The Gulf of Guinea off West Africa, another area of
study for Barry Katz, has yielded many surprises.
"We have found that many of the oils in the Gulf
of Guinea are not simply a result of a single episode of hydrocarbon
charging," Katz said. "We've had oils come in at relatively shallow
burial depths where we would expect them to be of relatively poor
quality and get positive surprises."
These high quality crudes seem to be the result of
a very complicated history of episodic hydrocarbon movement and
recent hydrocarbon movement into the reservoirs, Katz explained.
These newer oils into the reservoirs are not yet biodegraded and
improve the overall quality, mitigating much of the problems if
the oils were solely older, biodegraded crudes.
"We have seen places where the API gravity is as
much as 20 degrees higher than we would have expected simply based
on the depth of the reservoir," he said.
Unfortunately, not all the fields in the Gulf of
Guinea experience this same favorable changing history.
"Even in a given field, not all the reservoirs experience
this mixing of older and new crudes," he said. "It is a very complicated
process and something we are still trying to work out."
Katz said that within the same reservoir he and his
colleagues may see a difference of 15 to 20 degrees on opposite
sides of a fault.
"That complicates the commercial viability of a field
where you know the volumes are present, but you may not be able
to produce much of the oil with current technology," he said.
Scientists have not reached a point where they can
predict with accuracy where these crude quality changes occur in
the Gulf of Guinea, Katz continued.
"We are still pulling together data, but the approach
we are taking is to attempt to tie a better understanding of the
structural history of the basin to the basin's generation and migration
history using various modeling tools," he said.
"Are we seeing differences in the timing of when
a fault moves — and is that possibly causing the crude quality
variations? Are there places where the faults actually intersect
and hydrocarbon conduits could be turning things on and off?
"Those," he said, "are the types of questions we
hope to answer."
Unlike onshore and shallow water exploration, the key
to the deepwater hunt is not only finding oil — it's finding oil
in sufficient quantities and increasingly of sufficient quality
to produce commercially.
Consequently, a growing research effort is centered
on finding ways to predict oil quality long before the drill bit
starts turning.
"With increasing water depths, project economics
are much more sensitive to oil quality," said Barry Katz, a ChevronTexaco
fellow who is currently working on the problem of oil quality in
deepwater settings. "At these depths there are significant production
issues associated with poor quality, higher viscosity crudes that
operators do not face on the shelf or onshore. There is only so
many dollars per barrel you are able to discount for sulfur and
acid content before it is no longer an economically viable project.
"What we are trying to do is come up with better
ways of predicting pre-drill not only what the risks are in terms
of the absence or presence of hydrocarbons, but also the oil quality
risk."
Katz will present a paper at the AAPG International
Conference and Exhibition this fall in Cancun, Mexico, titled "Oil
Quality in Deepwater Settings — Concerns, Perceptions, Observations
and Reality." His talk is part of a session on geochemical aspects
of hydrocarbon quality and the prediction of producibility.
The primary focus of Katz' research is the cooler
surface temperatures at the mud line at deeper water depths.
"As a result of these cooler temperatures the entire
sedimentary column at similar depths is cooler, which allows for
bacterial activity to proceed at greater depths," he said. "Consequently,
we have potentially greater risks of biodegradation in deeper water
than we would in shallow water at the same drill depths."
Each deepwater region of the world also has unique
oil quality issues as well.
"For example, in the Gulf of Mexico we not only have
the biodegradation problems, but the nature of the source rocks
also seems to significantly influence the sulfur content of the
crude oil," he said. "There has been a great deal of work over the
last 15 years showing that, although we have only a limited number
of samples of what might be the source rocks in the Gulf, there
are a number of different facies, some of which yield a high sulfur
crude.
"Today, using piston core samples, we are working
to map out where we would anticipate those poor quality crudes to
occur."
Gaining Confidence
In simplistic terms, Katz said the work is focused
on identifying crudes associated with carbonate source rocks that
tend to lack iron, which means the organic matter is enriched with
sulfur.
Fortunately, the more typical shale source rocks
in the Gulf of Mexico contain iron and the sulfur is absorbed by
the iron as opposed to the organic matter, meaning sweeter crude
oils.
"Portions of the deepwater Gulf reservoirs appear
to have oils derived principally from carbonates within the section
and other areas are dominated by crudes derived from shales within
the stratigraphic sequence," he said. "We are trying to high grade
areas based on the differences in source rock facies to target the
higher quality oils."
The piston core samples — a tool Katz maintained
has improved over time as scientists have pinpointed where and how
to collect samples — recover small amounts of oil from minute surface
seeps. The oil is extracted from the core samples and then characterized,
looking for biomarker compounds that are indicative either of a
clastic environment or a carbonate depositional setting.
"We make a lot of inferences, one of which is that
there is some relationship between what we collect at the surface
and what is actually happening at depth," he said. "We also have
to infer that the characteristics we see in the surface seeped oil
are reflecting predominately the source, and we aren't looking at
more recent contamination or things that the oil picked up as it
migrated through the section. But in samples that have had a lot
of hydrocarbons present, we are reasonably sure we can make those
inferences with confidence, and there have been thousands of piston
cores collected in the deepwater Gulf.
"We have been pretty successful in correlating our
findings with deepwater discoveries, which provides additional confidence
in the work," he added. "These crude oil quality assessments are
becoming one of the key risk elements we look at when we make the
very first decision on whether to even acquire deepwater acreage:
How confident are we about the chance for crude oil you can produce?"
Global Approach
Oil quality has long been an issue in the deep waters
offshore Brazil, where Katz said scientists see a little bit of
all the issues experienced elsewhere around the world.
"We see multiple charging events where there are
mixed oils but with varying degrees of mixing," he said, "and we've
seen places where there is no strong evidence of mixing. We see
places where on one side of a fault within a given structure there
is higher quality crude than on the other side. The Petrobras giant
Roncodor Field is an excellent example of where there are reasonably
high quality crudes on one side of a fault and significantly lower
quality oils on the other side of the fault."
While there are some similarities among deepwater
regions around the world, each area also has unique properties.
"That's why this work is global," he said. "I think
the processes are the same, but we start with slightly different
geology. That, along with the nature of the initial source — that
is where it is located in the stratigraphic column and when it is
generating relative to the time the reservoirs are in place — play
a role in crude quality."
He said the same situations occur in shallow water
and onshore, but they are not as big a concern in those settings.
"Everything is amplified in deepwater, where the
well costs are approaching $40 to $100 million each," he noted.
"Companies have walked away from several 100 million barrels of
oil equivalent discoveries due to inadequate oil quality.
"Finding oil does not necessarily mean we make money.
In fact, we must always keep in mind this business is not about
finding oil, it's about making money."