In
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"Geophysical Corner" we discussed the basement fault
block pattern, and stated that the best (and only) way to map it in sedimentary
basins is with properly processed and interpreted aeromagnetic data.
This month we'll
try to substantiate the claim that many -- and possibly a majority -- oil
and gas fields are controlled by basement, with examples of several different
types of "purely" stratigraphic traps related to basement faults.
Some geologists may
concede that the evidence for underlying basement control is convincing.
Others will not.
I argue that if a
basement fault is in exactly the right location, and has exactly the right
strike direction relative to a stratigraphic feature of interest, it probably
is not coincidental.
It must be cause
and effect.
Oolite Shoal Over Basement Fault
Figure 1 shows the location of a southwest
Kansas oil field in Pennsylvanian oolitic limestones. The oolite bank
was deposited on the probable upthrown side of an underlying basement
fault.
A nearby fault mapped
from seismic data is also shown in red, as are the structural contours
on top of the Pennsylvanian limestone.
Pennsylvanian Algal Mound Over Basement
Fault
Figure 2 shows the relationship of a Pennsylvanian
algal mound field in Utah to an underlying basement fault.
Uplift of the north
edge of the basement block under the field could have raised the sea floor
to a shallow water environment, allowing the development of the algal
mound.
Offshore Bars Over Basement Faults
Figure 3 shows the relationship of Hartzog
Draw Field (that has produced approximately 220 million barrels of oil)
in the Powder River Basin, Wyoming, to an interpreted underlying basement
fault.
Swift and Rice (1984)
proposed that the sandstone reservoir in this field and other similar
fields in the basin were formed by the winnowing action of bottom currents
over sea floor highs.
The sea floor high
could have resulted from the raising of a basement block edge during late
Cretaceous (Laramide) compression.
Other nearby fields
showing similar one-to-one relationships to magnetically mapped basement
faults are:
- Dead Horse-Barber Creek.
- Nipple Butte-Holler Draw.
- Culp-Heldt Draw.
- Poison Draw.
- Scott.
- House Creek.
Fluvial Systems Along Basement Faults
Figure 4 shows the prolific Fiddler Creek
Field in the Powder River Basin, which produces from a lower Cretaceous
fluvial sand in the Muddy Formation, as it relates to an underlying interpreted
basement fault.
Fracturing and jointing
along this fault zone would have made the underlying rocks more susceptible
to erosion, creating a topographic low along which the river flowed and
deposited sands.
We have located four
other such correlations of fields in the Muddy Formation in this basin
with underlying basement faults:
- Clareton.
- Kitty.
- East and West Sandbar.
- The recent Vastar discovery
in 75W, 39N.
Of related interest,
several of the present day drainages in the basin, such as the Belle Fouche
and Little Powder Rivers, follow precisely along basement faults for long
stretches.
Shoreline Bars Along Basement Faults
Figure 5 shows the prolific Echo Springs-Standard
Draw-Coal Gulch late Cretaceous shoreline bar (>1tcf of gas) in Wyoming's
Washakie Basin, and its relationship to an interpreted basement fault.
Because of the manner
in which the sands are stacked, an up-to-the-west fault on the west side
of the field is expected (John Horne, pers. comm., 1998).
It is precisely here
that a magnetically mapped basement fault is located.
The throw on this
fault is minimal, perhaps a few tens of feet, as suggested by comparison
to faults controlling deposition in the similar Cardium Formation in the
Western Alberta Basin (Hart, 1997).
This small amount
of throw was below the limit of resolution of a 3-D seismic survey carried
out over the field's northern part in 1996-97 (Clawson and Favret, 1997).
Fracture Production
As discussed last
month, the creation of fracture reservoirs is closely related to fault
control -- but in fracture plays the amount of vertical fault movement
can be minimal. We have documented a number of cases where fracture production
is coincident with mapped basement faults.
In southern Ohio
in the Appalachian Basin, for example, a 250 percent increase in the average
gas production in a Clinton-Medina well resulted from drilling on a magnetically-defined
fracture intersection.
Our most compelling
fracture correlation has been in the Bakken play of North Dakota, where
we obtained production data on 158 horizontal wells and used a computer
program to calculate all EUR's in similar fashion. This data base was
compared to locations of magnetically defined basement faults. Wells drilled
in corridors 1.5 miles (2.4 kilometers) wide centered on basement faults
yielded 21 percent higher EUR's than those drilled farther away.
In the southeast
quadrant of the play this figure was 41 percent higher.
Many of these wells
were drilled parallel or sub-parallel to basement faults, and at the edges
of the corridors. We believe the production figures would have been higher
if the wells had been drilled with a knowledge of the locations and strike
of the basement faults beforehand.
Why? Because eight
wells drilled within a 0.75-mile radius of basement fault intersections
yielded EUR's 85 percent higher than wells away from intersections.
Comments On Other Magnetic Mapping
Techniques
How do the above
processing and interpretational techniques for basement compare to the
new "HRAM" methods?
"HRAM" stands for
"high resolution aeromagnetics," a technique that employs very tight flight
line spacing (100-500m) flown at low flight levels (50-150m), and generally
displayed as color-coded shade relief maps of total intensity.
It is claimed that
HRAM can map the traces of faults within the sedimentary section due to
magnetite formed along the faults and can locate areas of higher surface
diagenetic magnetite content related to micro seepage. Both of these claims
are speculative and controversial.
It is also claimed
that HRAM can locate pipelines and other cultural features, which is true,
but which have questionable value in exploration.
For basement mapping
there is no technical or computational advantage for the tight flight
line spacings and low level flying employed by HRAM.
The foregoing examples
and figures should demonstrate to exploration managers that the magnetic
method, properly applied, is an indispensable tool in almost any exploration
program.
Magnetics, however,
has not been generally used to map basement faults. Instead, the technique
has been applied mainly to peripheral problems of lesser importance, such
as depth estimation.
With the increasing
effectiveness of 3-D seismic, magnetics has thus fallen behind in use.