DEEP UNDERWATER, and deeper underground, scientists see surprising hints
that gas and oil deposits can be replenished, filling up again, sometimes
Although it sounds too good to be true, increasing evidence from the Gulf
of Mexico suggests that some old oil fields are being refilled by petroleum
surging up from deep below, scientists report. That may mean that current
estimates of oil and gas abundance are far too low.
Recent measurements in a major oil field show "that the fluids were
changing over time; that very light oil and gas were being injected from below,
even as the producing [oil pumping] was going on," said chemical oceanographer
Mahlon "Chuck" Kennicutt. "They are refilling as we speak. But whether this is
a worldwide phenomenon, we don't know."
Also not known, Kennicutt said, is whether the injection of new oil from
deeper strata is of any economic significance, whether there will be enough to
be exploitable. The discovery was unexpected, and it is still "somewhat
controversial" within the oil industry.
Kennicutt, a faculty member at Texas A&M University, said it is now clear
that gas and oil are coming into the known reservoirs very rapidly in terms of
geologic time. The inflow of new gas, and some oil, has been detectable in as
little as three to 10 years. In the past, it was not suspected that oil fields
can refill because it was assumed the oil formed in place, or nearby, rather
than far below.
According to marine geologist Harry Roberts, at Louisiana State University,
"petroleum geologists don't accept it as a general phenomenon because it
doesn't happen in most reservoirs. But in this case, it does seem to be
happening. You have a very leaky fault system that does allow it [petroleum] to
migrate in. It's directly connected to an oil and gas generating system at
What the scientists suspect is that very old petroleum - formed tens of
millions of years ago - has continued migrating up into reservoirs that oil
companies have been exploiting for years. But no one had expected that depleted
oil fields might refill themselves.
Now, if it is found that gas and oil are coming up in significant amounts,
and if the same is occurring in oil fields around the globe, then a lot more
fuel than anyone expected could become available eventually. It hints that the
world may not, in fact, be running out of petroleum.
"No one has been more astonished by the potential implications of our work
than myself," said analytic chemist Jean Whelan, at the Woods Hole
Oceanographic Institution, in Massachusetts. "There already appears to be a
large body of evidence consistent with ...oil and gas generation and migration
on very short time scales in many areas globally," she wrote in the journal Sea
"Almost equally surprising," she added, is that "there seem to be no
compelling arguments refuting the existence of these rapid, dynamic migration
The first sketchy evidence of this emerged in 1984, when Kennicutt and
colleagues from Texas A&M University were in the Gulf of Mexico trying to
understand a phenomenon called "seeps," areas on the seafloor where sometimes
large amounts of oil and gas escape through natural fissures.
"Our first discovery was with trawls. We knew it was an area of massive
seepage, and we expected that the oil seeps would poison everything around" the
site. But they found just the opposite.
"On the first trawl, we brought up over two tons of stuff. We had a tough
time getting the nets back on board because they were so full" of very
odd-looking seafloor creatures, Kennicutt said. "They were long strawlike
things that turned out to be tube worms.
"The clams were the first thing I noticed," he added. "They were pretty
big, like the size of your hand, and it was obvious they had red blood inside,
which is unusual. And these long tubes - 3, 4 and 5 feet long - we didn't know
what they were, but they started bleeding red fluid, too. We didn't know what
to make of it."
The biologists they consulted did know what to make of it. "The experts
immediately recognized them as chemo-synthetic communities," creatures that get
their energy from hydrocarbons - oil and gas - rather than from ordinary
foods. So these animals are very much like, but still different from, recently
discovered creatures living near very hot seafloor vent sites in the Pacific,
Atlantic and other oceans.
The difference, Kennicutt said, is that the animals living around cold
seeps live on methane and oil, while the creatures growing near hot water vents
exploit sulfur compounds in the hot water.
The discovery of abundant life where scientists expected a deserted
seafloor also suggested that the seeps are a long-duration phenomenon. Indeed,
the clams are thought to be about 100 years old, and the tube worms may live as
long as 600 years, or more, Kennicutt said.
The surprises kept pouring in as the researchers explored further and in
more detail using research submarines. In some areas, the methane-metabolizing
organisms even build up structures that resemble coral reefs.
It has long been known by geologists and oil industry workers that seeps
exist. In Southern California, for example, there are seeps near Santa Barbara,
at a geologic feature called Coal Oil Point. And, Roberts said, it's clear
that "the Gulf of Mexico leaks like a sieve. You can't take a submarine dive
without running into an oil or gas seep. And on a calm day, you can't take a
boat ride without seeing gigantic oil slicks" on the sea surface.
Roberts added that natural seepage in places like the Gulf of Mexico "far
exceeds anything that gets spilled" by oil tankers and other sources.
"The results of this have been a big surprise for me," said Whelan. "I
never would have expected that the gas is moving up so quickly and what a huge
effect it has on the whole system."
Although the oil industry hasn't shown great enthusiasm for the idea -
arguing that the upward migration is too slow and too uncommon to do much good
- the search for new oil and gas supplies already has been affected, Whelan and
Kennicutt said. Now, companies scan the sea surface for signs of oil slicks
that might point to new deposits.
"People are using airplane surveys for the slicks and are doing water
column fluorescence measurements looking for the oil," Whelan said. "They're
looking for the sources of the seeps and trying to hook that into the seismic
evidence" normally used in searching for buried oil.
Similar research on known oil basins in the North Sea is also under way,
and "that oil is very interesting. There are absolutely marvelous pictures of
coral reefs which formed from seepage [of gas] from North Sea reservoirs,"
Analysis of the ancient oil that seems to be coming up from deep below in
the Gulf of Mexico suggests that the flow of new oil "is coming from deeper,
hotter [sediment] formations" and is not simply a lateral inflow from the old
deposits that surround existing oil fields, she said. The chemical composition
of the migrating oil also indicates it is being driven upward and is being
altered by highly pressurized gases squeezing up from below.
This upwelling phenomenon, Whelan noted, fits into a classic analysis of
the world's oil and gas done years ago by geochemist-geologist John Hunt. He
suggested that less than 1 percent of the oil that is generated at depth ever
makes it into exploitable reservoirs. About 40 percent of the oil and gas
remains hidden, spread out in the tiny pores and fissures of deep sedimentary
And "the remaining 60 percent," Whelan said, "leaks upward and out of the
sediment" via the numerous seeps that occur globally.
Also, the idea that dynamic migration of oil and gas is occurring implies
that new supplies "are not only charging some reservoirs at the present time,
but that a huge fraction of total oil and gas must be episodically or
continuously bypassing reservoirs completely and seeping from surface sediments
on a relatively large scale," Whelan explained.
So far, measurements involving biological and geological analysis, plus
satellite images, "show widespread and pervasive leakage over the entire
northern slope of the Gulf of Mexico," she added.
"For example, Ian MacDonald at Texas A&M has published some remarkable
satellite photographs of oil slicks which go for miles in the Gulf of Mexico in
areas where no oil production is occurring." Before this research in oil
basins began, she added, "changes in reservoired oils were not suspected, so no
reliable data exists on how widespread the phenomenon might be in the Gulf
Coast or elsewhere."
The researchers, especially the Texas team, have been working on this
subject for almost 15 years in collaboration with oil industry experts and
various university scientists. Their first focus was on the zone called South
Eugene Island block 330, which is 150 miles south of New Orleans. It is known
as one of the most productive oil and gas fields in the world. The block lies
in water more than 300 feet deep.
As a test, the researchers attempted to drill down into a known fault zone
that was thought to be a natural conduit for new petroleum. The drilling was
paid for by the U.S. Department of Energy.
Whelan recalled that as the drill dug deeper and deeper, the project seemed
to be succeeding, but then it abruptly ended in failure. "We were able to
produce only a small amount of oil before the fault closed, like a giant
straw," probably because reducing the pressure there allowed the fissure to
In addition to the drilling effort and the inspection of seeps, Whelan and
her colleagues reported that three-dimensional seismic profiles of the
underground reservoirs commonly show giant gas plumes coming from depth and
disrupting sediments all the way to the surface.
This also shows that in an area west of the South Eugene Island area, a
giant gas plume originates from beneath salt about 15,000 feet down and then
disrupts the sediment layers all the way to the surface. The surface expression
of this plume is very large - about 1,500 feet in diameter. One surprise,
Whelan said, was that the gas plume seems to exist outside of faults, the
ground fractures, which at present are the main targets of oil exploration.
It is suspected that the process of upward migration of petroleum is driven
by natural gas that is being continually produced both by deeply buried
bacteria and from oil being broken down in the deeper, hotter layers of
sediment. The pressures and heat at great depth are thought to be increasing
because the ground is sinking - subsiding - as a result of new sediments piling
up on top. The site is part of the huge delta formed over thousands of years
by the southward flow of the massive Mississippi River. Like other major
deltas, the Mississippi's outflow structure is continually being built from
sands, muds and silts washed off the continent.
Analysis of the oil being driven into the reservoirs suggests they were
created during the so-called Jurassic and Early Cretaceous periods (100 million
to 150 million years ago), even before the existing basin itself was formed.
This means the source rock is buried and remains invisible to seismic imaging
beneath layers of salt.
In studying so-called biomarkers in the oil, Whelan said, it was concluded
that the oil is closely related to other very old oils, implying that it "was
probably generated very early and then remained trapped at depth until
recently." And, she added, other analyses "show that this oil must have
remained trapped at depths and temperatures much greater than those of the
present-day producing reservoirs."
At great depth, where the heat and pressure are high enough, she explained,
methane is produced by oil being "cracked," and production of gas "is able to
cause sufficient pressure to periodically open the fracture system [faults] and
allow upward fluid flow of methane, with entrapment of oil in its path."
Oil and Water
The underground sediments of a zone called South Eugene Island Block 330,150
miles south of New Orleans, is one of the world's most productive oil and gas
fields. It lies more than 300 feet deep in the Gulf of Mexico.
Buried salt layers gradually creep upward to form tall "salt domes, " which
can trap gas and oil coming up from deep below.
SOURCE: Woods Hole
[CORRECTION: A graphic yesterday accompanying a story on saltwater seeping
into North Shore aquifers should have shown the northernmost tip of Kings Point
in the Great Neck peninsula as having 1,000 milligrams of chloride per liter
of water. pg. A02 ALL 4/17/02]
Suspected Oil Reserves
Geologists now seek "seeps" of ancient oil leaking out through surface features
as clues to where to drill.
Oil Reservoir (8,250 feet)
Migration of new supplies of gas and oil from deeply buried sediments probably
occurs through cracks - faults - that create pathways leading upward.
Ancient salt barrier
Recent evidence suggests that ancient petroleum continues to squeeze up through
such faults. Some gas and oil get trapped on the way up, recharging existing
Oil dissolved in supercritical methane (25,146 feet)
Source rock with hot gass
SOURCE: Woods Hole Oceanographic Institution