- Below the Gulf of Mexico, hydrocarbons flow upward through
an intricate network of conduits and reservoirs. They start in thin layers
of source rock and, from there, buoyantly rise to the surface. On their
way up, the hydrocarbons collect in little rivulets, and create temporary
pockets like rain filling a pond. Eventually most escape to the ocean.
And, this is all happening now, not millions and millions of years ago,
says Larry Cathles, a chemical geologist at Cornell University.
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- "We're dealing with this giant flow-through system
where the hydrocarbons are generating now, moving through the overlying
strata now, building the reservoirs now and spilling out into the ocean
now," Cathles says.
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- He's bringing this new view of an active hydrocarbon
cycle to industry, hoping it will lead to larger oil and gas discoveries.
By matching the chemical signatures of the oil and gas with geologic models
for the structures below the seafloor, petroleum geologists could tap into
reserves larger than the North Sea, says Cathles, who presented his findings
at the meeting of the American Chemical Society in New Orleans on March
27.
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- This canvas image of the study area shows the top of
salt surface (salt domes are spikes) in the Gas Research Institute study
area and four areas of detailed study (stratigraphic layers). The oil fields
seen here are Tiger Shoals, South Marsh Island 9 (SMI 9), the South Eugene
Island Block 330 area (SEI 330), and Green Canyon 184 area (Jolliet reservoirs).
In this area, 125 kilometers by 200 kilometers, Larry Cathles of Cornell
University and his team estimate hydrocarbon reserves larger than those
of the North Sea. Image by Larry Cathles.
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- Cathles and his team estimate that in a study area of
about 9,600 square miles off the coast of Louisiana, source rocks a dozen
kilometers down have generated as much as 184 billion tons of oil and gas
- about 1,000 billion barrels of oil and gas equivalent. "That's 30
percent more than we humans have consumed over the entire petroleum era,"
Cathles says. "And that's just this one little postage stamp area;
if this is going on worldwide, then there's a lot of hydrocarbons venting
out."
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- According to a 2000 assessment from the Minerals Management
Service (MMS), the mean undiscovered, conventionally recoverable resources
in the Gulf of Mexico offshore continental shelf are 71 billion barrels
of oil equivalent. But, says Richie Baud of MMS, not all those resources
are economically recoverable and they cannot be directly compared to Cathles'
numbers, because "our assessment only includes those hydrocarbon resources
that are conventionally recoverable whereas their study includes unconventionally
recoverable resources." Future MMS assessments, Baud says, may include
unconventionally recoverable resources, such as gas hydrates.
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- Of that huge resource of naturally generated hydrocarbons,
Cathles says, more than 70 percent have made their way upward through the
vast network of streams and ponds, venting into the ocean, at a rate of
about 0.1 ton per year. The escaped hydrocarbons then become food for bacteria,
helping to fuel the oceanic food web. Another 10 percent of the Gulf's
total hydrocarbons are hidden in the subsurface, representing about 60
billion barrels of oil and 374 trillion cubic feet of gas that could be
extracted. The remaining hydrocarbons, about 20 percent, stay trapped in
the source strata.
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- Driving the venting process is the replacement of deep,
carbonate-sourced Jurassic hydrocarbons by shale-sourced, Eocene hydrocarbons.
Determining the ratio between the younger and older hydrocarbons, based
on their chemical signatures, is key to understanding the migration paths
of the oil and gas and the potential volume waiting to be tapped. "If
the Eocene source matures and its chemical signature is going to be seen
near the surface, it's got to displace all that earlier generated hydrocarbon
- that's the secret of getting a handle on this number," Cathles says.
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- Another important key to understanding hydrocarbon migration
is "gas washing," Cathles adds. A relatively new process his
research team discovered in the Gulf work, gas washing refers to the regular
interaction of oil with large amounts of natural gas. In the northern area
of Cathles' study area, he estimates that gas carries off 90 percent of
the oil.
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- Ed Colling, senior staff geologist at ChevronTexaco,
says that identifying the depth at which gas washing occurs could be extremely
useful in locating deeper oil reserves. "If you make a discovery,
by back tracking the chemistry and seeing where the gas washing occurred,
you have the opportunity to find deeper oil," he says.
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- Using such information in combination with the active
hydrocarbon flow model Cathles' team produced and already existing 3-D
seismic analyses could substantially improve accuracy in drilling for oil
and gas, Colling says. ChevronTexaco, which funds Cathles' work through
the Global Basins Research Network, has been working to integrate the technologies.
(Additional funding comes from the Gas Research Institute.)
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- "All the players are looking for bigger reserves
than what's on shore," Colling says. And deep water changes the business
plan. With each well a multibillion dollar investment, the discovery must
amount to at least several hundred million barrels of oil and gas for the
drilling to be economic. Chemical signatures and detailed basin models
are just more tools to help them decide where to drill, he says.
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- "A big part of the future of exploration is being
able to effectively use chemical information," Cathles says. Working
in an area with more oil by at least a factor of two than the North Sea,
he says he hopes that his models will help companies better allocate their
resources. But equally important, Cathles says, is that his work is shifting
the way people think about natural hydrocarbon vent systems - from the
past to the present.
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- (First published 2003)
- http://www.geotimes.org/june03/NN_gulf.html
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