Research dispatch by Bryan Nicholson
Figure 1: Depth to Marcellus Shale Base in Pennsylvania, Ohio, New York,
and West Virginia. The Marcellus Shale is a geologic rock unit that is the
focus for the recent natural gas boom in the region. Extraction of resources
from the Marcellus is also a source for controversy over environmental,
political, and economic impacts.
Image from PennState MCOR: http://marcellus.psu.edu/resources/maps.php
Image from PennState MCOR: http://marcellus.psu.edu/resources/maps.php
Journal Article:
Molofsky, L. J., Connor, J. A., Wylie, A. S., Wagner, T., &
Farhat, S. K. (2013). Evaluation of Methane Sources in Groundwater in
Northeastern Pennsylvania. Groundwater, 333-349.
Journal Author Association:
Molofsky, Connor, Farhat - GSI
Environmental Inc., Houston, TX
Wylie, Wagner - Cabot Oil and Gas Corporation, Pittsburgh, PA
Wylie, Wagner - Cabot Oil and Gas Corporation, Pittsburgh, PA
Background & Synopsis:
Controversy and debate surrounds
the Marcellus Shale, a geologic rock unit targeted for massive gas extraction
in the northeastern United States (see Figure 1 above, Pennsylvania is outlined
near the center). Methane is among the primary natural gas resources extracted
in the Appalachian Basin and is a source of heated discussions in the media and
community (example news pieces: News
1 & News 2). Recent
technological advances have allowed for more economically viable drilling and
extraction methods using hydraulic fracturing, or “fracking.” Hydraulic
fracturing occurs when small explosives are used to fracture source rocks deep
below the surface and a mixture of sand and chemicals are injected to prop the
fractures open for greater resource extraction. This method allows a single
drilling pad to perform the same work as 4 to 12 conventional drilling
locations (or up to 32 in extreme cases!), drastically reducing the surficial
footprint on the landscape. With this recent drilling boom the community has
become more aware of potential impacts on the environment and has often lead to
political clashes between communities and resource companies. Since the onset
of the boom, multiple claims have arisen for methane contamination of water
wells. One of the primary issues in the debates is a lack of definitive
accountability.
A lack of background data on water
quality often restricts determining if methane occurrence in drinking water is
natural or anthropogenic (human derived). An example of anthropogenic
contamination would be the application of salt to roads during and after a
snowfall event. In this case, the salt will melt the snow, become dissolved in
the liquid water and may eventually make its way into a nearby stream or other
body of water. As a result, the amount of salt (or sodium) in the water will
increase and may reach a point where the water is considered not safe for
consumption (for example, salty ocean water). Similar scenarios exist for a
multitude of elements and materials, each with their own designated safe
consumption level that is determined by either state departments of
environmental protection or by federal agencies such as the US EPA
(Environmental Protection Agency). Accordingly, if we wanted to determine if a
stream is being impacted by salt runoff or if it contains a natural level of
sodium, we would need to know what the water quality was like before road salt
application procedures were introduced in the area. Without accurate data on
historical water quality, it is often impossible to determine if water
contamination is a result from recent events, from the past or even from
natural occurrences.
The authors of a recently published
study
collected and analyzed regional geologic data, aquifer dynamics and geochemical
characteristics, and historic documentation on the occurrence of shallow and
deep hydrocarbons
in Susquehanna County, PA. Over 1,700 water wells in Susquehanna County (Figure
2) were used to determine the occurrence and distribution of methane.
Seismic data from this study in
Susquehanna County reveal two lateral zones of faulted rock units in the
subsurface separated by another zone of less fractured, impermeable geologic
units (see Figure 3c below). This impermeable zone acts as a cap, preventing the
upward migration of natural gas from the Marcellus Shale (about 5000-6000 ft.
below surface) into the aquifer (about 100-500 ft. below surface) in
Susquehanna County. However, the faulted rock units that contain hydrocarbons above
the impermeable zone (such as the Lock Haven formation, Figure 3b) and the lack
of an impermeable layer above the Lock Haven to stop methane migration, provide
the conditions for natural gas to migrate freely toward the surface. Moreover,
most water wells in Susquehanna County are open-holed, unsealed wells; while
this construction method provides a greater water draw volume at various depths,
little to no protection is provided against contamination. What results is an
open system where methane from the Lock Haven formation is able to flow freely into
water wells and local streams, explaining why water wells drilled into the
Catskill Aquifer (Figure 3b) may naturally contain methane.
Figure 3: (a)
Geographic map showing transect of wells in the cross section and seismic line.
(b) Generalized cross section of Upper and Middle Devonian formations in
Susquehanna and Wyoming County with an estimated location of groundwater drawn
in. The Marcellus shale (near the bottom of the image at around 7000 feet deep)
is actively being hydrofractured. The Lock Haven formation is known to contain
natural gas. (c) Seismic cross section with faulted units outlined. There is a separation between the two marked
faulted zones (outlined in red and orange) that prevents migration of natural
gas from the Marcellus Shale toward the surface. However, the upper faulted
zone (in red) is within the Lock Haven formation and provides a scenario where
natural gas in the Lock Haven is able to migrate through these faults toward
the Catskill formation and toward the surface. Images are from the article.
Historical water well data and
documentation point to the presence of methane near the surface and in water wells
as long as 200 years ago. This is evidenced by observations of bubbling springs
and ponds, as well as by early documentation of hydrocarbon gas shows during
historic drilling. Comparison of methane occurrence with surface topography
revealed higher dissolved methane in topographic lows, like valleys, and lower
methane concentrations in topographic highs, like hills. One plausible
explanation for this is the occurrence of dense fractures and structures under
streams and valleys that may allow connections to the Lock Haven formation
directly below the Catskill formation (see Figure 3b). Because the distance
between the Lock Haven and Marcellus Shale is about 3000 feet, there is little chance
of fluid migration from the Marcellus to the Lock Haven. Methane concentrations
of water wells in valleys within defined gas production areas (within 1 km of
drilling) show no statistical difference with those water wells located in
valleys of nonproduction areas (greater than 1 km distance from drilling). This
indicates that the regional occurrence of concentrated methane in valleys is a
natural phenomenon and helps explain varying observations of methane
concentrations between wells in the area.
Gases have unique chemical
signatures depending upon the rock units they formed in, and an analysis of
detected gases can be done to determine the geologic source unit of the gases.
The researchers performed an isotope and molecular analysis on methane in wells
located in Dimock Township (located near the southwestern corner of Susquehanna
County). Hydrogen (H) and Carbon (C) isotope analysis by the Pennsylvania DEP,
Cabot, Osborn et al., and the US EPA reveal that gases in the majority of the
water wells originate from formations above the Marcellus. The tested samples
indicate that the majority of the water wells that contain hydrocarbon gases
are consistent with gases found in formations above the Marcellus Shale. These
results indicate that methane found in the tested wells in Susquehanna County can
be explained without significant contribution from deep Marcellus Shale gas.
The findings from the study
indicate that methane concentrations in water wells in Susquehanna County,
Pennsylvania are common and are not a result of recent shale gas extraction but
are rather sourced by formations containing methane closer to the surface. Most
of the water wells in Susquehanna County are drilled into the Catskill Formation,
which contains faults and fractures that allow easier migration of methane into
the groundwater. Additionally, elevated methane levels in Susquehanna County
are associated with low topography areas like valleys and there is no
statistical difference between wells located near an active drilling site and
in a nonproduction area. Documentation of methane occurrence at the surface and
in wells in the area dates back over 200 years. Considering the findings of
this study and evidence provided, methane occurrence in water wells in
Susquehanna County is likely sourced from near surface formations above the
Marcellus Shale. While these results provide a compelling story for methane
sources in drinking water in Susquehanna County, it must be noted that these
findings cannot be automatically extrapolated to other areas; further research
must be conducted for other sites to determine linkages between hydraulic
fracturing and aquifer vulnerability in those areas.
Further reading:
Naturally Occurring Methane Found in Some Sullivan County Water
