North Carolina Shale Gas Baseline Groundwater Sampling Project

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The U.S. Geological Survey (USGS) South Atlantic Water Science Center is conducted an inventory of well records and baseline groundwater-quality sampling to better delineate areas of groundwater use and groundwater-quality characteristics prior to potential shale gas exploration in the Triassic Basins of Lee and Chatham Counties, North Carolina. The compilation of baseline groundwater-quality data in North Carolina provided an opportunity for comparison to data collected after drilling activities commence should shale gas exploration be allowed to occur.

The U.S. Geological Survey (USGS) South Atlantic Water Science Center is conducted an inventory of well records and baseline groundwater-quality sampling to better delineate areas of groundwater use and groundwater-quality characteristics prior to potential shale gas exploration in the Triassic Basins of Lee and Chatham Counties, North Carolina. The compilation of baseline groundwater-quality data in North Carolina provided an opportunity for comparison to data collected after drilling activities commence should shale gas exploration be allowed to occur.  (fig. 1). Shale Gas exploration has become economically viable in many areas of the United States (http://www.eia.doe.gov/oil_gas/rpd/shaleusa2.pdf) as a result of improved directional drilling capabilities and hydraulic fracturing ("fracking") techniques. The compilation of baseline groundwater-quality data in North Carolina is an opportunity for comparison to data collected after drilling activities commence should the State allow Shale Gas exploration to occur. In June 2011, the North Carolina Legislature passed House Bill 242 directing the North Carolina Department of Environment and Natural Resources (NC DENR) to study issues related to potential Shale Gas Exploration in the Triassic Basins of the State (http://www.ncleg.net/sessions/2011/bills/house/pdf/h242v2.pdf).

Overview:

NC Shale Gas Well Inventory Study Area
NC Shale Gas Well Inventory Study Area(Public domain.)

Interest in the Triassic Basin shale deposits in parts of Lee and Chatham counties has grown in recent years, with more than 9,000 acres currently (2012) leased within the Sanford sub-basin by oil and gas exploration companies. Drilling processes and waste disposal practices related to shale gas exploration in the United States and other parts of the world have sometimes led to contamination of groundwater resources.

Groundwater-quality conditions in the aquifer in the Sanford sub-basin are largely unknown. Only three wells have been sampled in the study area by the USGS during the 1950's and 1960's. Some primarily inorganic sample analyses are available from private, community, and non-community transient wells, but have not been compiled or summarized to date.

During this study, the USGS will:

  • Compile all available well construction and groundwater-quality data
  • Collect new baseline groundwater-quality samples in the study area

Baseline/reconnaissance groundwater-quality samples will be collected from private, community, and non-community transient wells in the Sanford sub-basin study area ahead of shale gas exploration in the North Carolina. These data could potentially be compared to post-shale gas production groundwater-quality samples from the same area should the State allow shale gas production in the near future.

Both well construction and groundwater-quality data collected as part of this study will be made available through an online interactive map and USGS Open-File Series report.

 

Geology:

Sanford sub-basin, and other Early Mesozoic (Triassic) Basins in North Carolina
Sanford sub-basin, and other Early Mesozoic (Triassic) Basins in North Carolina(Public domain.)

Geologic deposits within the Deep River Basin in North Carolina include the following sedimentary rock types: sandstone, conglomerate, shale, siltstone, claystone, coal, and small amounts of limestone and chert. These sedimentary rocks of the Deep River Basin were deposited as layers during early Mesozoic Era rifting of supercontinent Pangea. During rifting, the basin filled with clastic sediments, including alluvial fan, deltaic, lacustrine, and swamp deposits. These deposits also have been injected by mafic diabase dikes during the later Jurassic Period.

The Deep River basin is bordered to the east by Jonesboro Fault, a west-dipping, high angle, normal fault. Intra-basinal faults are also mapped throughout the basin.

The natural gas resource reservoir rock is an organic-rich black shale (black fine-grained clastic rocks) within the Cumnock formation in Lee and Chatham counties. The Cumnock Formation is described as dominantly a black and dark gray shale with associated gray sandstone and coal, approximately 230 to 250 meters thick. Historic coal mining in the area was conducted during the Revolutionary War period in the late 1700's through the post-World War II Era, and small strip mining operations were active in the area briefly during the 1980's. Methane is known to be associated with these black shale/coal sequences, as evidenced by the 1925 mine accident at the Coal Glen Mine in Chatham County.

 

Groundwater:

Generalized Cross Section of the Sanford Subbasin
Generalized Cross Section of the Sanford Subbasin(Public domain.)

The groundwater system in the Triassic Basin of North Carolina is part of a larger Early Mesozoic Basin aquifer within the Piedmont Physiographic Province of the eastern United States. The groundwater system is composed of weathered regolith material at land surface and underlying bedrock sedimentary rock layers . Differential weathering along lithologic contacts and bedding planes may enhance permeability in the aquifer. Additionally, secondary features including faults, joints, and diabase dikes may enhance permeability through openings, associated fracturing, or weathering near these features. The presence of diabase dikes suggests potential "cooking" of the natural gas, but can often be a boundary for subsurface flow and "pooling" of groundwater.

Study Area:

The study area was defined by prior exploration and gas shows (Reid and others, 2010) as having the most potential to develop shale gas, based on available core and test drilling data within the Sanford Sub-basin of the Early Mesozoic Basin aquifer.

The total area defined by this boundary is about 77 square miles, which includes the northwestern quarter of Lee County and extreme southeastern Chatham County along the Deep River. Data from the 2010 U.S. Census indicates that approximately 14,903 people live in the study area.

Data Inventory:

An inventory of available private and public water-supply well information within the 77-square-mile study area in parts of Lee and Chatham counties NC will be compiled from local County public health agencies and the NC DENR Division of Water Quality Aquifer Protection Section. Additionally, groundwater-quality analytical results will be compiled as available from the County public health departments and the NC Department of Health and Human Services. Both paper and electronic data will be assimilated to provide an assessment of well construction characteristics (date drilled, drilling company, total depth, casing depth, yield, and depth to productive zones) within the study area.

Groundwater Quality Sampling:

Picture of the Simpson well
(Credit: Jeff Reid, North Carolina Geological Survey. Public domain.)

A subset of about 50 wells that have available construction data (total depth, casing depth, yield, and year drilled) will be selected for the collection of groundwater-quality samples. Most samples will include analyses of dissolved gases and major ions. Depending on available funding, more detailed analysis will be done at a subset of these wells. Detailed sampling will include dissolved gases, methane and ethane isotopes, major ions, metals, volatile organic compounds (VOCs), semi-volatile organic compounds, radium isotopes, strontium isotopes, oxygen/deuterium/carbon stable isotopes, dissolved inorganic and organic carbon.