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Natural Gases in Ground Water near Tioga Junction, Tioga County, North-Central Pennsylvania-Occurrence and Use of Isotopes to Determine Origins, 2005

U.S. Geological Survey Scientific Investigations Report 2007-5085

By Kevin J. Breen, Kinga Révész, Fred J. Baldassare, and Steven D. McAuley

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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.

Table of Contents

     Purpose and Scope
     Description of Study Area
          Hydrogeologic Setting
               Structural Geology
               Aquifer Framework
          History of Gas Development and Gas Storage
     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
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

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