Unconventional Gas EarthCache
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This series of earth caches is based on the publication “Roadside
Geology Along the Alexandria to Ashland (AA) Highway.” The road
logs were published by the Kentucky Geological Survey to give the
public an appreciation of the geologic world around them.
Many geologists have referred to the AA Highway as a “treasure
trove” and “an outdoor classroom” in which to study diverse and
significant features such as faults, systemic rock boundaries,
fossils and ancient river markers. Buckle your seat belts and head
back in time and look for the clues as you head down the AA. Each
cache in this series will stop at a unique geologic formation and
will seek answers to some basic questions that should be easy to
calculate. Sizeable pull off areas are available at each stop in
the series. The calculations can be made from your car even, making
it handicap accessible!
Geologists call it continuous gas, but it is also called
unconventional gas, blanket gas or even weird gas. Whatever you
choose to call it, you must give it due respect for its growing
importance. The Department of Energy reports the share of
unconventional gas doubled from 17 percent of natural gas supplies
in 1990 to 35 percent in 2003. By 2025 it is projected to be 44
percent.
One of the places you find unconventional gas is in shale rocks
from the Devonian Period of geologic time. By current estimates,
the untapped shale gas resource in the US could total 500 to 700
trillion cubic feet! Down hole tools, 3-D seismic, geochemical logs
and special analysis software are being applied as operators begin
to see shale plays (prospects) more in terms of reservoirs than of
producing zones.
Millions of years ago, the remains of plants and animals decayed
and built up in thick layers. This decayed matter from plants and
animals is called organic material. It was once alive. Over time,
the mud and soil changed to rock, covered the organic material and
trapped it beneath the rock. Pressure and heat changed some of this
organic material into coal, some into oil (petroleum), and some
into natural gas.
Historically, conventional natural gas reservoirs have been the
most practical, and easiest, deposits to mine. However, as
technology and geological knowledge advances, unconventional
natural gas deposits are beginning to make up an increasingly
larger percentage of the supply picture.
So what exactly is unconventional gas? Substantial amounts of gas
have accumulated in geologic environments that differ from
conventional petroleum traps. This gas is termed unconventional gas
and occurs in tight, relatively impermeable sandstones, in joints
and fractures or absorbed into the matrix of shales, often of the
Devonian Period about 360 to 408 million years old, dissolved in
hot geopressure. Unconventional natural gas is more difficult and
less economical to extract, usually because the technology to reach
it is expensive.
One type of unconventional gas is called tight gas. It is trapped
in an unusually impermeable sandstone or limestone formation. The
problem is to get the low-permeability formation to release
sufficient gas to flow in economic amounts to the well bore.
Devonian shale is another form of unconventional gas. Shale is a
non-permeable rock, a clay compacted by pressure, where free gas is
stored in the rock pores or in natural fractures. As with other
unconventional types, the gas is stored continuously, and hydraulic
fracturing is used to make it flow freely.
Coal-bed methane is a third form of continuous gas. The coal-bed
gas reserves remain trapped in the coal seams as long as the water
table lies above it. To release the gas, a well is drilled and
water is pumped out to lower the water table and release the gas to
flow to the well bore.
The upper Devonian shale of the Appalachian Basin, which is known
by different names in different areas has produced gas since the
early 20th century. The shale is most commonly known as Chattanooga
or Ohio Shale. The carbon-rich Chattanooga-New Albany-Ohio Shales
constitute a present and potential economic resource.
Unconventional gas stored in the earth is produced differently than
conventional gas. It is stored uniformly in a formation that
extends over a wide area but is trapped in a rock formation that
requires additional resources to free it.
The search for natural gas begins with geologists locating the
types of rock that are usually found near gas and oil deposits.
Today their tools include seismic surveys that are used to collect
information about the rocks beneath. Scientists and engineers
explore a chosen area by studying rock samples, taking measurements
and studying the geomorphology of the area. If the site seems
promising, drilling begins.
Because oil and natural gas have a low density they will migrate
upward through the Earth and accumulate in a reservoir only if a
geologic structure is present to trap the petroleum. Geologic
structures wherein impermeable rocks occur above the permeable
reservoir rock are required. The job of petroleum geologists
searching for petroleum reservoirs, is to find conditions near the
Earth's surface where such traps might occur.
A trap is a configuration of rocks suitable for containing
hydrocarbons and sealed by a relatively impermeable formation
through which hydrocarbons will not migrate. Traps are described as
structural traps (in deformed strata such as folds and faults) or
stratigraphic traps (in areas where rock types change, such as
unconformities, pinch-outs and reefs). A trap is an essential
component of a petroleum system.
There are three basic forms of a structural trap in petroleum
geology- an Anticline Trap, Fault Trap, and Salt Dome Trap. The
common link between these three is simple- some part of the earth
has moved in the past, creating an impedance to oil flow.
An anticline is an example of rocks which were previously flat, but
have been bent into an arch. Oil that finds its way into a
reservoir rock that has been bent into an arch will flow to the
crest of the arch, and get stuck (provided, of course, that there
is a trap rock above the arch to seal the oil in place).
Fault traps are formed by movement of rock along a fault line. In
some cases, the reservoir rock has moved opposite a layer of
impermeable rock. The impermeable rock thus prevents the oil from
escaping. In other cases, the fault itself can be a very effective
trap. Clays within the fault zone are smeared as the layers of rock
slip past one another.
Salt domes form if enough heat and pressure are exerted. To get all
the way to the Earth's surface, salt has to push aside and break
through many layers of rock in its path. This is what ultimately
will create the oil trap.
A second major class of traps is the stratigraphic trap. Such traps
include a variety of sealed geologic containers capable of
retaining gas formed by changes in rock type such as pinch-outs,
unconformities, or reefs.
Devonian black shales underlie approximately two-thirds of
Kentucky. They serve as both the source and trap for large
quantities of natural gas. The US Geological Survey has estimated a
mean of 70.2 trillion cubic feet of undiscovered natural gas and a
mean of 54 million barrels of undiscovered oil are located in the
Appalachian Basin.
New technologies have been developed to drill, complete and
stimulate these wells. Shale gas has been produced for years from
shales with natural fractures. The shale gas boom in recent years
has been due to modern technology in creating extensive artificial
fractures around well bores.
Horizontal drilling is often used with shale gas wells . Some of
the gas produced is held in natural fractures, some in pore spaces,
and some is adsorbed onto the organic material. The gas in the
fractures is produced immediately. The gas adsorbed onto organic
material is released as the formation pressure declines. Because
the pipe that drives oil drills is surprisingly flexible, a
horizontal well can snake around to reach isolated pockets or
follow a reservoir that meanders across the terrain.
Hydraulic fracturing involves the use of water to break up the rock
and shale that holds reserves of natural gas. Water, mixed with
sand in some cases, is pumped into the ground at a high pressure,
and breaks the rock so that the reserves of natural gas may be
released.
Future energy resources of the United States, particularly gas,
will be found in what we consider today to be unconventional
reservoirs, especially low-permeability reservoirs in shales,
siltstones, fine-grained sands, and carbonates.
Travel to Mile Marker 11 of Kentucky Route 9/10 on the AA Highway
in Lewis County. The Ohio Shale here is of the Upper Devonian unit.
It is extensively exposed in an outcrop belt extending from
northeastern Ohio into southern Ohio and into Kentucky. The
formation here is of the older Devonian Period Ohio Shale below
with younger Mississippian Period Rock above. The total formation
exposed from the bottom to the top is roughly 270’.
To gain credentials as a geocaching geologic sleuth, answer the
following and email us. Then post a picture of the unconventional
gas potential shale with your log.
1. Estimate the amount of rock exposed from the two geologic
periods represented.
a. Mississippian Rock 50 feet/Devonian Shale 220 feet
b. Mississippian Rock 70 feet/Devonian Shale 200 feet
c. Mississippian Rock 90 feet/Devonian Shale 180 feet
2. Shoot an elevation for the outcrop of Devonian Shale here.
It is likely that other energy resources also await discovery or
rebirth in the US as new technology allows us to locate, define,
interpret and extract them economically. The Natural Gas Plays on
Devonian shale and other unconventional gas sources could be the
energy solution for the future. Through use of technology,
geologists working with the gas industry are turning previously
uneconomic resources into proven reserves and production.
Now buckle up again and get ready for another new geologic
adventure along the AA Highway- Kentucky’s gateway to the past.
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