AEROMAG SURVEY FINDS FEATURES ALONG CENTRAL TEXAS OVERTHRUST

E. Gerald
Rolf Consultant
San Antonio
Kenneth O. Seewald
Seewald Energy Co.
San Antonio

The Ouachita thrust belt of central Texas appears to have great potential based on a recent aeromagnetic survey.

The Strawn basin of central Texas has had periodic, selective seismic surveys and limited drilling (Fig. 1).

About 60 wells have been drilled to Ordovician Ellenburger (one well per 96 sq miles). Only 26 of these penetrated more than 200 ft of Ellenburger, and of that number only eight drilled through Ellenburger (one well per 730 sq miles).

It would be safe to say the Ellenburger has not been properly evaluated. Some of the best recovery zones in West Texas fields are more than 600 ft below the top of Ellenburger.

Although there is no commercial production to date, there has been a significant gas reserve confirmation. The Kirby 1 Box in Williamson County, drilled in 1981, confirmed what the Shell 1 Purcell tested in 1954.

It is situated on a highly faulted, gas charged anticline having more than 450 ft of gas column with 150 ft of net porosity and recoverable reserve potential of 60-100 bcf (Fig. 2).

The Kirby well was completed in 1982 with a flow rate of 250 Mcfd of gas. The well was shut in until 1989, when a three week flow test averaged more than 800 Mcfd. The well is again shut in.

STRUCTURE

A review of seismic, gravity, and a recently completed aeromagnetic survey indicate the Strawn basin of central Texas contains numerous structures.

The structures vary from basement related horst blocks and fault closures to tight, elongate faulted folds associated with the frontal thrust fault zone (Fig. 3).

Throws associated with basement faulting vary from 100 ft to more than 600 ft. Dips of 3-6 are common.

Little information is available to identify the amount of Ellenburger erosion that may be associated with the various horst and fault blocks.

What is known is that the Mississippian Barnett shale source rock is in unconformable contact with the Ellenburger. There is some evidence that Pennsylvanian age reef development may be present in association with some of these horst blocks.

STRATIGRAPHY

Except for erosion due to local horst block uplift, it appears that a complete Ellenburger section is present within the basin consisting of the Honeycut, Gorman, and Tanyard formations.

Known thickness of these units is on the order of 3,000 ft. The Ellenburger is a non-bioclastic, massive to thin bedded, fine grained, gray limestone alternating with dark gray, fine to coarse crystalline dolomite.

Low energy sedimentary features include relict bioturbation, graded laminae, and rip up clasts redeposited as flat pebble breccias. The dolomite zones are more abundantly fractured.

In the Atlantic 1 Lewis there is a correlation of oil saturation with dolomite beds. The black Barnett shale appears to be a rich organic hydrocarbon source rock.

To the north in Wise County excellent high volume gas well completions from the Barnett shale have been announced. The presence of Viola and/or Simpson age rocks have not been reported except in a few northeastern penetrations.

Overlying the Barnett shale are the Pennsylvanian Marble Falls, Big Saline, and undifferentiated Atoka-Strawn shales. Few sands are present. The Pennsylvanian conglomerates that produce to the north have not been encountered.

EXPLORATION INCENTIVES

As an exploration objective, the central Texas Ellenburger has many important incentives:

Drill depths to evaluate the Ellenburger range from 5,000-15,000 ft.
The Ellenburger is a normally pressured, fractured reservoir with 3-9% porosity and permeability as low as 0.01 md. There is a possibility for application of the tight gas tax incentive.
The area is mostly open countryside, making the costs to acquire seismic data much cheaper than any other portion of the Ouachita Overthrust trend. Surface elevation ranges from 550-1,150 ft with local relief generally less than 100 ft.
Existing seismic data, although reliable, can be difficult to interpret. With recent improvements in acquisition, processing, and velocity analysis, newly acquired seismic is much more definitive.
There is a skeletal gas pipeline system in place with one 20 in. north-south line along Interstate 35 highway and five east-west 6 in. and 10 in. lines covering the eastern two thirds of the basin.
Shows of oil and gas have been recorded in many of the previous tests drilled in the area. Commercial gas has been reported at a recent Johnson County reentry, and a pipeline is being laid. Drillstem tests are rare, and those wells tested have been noncommercial principally because of low permeability.
Several wells have flowed salt water to surface, indicating commercial reservoir zones within the Ellenburger. Because of the zonation and compartmentalization within the Ellenburger, recovering water in an upper porous zone does not condemn the lower zones.
Acreage in central Texas can be leased with excellent terms. Lease bonus of $15-25/acre for 5 year and 10 year paid up leases are available. Royalties are 1/8 and/or 3/16. Rentals, if not paid up, are $1/year/acre.

CONCLUSIONS

A recent 6,000 sq mile detailed aeromagnetic survey has been flown covering a 135 mile by 44 mile area.

Review of these data indicates numerous basement related Ellenburger-Marble Falls structures and fault closures (Fig. 4).

The use of these data to limit a modern seismic program to specific prospect areas is considered an important cost effective feature for the basin.

Finally, modern technology in the form of improved fracture applications, horizontal drilling, and 3-D seismic programs if applied to Ellenburger exploration in the Strawn basin brings an exploration opportunity to all who are looking for major gas reserves at low costs.