Case of the Bad Maturity Data
1 in a series of illustrative geochemical real case
well-worn sign on the door read Rocky Vale, Private
Geochemist. Inside, sitting at an old wooden desk
amid a clutter of books and papers, peering at
geochemical data on a computer screen was the
man himself, Mr. Rocky Vale.
Decades of fast food had rendered him rather stout
and years of sun avoidance left his bald head looking
like a white-washed cannon ball. Studying geochemical
data and gazing into computer screens had taken its
toll physically, but mentally Vale was sharper than
a splinter on a park bench. The real insiders within
the industry knew Vale and his odd assortment of associates
were crackerjacks when it came to geochemical problem
if on cue, the telephone on Vale's desk rang. "Rocky,
is that you?" a gruff and somewhat twangy male
voice inquired. "Well, if it isn't Arcata Bottoms,
our favorite client," exclaimed Vale with a certain
wryness to his voice. "How have you been? Is
that story about you and the horse they were telling
at the geological society meeting true?"
ahem, some if it," Arcata confessed sheepishly,
"but that's not why I'm calling. I have a geochemical
problem. It seems one of my new clients had a large
number of wells on their concession analyzed with
vitrinite reflectance, but a maturity map of the oil
source rock developed from that geochemical analysis
doesn't fit the distribution of oil occurrences. According
to the resulting interpretation large oil fields were
often being associated with the 'immature' source
rocks. The explanation given was that 'somehow' oil
must have been generated "early" from the
source rocks in the basin studied. The oils, however,
were mature by every geochemical technique available
(eg. gc, gc/ms, etc.), and basin geology showed that
migration from more mature oil source rocks outside
the concession area could not have taken place. As
their consultant, they are looking to me for answers.
What should I tell them, Rocky?"
like something ve hear all too often." replied
Rocky thoughtfully as he tilted back in his chair.
"Let me call in our lab manager and get him on
the speakerphone. You know Dr. Louis Gravel, don't
was already entering the room and caught part of the
conversation. "Vot do you vant?," impatiently
asked the slender, quiet, gray ponytailed, German
scientist who had daily experience in dealing with
misguided petroleum professionals. He pushed back
his disheveled hair and settled himself into a chair.
making Bottoms repeat his problem, the bespectacled,
white coated doctor let out a sigh and began in his
very best English, "Too many of our low-cost
competitors cut corners and make quick analyses without
thinking like scientists. Like you said, it happens
all too often. I suppose some one dimensional basin
modeling vas attempted on several vells but the reported
maturities could not be derived from any reasonable
modeling permutation: they vere alvays too low. Is
that pretty close to what has been happening, Mr.
amazing, simply amazing," blurted Bottoms with
way too much awe in his voice. "You practically
read my mind. How do you do that, and just what the
heck is going on down there?"
"I never realized how easy it is to ruin a geochemical
analysis without even trying!"
it's easy," said a sanguine Gravel rolling his
blue-green eyes at the ceiling in an exaggerated movement.
"Let me explain to you something about vitrinite
reflectance, computed oil maturity and how the suppression
of vitrinite reflectance can result in a faulty low
is an organic matter component derived mostly
from lignin and cellulose of vascular plants.
It increases in light reflectivity as organic
matter in rocks is buried (e.g. lignite, bituminous,
and anthacite coals) and this property is used
to determine sedimentary rock maturity. Precise
vitrinite relectance is measured with a microscope
and calibrated to the oil and gas generation and
preservation maturity zones.
is fluorescent vitrinite that contains marine
or lacustrine lipids in its structure, usually
due to deposition in anoxic or suboxic environments.
When the organic matter reaches oil generation
maturity, the reflectivity of lipid-rich vitrinite
is reduced (suppressed). This can result in anomalously
low reflectance measurements which must be corrected
to obtain the true maturity.
the oil vindow," he continued, settling into
his lecture room pace, "the reflectance of vitrinite
deposited in reducing marine or lacustrine environments
is often lower (eg. 0.15-0.55%Ro) compared to vitrinite
deposited in more oxic environments. This can be due
to: the incorporation of lipids into the vitrinite
formation of vitrinitelike macerals from algae, marine
grasses, etc. the impregnation of generated bitumen
or incorporation of migrated oil, or several other
causes (Mukhopadhyay, P.K. 1994, Vitrinite Reflectance
as a Maturity Parameter, Chapter one, p. 7-12, (book)
ACS Symposium Series 570)
lipid reflectance suppression is not recognized, the
reported vitrinite reflectance maturities vill be
too low and a mature source rock may be identified
as immature. Are you vid me so far?".
Porady, jak leczyć
waiting for a reply, after all it was Arcata Bottoms
on the other end of the line, he continued. "The
problem can be recognized in a maturity profile of
a vell by anomalously low 'maturities' of oil source
rocks that have generated oil compared to the section
above and below, but is often overlooked if only the
oil source rocks themselves vere analyzed. Fortunately,
lipid-rich vitrinite fluoresces and can easily be
identified if the time is taken to analyze the samples
properly. Unfortunately this isn't alvays done by
some low cost geochem shops.
reflectance suppressed vitrinite is very common but
is not alvays recognized due to microscopist inexperience
or failure to use fluorescence ven doing reflectance
work. Several research studies have sought to calibrate
suppressed vitrinite reflectance to normal vitrinite
reflectance (Mukhopadhyay, 1994)."
Bottoms interrupted. I never realized how easy it
is to ruin a geochemical analysis without even trying!
How come I never heard of any of this before?"
and even Vale who was still quietly listening in,
could think of at least a dozen reasons, but always
professionals, both restrained any impulse to elaborate.
"You must have been busy discovering the North
Slope," Vale finally butted in with a mock seriousness
that Bottoms misinterpreted as high praise.
Bottoms," Gravel continued, "tell your client
that H.B. Lo of Exxon, based partially on microscopy
work done by DGSI, related the degree of reflectance
suppression to the hydrogen index of oil source rock
and non source rock shales (Lo, H.B.,1993,Organic
Geochemistry, Vol. 20, No.6, p. 653657). He found
that the degree of suppression is related both to
the hydrogen index of the immature source rock and
to the maturity. (Click here for the figure 432x301).
suppression increases with oil source rock quality
and is greatest at mid oil vindow maturities. DGSI
experience has shown this model to be very reliable
although some modifications have been made since Dr.
Lo's original work was published."
(Lo, H.B., 1998, Abstracts of the 15th Annual Meeting
of TSOP, p. 42-43.)
"The lesson here is obvious: If it's worth doing,
it's worth doing right."
thank you Dr. Gravel, and you too Rocky." Bottoms
was relieved and preening that he had made the right
decision to call DGSI. "You guys certainly know
how to make an old consultant like me look good in
front of my clients. If I listen to you more often,
I'll be able to double my rates. What do you suggest
I do now?"
again, several rather unprofessional equine images
appeared in Vale's mind which were quickly suppressed
while Gravel's thoughts were already on his next project.
Gravel rose and in his odd, stilted kind of walk,
left the room, an untied shoelace trailing him into
scratched his shiny domed head, folded his fingers
over the hill of his belly and with his eyes closed,
slipped into his 'private eye voice. "In a similar
horror story of what can happen if reflectance suppression
is not recognized, I seem to recall that DGSI was
asked to reanalyze some of the wells. The result was
soon obvious: the calculated vitrinite reflectance
maturities were too low in wells where the oil source
rock was in the oil window because they were derived
from fluorescing, lipid-rich vitrinite. Corrections
were made with the help of DGSI and a new source rock
maturity map was prepared which fit the distribution
of oil production and oil shows almost perfectly.
And best of all, it led to a new discovery that would
not have been found if the original source rock maturity
map had been relied upon. And Arcata...."
lesson here is obvious: If it's worth doing, it's
worth doing right. Poor quality data can be worse
than no data at all and you (almost always) get what
you pay for. Keep this in mind when comparing prices
and geochemical laboratories." And with that
said, Vale ruminated on the sad state of technical
fuzziness that Arcata Bottoms worked in while waiting
for the kind hearted, but dimly lit bulb at the other
end of the line to respond. Vale knew whatever Bottoms
said would surely give Vale's entire team of misfits
something to chuckle about on their way home from
then the intercom buzzed. It was Madam Marvelous,
office manager and everyone's favorite confidant.
Stiletto heels not withstanding, Marvelous was the
glue that kept the office together. In a silken and
even tone she breathed, "Call on line 2 regarding
diesel oil contamination of hydrocarbon samples. Can
you take it?"
Vale sighed, gruffed out a frazzled yes, bid good-bye
to Bottoms and pressed the line 2 button on the phone.
"Hello, can I help you?" asked Vale. TO
BE CONTINUED in case number two.
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