2. 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.
3. 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
4. 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
5. 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
6. 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
7. 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
8. 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
9. 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
