobe Acrobat PDF Reader, it is available for free download from Adobe Systems Incorporated.
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In January 2001, State oil and gas inspectors noted bubbles of natural gas in well water during a complaint investigation near
Tioga Junction, Tioga County, north-central Pa. By 2004, the gas occurrence in ground water and accumulation in homes was a safety concern;
inspectors were taking action to plug abandoned gas wells and collect gas samples. The origins of the natural-gas problems in ground water
were investigated by the U.S. Geological Survey, in cooperation with the Pennsylvania Department of Environmental Protection, in wells
throughout an area of about 50 mi2, using compositional and isotopic characteristics of methane and ethane in gas and water wells. This
report presents the results for gas-well and water-well samples collected from October 2004 to September 2005.
Ground water for rural-domestic supply and other uses near Tioga Junction is from two aquifer systems in and adjacent to the Tioga River
valley. An unconsolidated aquifer of outwash sand and gravel of Quaternary age underlies the main river valley and extends into the valleys
of tributaries. Fine-grained lacustrine sediments separate shallow and deep water-bearing zones of the outwash. Outwash-aquifer wells are
seldom deeper than 100 ft. The river-valley sediments and uplands adjacent to the valley are underlain by a fractured-bedrock aquifer in
siliciclastic rocks of Paleozoic age. Most bedrock-aquifer wells produce water from the Lock Haven Formation at depths of 250 ft or less.
A review of previous geologic investigations was used to establish the structural framework and identify four plausible origins for natural
gas. The Sabinsville Anticline, trending southwest to northeast, is the major structural feature in the Devonian bedrock. The anticline, a
structural trap for a reservoir of deep native gas in the Oriskany Sandstone (Devonian) (origin 1) at depths of about 3,900 ft, was explored
and tapped by numerous wells from 1930-60. The gas reservoir in the vicinity of Tioga Junction, depleted of native gas, was converted to the
Tioga gas-storage field for injection and withdrawal of non-native gases (origin 2). Devonian shale gas (shallow native gas) also has been
reported in the area (origin 3). Gas might also originate from microbial degradation of buried organic material in the outwash deposits
(origin 4).
An inventory of combustible-gas concentrations in headspaces of water samples from 91 wells showed 49 wells had water containing combustible
gases at volume fractions of 0.1 percent or more. Well depth was a factor in the observed occurrence of combustible gas for the 62 bedrock
wells inventoried. As well-depth range increased from less than 50 ft to 51-150 ft to greater than 151 ft, the percentage of bedrock-aquifer
wells with combustible gas increased. Wells with high concentrations of combustible gas occurred in clusters; the largest cluster was near the
eastern boundary of the gas-storage field. A subsequent detailed gas-sampling effort focused on 39 water wells with the highest concentrations
of combustible gas (12 representing the outwash aquifer and 27 from the bedrock aquifer) and 8 selected gas wells. Three wells producing
native gas from the Oriskany Sandstone and five wells (two observation wells and three injection/withdrawal wells) with non-native gas from
the gas-storage field were sampled twice. Chemical composition, stable carbon and hydrogen isotopes of methane (δ13CCH4
and δDCH4), and stable carbon isotopes of ethane (δ13CC2H6) were analyzed. No samples could be
collected to document the composition of microbial gas originating in the outwash deposits (outwash or “drift” gas) or of native natural gas
originating solely in Devonian shale at depths shallower than the Oriskany Sandstone, although two of the storage-field observation wells
sampled reportedly yielded some Devonian shale gas. Literature values for outwash or “drift” gas and Devonian shale gases were used to supplement
the data collection.
Non-native gases from wells in the gas-storage field and native gases from wells producing from the Oriskany Sandstone were similar in chemical
composition; methane (volume fraction ranging from 94.5 to 97.2 percent) and ethane (volume fraction ranging from 2.0 to 2.6 percent) were predominant.
Isotopic composition data for storage-field gases (median δ13CCH4 of about -44.1 per mil, δDCH4 of -168
per mil, and δ13CC2H6 of -32.7 per mil) were different than gases from the Oriskany Sandstone
(median δ13CCH4 of -34.6 per mil, δDCH4 of -159 per mil, and δ13CC2H6 of -40.4
per mil). Both Oriskany Sandstone and storage-field gases were thermogenic. Compositions of gases from storage-field observation wells were
intermediate to, and likely related to mixing of, native gases from the Oriskany Sandstone and non-native gases from the storage-field
injection/withdrawal wells.
In water-well samples, methane and ethane were the only hydrocarbons detected at reportable concentrations. Methane concentrations as high as
44.8 mg/L (milligrams per liter) were measured and methane concentrations were greater than 25 mg/L in 38 percent of the 39 samples. The
δ13CCH4 values were measurable in 35 well waters and had a bimodal distribution with modes at -65 per mil (14
wells) and -40 per mil (21 wells). Gas in water samples from the 14 wells in the -65 per mil mode had a small measure of microbial gas (outwash or
“drift” gas) in the isotopic signature as determined by carbon-14 content of methane. The microbial gases were found chiefly in bedrock-aquifer well
waters; 10 water wells representing upland and valley settings were along the northern flank of the Sabinsville Anticline. Waters with microbial gases
contained traces of ethane (volume fraction of 0.01 percent or less) that were too small for determination of δ13CC2H6.
Gases from the 21 water-well samples in the -40 per mil mode for δ13CCH4 were thermogenic. The δDCH4
and δ13CC2H6 values for the 21 samples also showed thermogenic signatures. The thermogenic gases were found
chiefly in a 17-well cluster on the axis of the Sabinsville Anticline at the eastern margin of the gas-storage field. This cluster corresponds with
the cluster of wells with high concentrations of methane from the combustible-gases inventory. An observation well for the gas-storage field, TW805,
was nearest to the cluster and three water wells in the cluster contained gases that nearly matched the stable carbon and hydrogen isotope
composition in TW805. All the water wells had gas signatures indicating mixing of gases from different origins; however, the overall isotopic
composition of methane and ethane showed that the gases in water wells at the eastern margin of the gas-storage field were principally thermogenic.
The δ13CCH4 and δ13CC2H6 values of the majority of thermogenic gases from
water wells either matched or were intermediate between the samples of storage-field gas from injection/withdrawal wells and the samples of
storage-field gas from observation wells.
Proximity to the axis of the Sabinsville Anticline and the eastern margin of the gas-storage field correspond to the presence of thermogenic gas in
water wells. Of the water-well gases with a thermogenic signature, about half are from outwash-aquifer wells and half from bedrock-aquifer wells.
Of the bedrock-aquifer-well gases with a thermogenic signature, the majority are from wells drilled into bedrock beneath the Tioga River valley.
Clay layers in the main Tioga River valley may play a role in keeping gas migration confined to the deep water-bearing zones of the outwash aquifer
and the underlying bedrock aquifer.
Isotopic signatures have been used successfully in this study to help discern the origin of the gases in water wells near Tioga Junction. The
thermogenic gas found in water wells does not match the composition of native gas from the Oriskany Sandstone. Mixing of Oriskany gases with
storage-field gases has occurred, and there was also evidence for mixing of a microbial component of gas in some water wells. The possibility of
three or more end-member compositions and many possible mixing scenarios for gases complicate the data interpretation. The lack of samples solely
representing native shallow Devonian gas and the small number of storage-field gas samples places some limits on making firm conclusions about the
origin of the methane in ground water. The weight of the evidence, however, points to storage-field gas as the likely origin of the natural gases
found in water wells near Tioga Junction.
Abstract
Introduction
Purpose and Scope
Description of Study Area
Hydrogeologic Setting
Structural Geology
Aquifer Framework
History of Gas Development and Gas Storage
Methods
Gas-Well Data Collection
Water-Well Data Collection
Data Interpretation
Nomenclature for Isotope Ratios
Isotopic Signatures of Methane in Natural Gas
Laboratory Analyses
Quality Assurance
Occurrence and Origins of Natural Gases in Ground Water
Occurrence of Natural-Gas Concentrations in Ground Water
Relation of Combustible-Gas Occurrence to Chemical Characteristics of Ground Water
Relation of Combustible-Gas Occurrence in Ground Water to Hydrogeologic Setting
Use of Isotopes to Determine Origins of Natural-Gas Concentrations in Ground Water
Isotopic Characteristics and Origins of Oriskany and Storage-Field Gases
Isotopic Characteristics and Origins of Methane and Ethane in Ground Water
Limitations of the Available Data
Summary
Acknowledgments
References Cited
Appendix: The problems with methane in water wells
This report is available online in Portable Document Format (PDF). If you do not have the Adobe Acrobat PDF Reader, it is available for free download from Adobe Systems Incorporated.
View the full report in PDF 10.2 MB
For more information about USGS activities in Pennsylvania contact:
Director
USGS Pennsylvania Water Science Center
215 Limekiln Road
New Cumberland, Pennsylvania 17070
Telephone: (717) 730-6960
Fax: (717) 730-6997
or access the USGS Water Resources of Pennsylvania home page at:
http://pa.water.usgs.gov/.
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