Distribution of the Marcellus Shale
Depositional environment during the Middle Devonian
Marcellus... These days there is a lot of talk and activity regarding the Marcellus. Good, bad and misunderstood.
The Marcellus Formation is an organic-rich black shale with the potential to supply a great deal of natural gas. The Marcellus shale deposit extends across New York, Pennsylvania, Ohio, Maryland, West Virginia and Kentucky. Known to contain large volumes of natural gas, the resource was, until recently, not economical to recover. However, higher energy costs and new technology have recently made the Marcellus an important gas play. Reported estimates of recoverable gas reserves range from 8 to 50 trillion cubic feet (Tcf). Add in the close proximity to the northeast's major population centers, and it is understandable why energy companies are clamoring for a piece of this gas play.
The Marcellus Shale was deposited as an organic-rich mud across the Appalachian Basin in a marine (inland sea) environment during Middle Devonian time (~390 million years ago). The mud deposit was later buried by a thick sequence of younger Devonian sediments washed off ancient mountains as a river delta into the inland sea. The basin floor subsided under the weight of the sediment, resulting in a wedge-shaped deposit that is thicker in the east and thins to the west. Over the next several million years compaction turned the mud to shale, mountain building forces tilted and folded the sequence of sedimentary rock, erosion forces lowered mountain tops forming new sedimentary rock layers, until finally you are left with today's geological setting.
In Pennsylvania, the depth to the top of the Marcellus shale ranges from 0 feet where it crops out in central Pennsylvania to over 9,000 feet in parts of southwestern and northeastern Pennsylvania. The thickness of the Marcellus shale ranges from about 20 feet in northwestern Pennsylvania to several hundred feet in central and northeastern Pennsylvania.
Petroleum Geology 101
The rich organic matter deposited at the bottom of the oxygen-deficient sea bed was buried, compressed and heated deep within the Earth over geologic time, forming hydrocarbons, including natural gas. The oil and natural gas originate in the source rock (organic mud) and migrate upward through overlying rock layers and to the ground surface and atmosphere. When the migrating oil and gas becomes trapped in the rock, pockets or reservoirs are formed.
A conventional petroleum system is made up of distinct units: the source rock, which is the organic-rich rock layer that produces oil and/or gas; the reservoir where the hydrocarbons migrate into and are stored; and, the cap rock, which is an impermeable layer that keeps the hydrocarbons from migrating further, trapping them in the reservoir where they can accumulate to recoverable quantities. Conventional oil and gas systems are the classic scenarios that occur in isolated reservoirs associated with stratigraphic or structural traps such as anticlines and salt domes.
Long known to be a source rock for many conventional oil and gas reservoirs in the Appalachian basin, the Marcellus shale is now being explored as an unconventional reservoir.
An unconventional petroleum system is one in which the reservoir, the trap, and the source rock are the same rock unit. Hydrocarbons originating in the source rock remain trapped within the shale's micropores and fractures which act as the reservoir, confined by the low permeability of the surrounding shale matrix. Unconventional petroleum systems therefore can extend the length of the shale formation, consisting of continuous, wide spread deposits. In the past, however, the low permeability conditions of the shale reservoir made removal of hydrocarbons economically unfeasible.
Horizontal Drilling & Hydraulic Fracturing
Historically natural gas deposits in the unconventional systems of deep shale formations were not economically recoverable. Today, hydraulic fracturing or fracing (pronounced frak-ing) combined with horizontal drilling, is a technological advancement that allows recovery of natural gas from these deep shale formations like the Marcellus.
The current Marcellus shale play began in 2004 when a gas exploration company looking at an underlying conventional reservoir, experimented in the Marcellus shale with drilling and hydraulic fracturing techniques borrowed and revised from those used on the Barnett Shale gas play in Texas.
In a typical oil and gas conventional system, one vertical well is drilled into the reservoir and the oil and/or gas is removed. In an unconventional system this is not an economically feasible technique. However horizontal drilling has changed that. A horizontal well begins by starting a vertical well bore to the target depth and then steering the drill bit in a horizontal direction through the plane of the source bedrock layer. Horizontal wells can extent about 4,000 feet.
However, with the low permeability of the shale, the horizontal well alone does not allow recovery of significant amounts of gas from the rock. Fracing is the process of creating openings ('fractures') in the reservoir rock to allow natural gas to flow. Hydraulic fracturing has been used by both the oil and gas, and water supply industry for many decades. For deep shale gas plays, water, sand and other additives are pumped under high pressure into the shale to create wider, interconnected fractures. A hydraulic fracturing operation is normally performed once in the life of a well. The newly created fracture system allows natural gas to flow into the wellbore and be collected at the surface.
With this technique, one recovery well started from a single vertical well location, or multiple wells on the same pad, collect gas across a large horizontal extent.
Taylor GeoServices (TGS) provides expertise and resources to assist in a wide range of operations associated with the Marcellus Shale. TGS geologists and scientists understand the diverse complexities of resource exploration, environmental protection, regulatory permitting, and land restoration. With offices in the Lewisburg, Scranton and York areas we are positioned to assist our clients throughout the Marcellus play.
Contact Jim Taylor
Our services include:
Baseline Water Quality Evaluations
Water Sampling & Analysis
GIS Mapping & Resources
Environmental Impact Studies
SRBC & DRBC Technical Support
Manage Flowback Water
Pad Stormwater Management
Drill & Frac Water Sourcing
Well Sighting & Design
Water Well Evaluation
E & S Management
Like any industry, there are both benefits and environmental concerns to be aware of with the Marcellus play. A few of the concerns that can be addressed with planning and proactive measures include groundwater protection, surface water protection and land restoration.
As with any drilling or construction operation there will be a lot of activity. Site access, exploration, clearing, grading, drilling, fracing all create an impact to the property. Know your lease agreement, protect sensitive areas, establish baseline water quality, and have a restoration plan, are things that can be done up front, prior to drilling, to minimize adverse impacts during the construction operations and after drilling is completed.
Drinking Water Protection
The process of deep shale hydrofracturing is not likely to directly impact groundwater or potable water supplies. Drilling targets the narrow layer of Marcellus Shale at depths around one mile below the ground surface. Useable groundwater and water supply wells are located in aquifers hundreds of feet below the ground surface. Therefore there are thousands of feet of alternating sedimentary lithologic sequences (bedrock layers) separating the target shale layer from the water well aquifers. At these great depths and the associated pressures, there are very few permeable fractures that are available to transport water, and fractures that are present are not interconnected for any significant length. This is why the shale reservoir needs to be fractured and propped in order to remove the gas. Even induced fractures from the 'fracing' process do not propagate large distances, let along thousands of feet. Bedrock layers such as shale and siltstone also create barriers to groundwater flow. These factors all combine to greatly limit the flow of water, so that a time frame for water to flow from the Marcellus upward into the shallow drinking water supply, may be on the order of hundreds to thousands of years.
Reported impacts to groundwater from fracing are typically associated with shallow gas resources outside the Marcellus shale play.
While hydrofracing itself is not likely to impact groundwater, there are certainly other activities associated with the drilling and fracing that can potentially impact groundwater. These include poor well casings, surface runoff, improper handling, storage and treatment of return water.
Prior to starting any drilling operation it is important to gather water samples from water wells and the aquifer to establish baseline groundwater quality. That way if a suspected problem arises in the future, post drilling samples can be collected and compared to the baseline.
Other measures to reduce potential groundwater impacts include oversight during drilling and casing installation; proper design, permitting, installation and maintenance of stormwater runoff and erosion control.
Water Use in Marcellus shale gas development
Water is an essential component of deep shale gas development. Water is used for drilling, where a mixture of clay and water is used to carry rock cuttings to the surface, as well as to cool and lubricate the drill bit. Drilling a typical Marcellus deep shale gas well requires approximately 100,000 gallons of water. Water is also used in hydraulic fracturing, where fracing a typical horizontal deep shale gas well requires an average of five and a half million gallons per well. Sources of water can include rivers, creeks, lakes, and groundwater. However in remote drilling areas these quantities of water are not easily available so water is typically transported by truck to drilling locations and stored in tanks or impoundments prior to being used. Temporary pipelines to transport water supplies can also be used. Some alternative sources of water being looked at include wastewater treatment plant discharge, the reuse of frac water or even acid mine drainage.
Frac fluid is mostly water with additives and surfactants for friction reduction and to prevent fouling. Newly created fractures are "propped" open by sand or other proppants, included with the frac fluid. The injected water that returns out of the formation and back to the groundsurface is termed flowback water. Around 40% of the frac water can return as flowback water and must be contained and treated. The problem with flowback water is not the initial chemistry of the frac water, but the constituents picked up while in contact with the shale reservoir. Flowback water is very high in total dissolved solids (TDS). It also contains small percentages of hydrocarbons, metals, and radioactive material. With limited cost effective methods for mitigating the high volume, high TDS/salinity of flowback water, treatment is typically achieved by transporting the water to a public wastewater treatment plant. Advanced treatment technologies are highly sought after for both economical and environmental reasons.
Contact Jim Taylor, P.G. for additional information on our Shale Gas Services
Taylor GeoServices, Inc. * 38
Bishop Hollow Road, Suite 200 * Newtown Square, PA 19073