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Oil Fields' Free Refill / Seepage from the depths in some reservoirs hints at greater resources than expected

DEEP UNDERWATER, and deeper underground, scientists see surprising hints

that gas and oil deposits can be replenished, filling up again, sometimes

rapidly.

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

great depth."

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

Technology.

"Almost equally surprising," she added, is that "there seem to be no

compelling arguments refuting the existence of these rapid, dynamic migration

processes."

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,"

Whelan said.

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

rock formations.

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

collapse.

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)

Fault

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 fields.

Oil dissolved in supercritical methane (25,146 feet)

Source rock with hot gass

(30,178.5 feet)

SOURCE: Woods Hole Oceanographic Institution

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