ARK-LA MEGAREGIONAL SEISMIC DATA AVAILABLE

Allen J. Bertagne, Claude Vuillermoz
CGG American Services Inc.
Houston

A 253 mile seismic line extending from Franklin County, Ark., to Red River Parish in west-central Louisiana was made available to the industry earlier this year (Fig. 1).

This line, NS5-1, crosses several areas of current exploration interest including the eastern Arkoma basin, the State Line graben trend, and the North Louisiana salt basin.

It also traverses several frontier areas including the southern Ouachitas and the Eagle Mills/Morehouse basin.

The seismic line has been interpreted using available geologic data and the results presented in several documents. These documents demonstrate the seismic expression of existing plays and identify possible future exploration targets.

Line NS5-1 is the first of many long (Megaregional(TM)) profiles that will eventually cross the U.S. from north to south and east to west (Fig. 2).

The data compilation and seismic interpretation has been undertaken by CGG on behalf of an industry consortium consisting of more than 20 oil companies. Several other service companies are also participating in various aspects of the project.

The Megaregional grid and supporting information will provide a unique perspective on the petroleum geology of North America and should lead to the development of new exploration plays.

PROJECT CONSORTIUM

In June 1989, CGG established the Megaregional Project Consortium, an industry group that now includes more than 20 oil companies.

Specific objectives of the Megaregional Project are to:

Compile regional seismic and geologic data along transects that will eventually cross the entire U.S.
Interpret the data to obtain an improved understanding of the regional geology of the U.S.
Observe the seismic expression of known exploration plays and identify possible future exploration plays.

Because of the high costs of new acquisition, Megaregional lines are compiled from existing seismic data. The data consist of three types: data contributed by consortium members, data from CGG's non-exclusive library, and, in certain instances, public domain data from COCORP.

In the Gulf Coast area, the optimally located seismic lines are identified using a computerized seismic database provided by Seis-Scan Inc.

The individual lines that make up a Megaregional profile are of different vintages and were acquired using different equipment and parameters. The data are processed to a uniform scale and similar appearance and are then merged.

The resulting sections, although not always state-of-the-art, allow the consortium to achieve its major objectives. This approach also allows the lines to be prepared relatively quickly and inexpensively.

In certain instances, tapes are unavailable for processing; only paper prints have been preserved. In such cases, the information on the paper sections can be optically scanned, then digitally reconstructed and stored on tape. This work is being done for the consortium by Spectrum Geophysical Services.

The reconstructed data can then be processed using standard geophysical techniques. Results of the optical scanning performed to date have been very satisfactory.

After the geophysical data are compiled, the relevant geologic information is posted on the line, including surface geology, well data, formation tops and production information. The well and production data are provided by Geomap Co. Also shown are velocity control points where geologic and geophysical data can be tied.

Supporting information includes location maps, stratigraphic columns, and production keys. These documents allow a geologist to begin the interpretation and exploration process immediately rather than spending long hours on data compilation.

INTERPRETATION

Several interpretive documents are also prepared (Fig. 3).

Interpretations are based on the seismic and other information described in the preceding paragraph, plus:

Structure maps and production maps at a scale of 1 in. = 4,000 ft, provided by Geomap Co.
Synthetic seismograms provided by Oklahoma Seismic and Geobyte Images
Check-shot surveys provided by Velocity Databank.

Considerable volumes of public domain data are also reviewed and incorporated into the interpretation.

After Megaregional documents have been prepared they are held exclusive to the consortium members for 3 months before becoming available to the industry at large.

ARKOMA BASIN LINE N35-1

The northern half of Line NS5-1 traverses the Arkoma basin and the Ouachita thrust belt of western Arkansas (Fig. 4).

In the Arkoma basin, the contrasting structural styles of the shallow Atokan section and the deeper Cambrian-Mississippian section are well displayed.

The shallow section consists of broad synclines and tight anticlines cored by triangle zones. These shallow folds are also present in the Oklahoma part of the Arkoma basin. Deep Arbuckle (Cambrian-Ordovician) blocks are interpreted to be bounded by high angle reverse faults in western Arkansas. These fault blocks are on trend with and similar to the structure currently being drilled at the Oxy USA 1 Danville, in 33-5n-22w, Yell County, Ark.

Key wells near the Arkoma basin part of the line include the Mobil Oil Corp. Isom, a 12,500 ft Arbuckle test in 6n-27w, and a 17,000 ft Hunt Oil Co. well in 3n-27w that drilled a large north-vergent Atokan thrusted structure.

Production in this part of the Arkoma basin is largely from lower and middle Atokan deltaic sands.

CENTRAL, SOUTHERN OUACHITAS

Seismic data quality in the core of the Ouachitas is fair to poor.

However, by using the surface geology and the few deep Cambrian-Ordovician events that are visible, it is possible to reconstruct the overall geological framework.

Highly folded and faulted lower Paleozoic basinal sediments ("Ouachita facies") are thrusted over time-equivalent shelf sediments. The thrusting is of Desmoinesian (Pennsylvanian) age and was followed by latest-Pennsylvanian to Permian uplift that "bowed up" the pre-existing thrusted structures.

Large Morrowan and Atokan fault-cored folds are present to the south of the complex "Ouachita facies" structures described above.

Key wells in the southern Ouachitas include the Shell Arrivet in 5s-25w and Sohio Stewart in 9s-26w. The former appears to have been an Arkansas Novaculite (Devonian-Mississippian) test, whereas the latter tested a Jackfork (Morrowan) anticline.

EAGLE MILLS BASIN

Line NS5-1 crosses the Eagle Mills basin of southern Arkansas.

It is within a few miles of several key wells, including the Humble Royston in 10s-24w and the McAlester Buchanan in 11s-22w. The seismic data allow the Pennsylvanian marine sediments of the Morehouse formation and the Triassic Eagle Mills formation to be identified.

The Eagle Mills consists of redbeds and volcanics that have generally been interpreted as rift-fill sediments.

The seismic data from Line NS5-1 show no major normal faults, suggesting that a rift model is not applicable to southern Arkansas.

ARKANSAS-NORTH LOUISIANA

The onlap of Jurassic and Cretaceous sediments onto the southern Ouachitas is well defined on Line NS5-1, as is the southward thickening of the Mesozoic sedimentary section,

Several low amplitude salt cored folds are present in southern Arkansas; these folds commonly produce from the Smackover.

The State Line graben trend is a salt-related feature that displays a major down-to-the-south fault and smaller down-to-the-north faults. There is considerable "growth" of Smackover/Norphlet age sediments across this fault.

Exploration continues in the area, and recent wells have produced gas and condensate from the Jurassic Haynesville at depths of approximately 11,000 ft (OGJ, Feb. 25, 1991, P. 53).

In the North Louisiana salt basin, Line NS5-1 crosses numerous salt features, including pillows, turtle structures, and the Minden dome (the only productive dome in the basin).

Productive horizons along this part of the profile include the Cotton Valley sandstones and several Lower and Upper Cretaceous units.

THE FUTURE

Line NS5-1 is the first Megaregional line released to the industry.

The southward continuation of this line (NS5-2), which extends to the East Cameron area offshore, will be released in December.

Line NS5-2 crosses the southern North Louisiana salt basin, the Sligo, Glen Rose, and "Edwards" reef trends, and the growth fault and salt-related structures influencing the Paleocene to Miocene producing trends of southern Louisiana.

The line passes within 2 miles of the 1989 Oligocene Miogypsinoides discovery of Chalkley Deep field (OGJ, Sept. 30, p. 104).

Work is proceeding on Line NS5-3, which extends to the southernmost part of the Keathley Canyon area off Louisiana. Line NS5 will eventually extend from the Arkoma basin across the deep Gulf of Mexico to the Yucatan shelf. Other lines of interest that are currently being worked on include:

A segment from the Black Warrior basin to the Gulf of Mexico
A line from the Permian basin to the Gulf of Mexico passing through the vicinity of the Llano uplift
A segment from the Anadarko basin to the Permian basin
An east-west line across Nevada, Utah, and Colorado.