By PARKER GAY
Maps: It's the Basements Fault
Editor's note: Parker Gay is with Applied Geophysics, Salt Lake City, Utah.
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In last month's "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:
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:
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).
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.