Some projections put the United States natural
gas needs at 30 trillion cubic feet of gas in the next 20 years.
But if domestic production is to be a big part of that projection,
exploration strategies are going to have to dig deeper.
That's true in basin-centered gas plays in the Rockies, it's true
in the Gulf of Mexico and it's especially true in the eastern Midcontinent,
where producers have barely touched any zone below 4,000 feet.
"In most areas of the eastern Midcontinent, the vast majority
of oil and gas tests have penetrated only a few thousand feet,"
said James A. Drahovzal, geologist and head of the Energy and Minerals
Section of the Kentucky Geological Survey.
"Not only has this limited amount of data obscured our vision
of the deep economic potential," he said, "but it has also provided
us with a poor understanding of the early geologic history of the
region."
But seismic data acquired over the past 10 years — along with
a few key deep tests — have begun to shed new light on the deep
structure and stratigraphy of the eastern Midcontinent.
One of the more significant changes in scientists' understanding
of the region has been the discovery of geographically extensive
sedimentary rocks below the Precambrian unconformity, which has
shifted the economic basement substantially deeper.
Some of these sedimentary rocks are tectonically complex, since
they are part of both rift-basin settings and fold and thrust belts
that were previously unknown or poorly understood.
The area's deep Cambrian basins also are becoming better understood
as a result of seismic data and some deeper wells — and mapping
of the deep structures, Drahovzal said, is providing a better picture
of the control they exert on the development and distribution of
important shallow prospects.
New Clues to Consider
This deeper sedimentary section was first uncovered about 13 years
ago when the Ohio Geological Survey drilled a well in Warren County
in southern Ohio, on the crest of the Cincinnati Arch. The well
was expected to reach crystalline basement at a depth of about 3,500
feet, but instead penetrated nearly 2,000 feet of a red lithic arenite
below the Cambrian Mount Simon Sandstone.
The discovery generated widespread attention, and the Cincinnati
Arch Consortium was formed to study the new basin — funded by six
companies interested in the potential for hydrocarbon source rocks
or reservoirs in the new formation.
"Prior to this well, everyone believed that Pre-Cambrian granites
underlaid the area," Drahovzal said. "We really didn't know what
this discovery meant at the time, and we still don't have a good
handle on the age of this sandstone. It may simply be an older part
of the Cambrian section, but many of us think it is Precambrian.
"Of course, this is still controversial," he added, "but our best
guess is it's Mesoproterozoic, which would make it in the range
of one billion years or more old."
This thick section, called the Middle Run Formation, fills part
of what has become known as the East Continent Rift Basin.
The basin's extent has not been defined — data is fairly limited
— but about 14 wells west of the Grenville Front have now penetrated
this lithic arenite.
Another important well was drilled about four years ago in Hart
County, Kentucky, on the west flank of the Cincinnati Arch. The
KII No. 1 Brooks drilled 1,815 feet of what appears to be the lower
part of the Middle Run Formation. The basal 200 feet in this well
is a cleaner quartz arenite.
Plus, the well had a blow of gas that spewed mud onto the rig
floor.
In the last several years two additional wells in the area have
encountered this red lithic arenite.
Whetting the Appetites
The East Continent Rift Basin is bounded on the east by the Grenville
Front and possibly on the west by normal block faulting. Gravity
and magnetic data suggest that the basin may be connected on the
northern end with the Midcontinent Rift System in southern Michigan.
The southern boundary is poorly defined, but the basin likely
extends into northern Tennessee.
Magnetic modeling and seismic data indicate that the basin deepens
eastward, reaching depths of 27,000 feet on its eastern side under
the Cincinnati Arch.
"These nuggets of information whet our appetite about what appears
to be a fairly extensive new sedimentary section," Drahovzal said.
"There is a possibility that this sandstone could contain natural
gas, even though it is very old."
What's needed now is more study to identify structures in the
section.
"We now have this basin that covers large parts of western Ohio,
eastern Indiana and much of western Kentucky with evidence of a
deep sedimentary section." he said.
"We're not sure how thick this interval is, but in some places
it is anywhere from two to more than seven kilometers."
As part of this work, an ancient eroded mountain belt called the
Hoosier thrust belt was discovered lying in the western part of
the East Continent Rift Basin beneath the English Basin. The English
Basin is interpreted as a Mesoproterozoic and Neoproterozoic depocenter.
Both proprietary and published seismic sections show a complex structural
basin, part of which has been cut by west-vergent Neoproterozoic
thrusting that has truncated the earlier east-vergent Hoosier thrust
belt.
The Hoosier thrust belt appears to have formed as the result of
contraction developing during the earlier of the several episodes
of Proterozoic orogeny.
The north-south thrust and fold structures of the eroded mountain
belt form potential natural gas traps. These structures were first
recognized in northern and western Kentucky, but recent seismic
data in southern Indiana has greatly enhanced the understanding
of the structures' distribution.
Obviously, a better understanding of these deep Proterozoic sequences
and their effect on overlying Paleozoic rocks is important to future
petroleum resource exploration.
For example, Drahovzal said seismic data from the Hoosier thrust
belt in southern Indiana indicates a ramp anticline structure with
a flat spot at its crest that could indicate an oil/water or gas/water
contact.
"This is a beautiful anticlinal structure at about 12,000 feet,
and it's never been drilled," he said. "It looks as if it could
be composed of sedimentary sandstone, but we're not certain if this
is part of what's now known as the Middle Run Formation in the East
Continent Rift Basin or not."
Deep Fan Deposits
This newly discovered basin isn't the only area with deeper potential
in the eastern Midcontinent. The Rough Creek Graben is an asymmetric
basin containing up to 18,000 feet of pre-Knox Cambrian sedimentary
fill near its northern boundary, the Rough Creek Fault Zone, and
it may be an important target for deep natural gas.
Most of the pre-Knox drilling has been near the northern boundary
fault. However, the fault zone was reactivated at least twice since
its initial formation in the Cambrian — once during the Ouachita
orogeny, when high-angle reverse faulting took place, and again
during the Jurassic, with normal faulting as the Gulf of Mexico
began to open.
The past exploration strategies are questionable, because the
reservoirs near the boundary fault are certainly breached as evidenced
by the extensive faulting by dead oil shows.
That doesn't mean the Rough Creek Graben is devoid of potential
structures. There is strong seismic evidence that the graben has
basin-floor fan complexes that could be excellent hydrocarbon reservoirs
just as they are in other parts of the world.
Drahovzal and other scientists believe these reservoirs would
be the most prospective in the areas of the graben away from through-going
fault zones where the hydrocarbons could be preserved. Both oil
and gas shows in shallower Cambrian rocks suggest that Cambrian
source beds are present in the deep part of the rift.
The southern updip edges of both the northern and southern fan
complexes should be optimum positions for possible production. The
fans are very deep in the basin's western part — 22,500 to 30,000
feet below sea level — and there is only sparse seismic data in
the eastern part of the Rough Creek Graben, but the depth to basement
shallows in that direction and similar fan complexes are probably
present in that area.
Additional data and further study are necessary to confirm this
potential.
"We do see evidence of these very deep fan deposits on seismic
in the western part of the Rough Creek Graben," Drahovzal said.
"They appear as mounded reflectors. Fan complexes are of 10 excellent
reservoir rocks. We are just now getting seismic data in areas where
the fan deposits shallow to the east, and we hope this new data
will prove these structures are present there as well."
My Old Kentucky Rome
Kentucky's Rome Trough, a graben that contains a thickened section
of the Cambrian-age Rome and Conasauga formations, is another area
with deep gas potential.
A research consortium of the Kentucky, West Virginia and Ohio
geological surveys currently is completing a two-year study of the
pre-Knox stratigraphic interval in the Rome Trough. The study is
funded by the Department of Energy and four energy companies.
Unlike other deep targets, the Rome Trough has actually achieved
some production, specifically from the Homer Field in Elliott County,
Kentucky.
Drahovzal said seismic indicates some structural settings in the
pre-Knox interval that could contain hydrocarbons. One example is
adjacent to the Rockcastle River Uplift.
"The south side of the Rome Trough is defined by the Rockcastle
River fault system in southeastern Kentucky," he said. "This fault
system was active during the Cambrian as evidenced by the thickened
Cambrian — Rome and Conasauga — units to the northwest."
He said that during the Cambrian, the fault was likely a normal
fault formed during a period of extension. Later — presumably during
the Alleghanian — oblique, compressional stresses from the southeast
resulted in the formation of a transpressional fault zone that uplifted
the southeast block to form the Rockcastle River Uplift.
Compressional stresses also resulted in the faulting and folding
within the graben to the northwest.
"These folds on the downthrown side of the Rockcastle River fault
zone could be prospective as source rock is known to be present
in the Rome Trough and production is present to the north in Kentucky
within the Rome Formation," Drahovzal said. "Near the crest of the
anticline, in the upper part of the Rome Formation, is a possible
turbidite."
The depth to the top of the Rome in this area is a little less
than 8,000 feet.
"It is these kinds of prospects that will need to be examined
further and tested if we are to reach a 30 Tcf natural gas future,"
he commented.
"The deep structures of the eastern Midcontinent have the potential
for containing reserves of natural gas, and future exploration strategies
will need to consider them."