shale geology & background

Walden Associates

Technical, Regulatory and Public Policy Issues

Hydrofracking Shale for New Energy

Geology and Background

Walden Associates

Geology of Shale Gas

• Shale is a fine-grained sedimentary rock formed from silt, clay and fine sand.

• Gas and oil producing shale formations are often thousands of feet thick and were formed in deep marine basins

• Natural gas and oil are derived from organic material that make up part of the shale.

• These organics from hydrocarbons as the sediments are subjected to great heat and pressure in the earth.

Walden Associates

• Natural gas may also be produced where shale and other sedimentary rocks are interbedded with coal. This is called “coal-seam methane”.

• Natural gas is primarily composed of methane and other gasses, including butane, propane as shown on the next slide.

Geology of Shale Gas

Walden Associates

Typical composition of natural gas From naturalgas.org

Methane CH4 70-90%

Ethane C2H6 0-20%

Propane C2H8 0-20%

Butane C4H10 0-20%

Carbon Dioxide CO2 0-8%

Oxygen O2 0 – 0.2%

Nitrogen N2 0-5%

Hydrogen sulphide H2S 0-5%

Rare gases AR, He, Ne Trace

*Gases in percent by volume

Natural Gas Composition

Walden Associates

• The organic sediments are converted to oil and gas by thermal breaking of carbon bonds in the organic materials making up part of the shale.

• Natural gas may occur along with oil and this is termed “wet gas”.

• Wet gas may be found in parts of gas-producing shale formations and not in others (“dry gas”).

• Oil is formed under generally lower temperatures and pressures than natural gas.

Natural Gas and Oil

Walden Associates

• Oil-field natural gas is typically trapped at the top of oil reservoirs in porous rock

• In oil fields, natural gas is often burned (“flared”) and not recovered

Natural Gas and Oil

Walden Associates

• Deep Natural Gas – Is typically found at great depths (>15,000 ft) and is more expensive to drill for and recover. The geology may include various sediments such as sandstone, limestone and shale.

• Shale Gas – May occur at any depth and the gas is transmitted along the most permeable pathways, such as bedding planes and fractures.

Natural Gas Shale Producing Geologic Formations

Walden Associates

• Tight Gas – As the name implies, this gas is trapped in “tight” geologic formations that call for special techniques, including hydrofracturing (“fracking”) to extract the gas.

• Coal Bed Methane – Gas that is associated with coal-bearing sedimentary rocks. It can be extracted economically but is generally regarded as a nuisance or even a hazard to coal mining.

Natural Gas Shale Producing Geologic Formations

Walden Associates

• Different types of gas producing units, as described above, may occur in the same geologic formation, such as the Bakken, Barnett, Montney, Haynesville, Marcellus, and most recently the Eagle Ford, Niobrara and Utica Shales.

• This can imply different technical and economic advantages or disadvantages to gas recovery over the area of the formation and the potential for environmental impacts to gas drilling and production.

Natural Gas Shale Producing Geologic Formations

Walden Associates

Walden Associates

Geologic Schematic Showing Occurrence of Different Natural Gas Sources

Walden Associates

Geology of the Marcellus Shale

Walden Associates