In today's high-tech world many exploration prospects
never get off the ground without the latest techniques and gadgetry
— but at least one geologist is out to show there is still a place
for good old-fashioned geologic legwork based on inexpensive information
like gravity and magnetic data.
He's working the Rocky Mountain basins with these tried and true
exploration methods, and is achieving some revealing results.
Indeed, a recent study focusing on a productive region of the Denver
Basin uncovered some interesting conclusions that could lead to
additional prospects in the basin.
"I don't know that any major oil company has looked for the D and
J sands (Cretaceous-age D and J sandstones) in 20 years or more
because the field sizes are so small," said W. Richard Moore, a
Crawford, Colo., independent. "But, for independent companies new
accumulations can be very economic — paying out very quickly as
a result of modest drilling costs."
Moore presented a paper on his findings ("New Exploration Techniques
and Opportunities, Denver Basin") at last year's Rocky Mountain
Section meeting in Laramie, Wyo.
"If oil companies believe this story, they can use the combination
of gravity maps, geothermal gradient maps and the available well
control to locate exploration prospects and then more wisely spend
their seismic dollars to further define drilling targets," Moore
said.
"What I've tried to do with this study is develop a very preliminary
exploration tool that can be followed up with more expensive sophisticated
technology like seismic."
Getting Started
The Denver Basin was very active in the 1950s and '60s, primarily
for the Cretaceous-age D and J sandstones. The fields are typically
stratigraphic traps and relatively small in size, but drilling costs
are extremely inexpensive, making even modest accumulations economic.
The D and J sands are found from 5,000 to 5,500 feet and individual
wells make on average 100,000 barrels of oil. Field sizes are typically
about 10 wells.
Moore has been conducting regional studies of Rocky Mountain basins
looking for basement fracture zones using gravity, magnetics and
geothermal gradients.
He believes he has identified at least one area in the northern
Denver Basin where the distribution of D and J fields can be explained
at least in part by his efforts.
"I think it is very important to identify these areas of basement
fracture zones, because I think these fracture zones control the
location of stratigraphic and structural traps," Moore said. "These
fracture zones work their way up through the sedimentary section,
changing the facies of the rock and causing stratigraphic traps
to form. They also impact structural traps through movement along
the fractures and via basement relief.
"Pinpointing the location of these fracture zones is extremely
important in localizing areas with trapping mechanisms," he added.
Moore studied an area covering Banner, Kimball and portions of
surrounding counties in southwestern Nebraska. He keyed on this
area, where the D sand appears to be the same depositional environment
and the same thickness, but there is only one concentration of D
sand stratigraphic traps.
"That production happens to be right on one of these gravity minimums
and an area of high geothermal gradient," Moore said. "I believe
basement fractures control the formation of traps in this area,
and I wanted to more thoroughly map the area to test that hypothesis."
This is much the same approach he took in the Williston Basin a
couple of years ago, when he did a basin-wide correlation between
Bouguer gravity, geothermal gradients and the location of oil fields.
This time he wanted to focus on a more localized area with well
control to test his approach
"Once you identify an area with a high density of sandstone traps
using gravity, which is a very inexpensive tool that is readily
available to anybody, you can pinpoint other areas in the basin
close to these productive pods where there might be strong concentrations
of D sand and even Niobrara and Paleozoic fields," he said.
The fractures permeate the entire sedimentary section, according
to Moore, so they would control the stratigraphy of each productive
zone.
"What prompted me to do this recent study was that I had these
regional gravity maps and I had seen indications in the Williston
Basin that I still didn't fully understand what they meant," he
said.
He read a paper written in the 1980s on vitrinite reflectants of
the Niobrara, indicating some areas of very anomalously high vitrinite
reflectants in that formation, which to him correlated with areas
of very high geothermal gradient.
"So, I put together a couple of maps in the Denver Basin while
referring to that paper and recognized that in this area of Nebraska
there is a wonderful correlation between high vitrinite reflectants
and Bouguer gravity minimums," he said. "Then I realized this was
an area with a concentration of D sand production.
"It was this very localized area of the Denver Basin that convinced
me that there is a correlation between Bouguer gravity, geothermal
gradients and oil fields all across the Rockies."
'It Worked'
Moore constructed a geothermal gradient map of the area using bottom
hole temperatures recorded by wireline logs. Gravity and magnetic
data were acquired from public sources.
Comparison of these maps show that areas of high geothermal gradient
coincide with areas of Bouguer gravity minimums, magnetic positives
and concentrations of oil fields. Few fields exist in areas of low
gradient and gravity positives.
"This paper on a basin-wide study of vitrinite reflectants really
piqued my interest," he said.
Moore then purchased all the available gravity data in the area,
contoured the area and made a detailed gravity map as well as his
own geothermal gradient map based on well control in the area and
his own vitrinite reflectants map based on Niobrara log resistivity
data.
"I wanted to convince myself that what had been mapped on a basin-wide
basis could also be applied locally — and sure enough, it worked,"
he said.
"Some local areas popped up with very strong correlations between
Bouguer gravity minimums, areas of high geothermal gradient and
oil production that weren't seen on the regional work."
He believes these areas of high geothermal gradient reflect areas
of fractured basement rock that have conducted heat into the sedimentary
section. Modeling indicated that the fracture zones have reduced
the density of the basement, which explains the Bouguer gravity
and magnetic anomalies.
"I saw very strong correlations between areas of Bouguer gravity
minimums and areas of high geothermal gradient," he said, "and I
believe that ties to areas of the basement that are strongly fractured,
so that heat conducts up through fractured basement rock compared
to areas of non-fractured basement flanking these productive areas."
These fractures, he believes, are key to hydrocarbon accumulations.
"In addition to creating the trapping mechanism, the fracture zones
create migration pathways for hydrocarbons generated further down
dip," he said.
"Without basement fractures you don’t get migration and you
don't get trap formation."
Other Areas
Moore pointed out an example of how these fracture zones impact
trap formation:
"Much of the productive zones in the Denver Basin are channel sands
that run right down dip, with fluvial systems back to the east and
the seaway to the west," he said. "There has to be a break in the
channel to create a trap. These intersecting fracture trends can
change the channel direction up to 90 degrees and it's these bends
in the channel systems that form traps. Without the fracture systems
the channels go straight down dip with nothing to trap the hydrocarbons.
"Also, there are a lot of structural noses caused by movement over
these same basement fracture systems."
The productive area centered in Nebraska's Banner County was right
in the heart of the D and J play in the 1950s, where a multitude
of wells was drilled in the area.
The high concentration of D and J sandstone fields that resulted
can't be explained by stratigraphy or structure.
"I believe these techniques explain why the fields are there,"
Moore said. "These fracture zones have localized D and J sandstone
fields by causing abrupt changes in the environment of deposition
of the sands, therefore causing stratigraphic traps. Adjacent areas
of limited basement fractures are notably void of traps and production."
Not only can existing producing trends in the D and J sands be
explained, Moore said, but other areas in the northern Denver Basin
can be identified that have the same high potential for production
from the Cretaceous D and J as well as the deeper and shallower
objectives.
The Silo Field along this trend in Nebraska, for example, illustrates
that these same elements impact all the productive zones. The field
is a large fractured Niobrara field that was one of the basin's
first to be drilled using horizontal technology about 10 years ago.
"This study clearly shows that, in the area centered around Banner
County, pods of production are associated with a high geothermal
gradient, Bouguer gravity minimums and magnetic positives, which
are indicative of basement fracture zones," he said.
In areas void of the fractures production is virtually non-existent.
"This is a very inexpensive first wave exploration tool," he added,
"that can indicate areas of interest for further study."
In addition to new prospects in the D and J and Niobrara, Moore
also has defined some exploration prospects in the Paleozoic section
as a result of this study.
The Paleozoic sands haven't gotten much attention despite some
tremendous wells in the zone.
"Nobody had developed a good exploration strategy for the Paleozoic
sands," he said. "This study might push that effort a little further
down the road."
The First Step
Admittedly, the lack of well control as you step away from existing
production does pose some problems.
"In areas of little well control, you have to map gravity because
it is difficult to map the geothermal gradient without log data,"
Moore said. "But just using gravity I see at least two areas adjacent
to this area of high D and J sand production. The existing fields
have uncovered in the neighborhood of 10 million barrels of oil,
and the two adjacent areas could have at least that much oil in
the D and J sandstones, based on the gravity and regional mapping
I've done."
That doesn't take into account the potential in the Niobrara and
the Paleozoic formations, he added.
"If oil companies believe this story, they can use the combination
of gravity maps, geothermal gradient maps and the available well
control to locate exploration prospects and then more wisely spend
their seismic dollars to further define drilling targets," he said.
"What I've tried to do with this study is develop a very preliminary
exploration tool that can be followed up with more expensive sophisticated
technology like seismic."