New well may open production in Marfa basin, Tex.

Scott L. Montgomery
Petroleum Consultant
Seattle

A recent well drilled in Presidio County, Tex., may represent the first successful oil discovery in the Marfa basin. This complex, highly faulted province portion of the Permian basin has thus far resisted all attempts at establishing hydrocarbon production.

The new well, the 1-480 Barrett, in Sec. 480, Block 8, GH&SA Survey, drilled in late 1996 by Steve Knox Oil Co., Edmond, Okla., seems poised to change this.

The 1-480 Barrett is located about 14 miles southeast from the town of Marfa (Fig. 1 [103,745 bytes]), along the faulted, northern flank of the basin. The well was drilled to a TD of 5,120 ft, penetrating to the lower Leonardian or upper Wolfcamp.

Four zones between 3,810 and 4,068 ft measured depth were perforated, three in the "Road Canyon" formation at 3,810-3,980 ft and one in the "Skinner Ranch" formation at 3,980-4,200 ft (Fig. 2 [176,575 bytes]). Two zones, both underpressured, have tested oil: a sandstone at 3,810-26 ft and a dolomite at 3,844-64 ft.

The sandstone was swabbed at a rate of 4 bbl/hr of oil, or 96 b/d, following perforation. Though a pumping unit was installed, production could not be established due to the high clay content (20% kaolinite) of the sand. The dolomite zone, on the other hand, has consistently yielded 60-70 b/d of fluid, of which 1-5 b/d has been oil.

The produced oil from both zones is reported to be 33.5° gravity, sweet crude. Other zones tested in the "Road Canyon" and "Skinner Ranch" formations were wet.

"Road Canyon" and "Skinner Ranch" are designations carried from the Marathon basin-Glass Mountains area to the east, where in situ Leonardian and Guadalupian deposits onlap and overlie the Marathon allochthon. In the 1-480 Barrett well, these two formations are tentatively applied to an interval of interbedded sands and carbonates of Guadalupian (Permian) age, regionally equivalent to the San Andres-Grayburg/Cherry Can- yon units of the Delaware basin.

Sidewall cores were taken through much of the total perforated interval. No data from related analyses are yet available.

The nearest wells to the 1-480 Barrett are the Prue Oil 1-503 Lane in Sec. 503 and the M&W 1-396 McGuire in Sec. 396. The operator of the latter well reported dead oil shows in a carbonate zone above the Wolfcamp.

At present, plans for the Barrett lease include continued testing of the initial well plus the drilling of a second test approximately 11/2 miles to the southwest.

Lost circulation in the productive dolomite zone convinced the operator to allow the well to flow naturally for several months in order to recover most or all of the "stolen" fluid. Following this flow-back period, both productive intervals will be frac'd and tested. It is hoped that commercial rates of production will be established at that time (late 1997-early 1998).

Marfa basin potential

In general, the hydrocarbon potential of the Marfa basin has been judged both poor and excellent over the years (compare, for example, Luff1 and DeJong and Addy.^2-3

The small number of exploratory wells in the province, coupled with the unavailability of detailed data, has helped ensure continued debate along these lines.

Most published studies are regional in nature. In addition, geophysical exploration, employing seismic, gravity, and magnetic data, are hindered in the Marfa region by a thick mantle (up to 4,000 ft) of Tertiary volcanic rocks that cover over 80% of the total basin area.

Unlike other segments of the Permian basin, such as the Delaware, Midland, and Val Verde basins, which ceased significant tectonic activity by the late Permian, the Marfa basin was subjected to deformation during the Late Cretaceous-Early Tertiary (Laramide), as a result of the eastward-verging Chihuahua overthrust, and in the Mid-Late Tertiary by Basin and Range faulting.

It is often supposed that most prospective structures in the Marfa region have been fault-breached or that reservoirs have been flushed.³Other arguments against basin potential have focused on the high paleo-geothermal gradient indicated by the widespread occurrence of Tertiary volcanics and intrusives.

Existing data, however, suggest that the basin is over 20,000 ft deep with up to 50% of the total sedimentary thickness consisting of Pennsylvanian-Wolfcampian flysch with source and reservoir potential.

Most of this detritus was derived from the advancing Ouachita-Marathon thrust complex to the south and east. It might be noted that similar Wolfcamp deposits have been found gas/condensate productive across a broad portion of the Val Verde basin to the east. DeJong and Addy³ indicate that a significant portion of the Permian section of the Marfa basin lies within the gas and oil windows. The basin has the overall form of a deep, steep-sided graben whose northern margin acted as a shelf-edge margin during the middle and upper Permian. The 1-480 Barrett well is located along this margin, possibly on a horst block trend.

Previous seismic surveys and regional mapping in this area reportedly identified a shelf margin, reef-type buildup1 similar to other Leonardian-Guadalupian shelf-margin buildups productive in the Delaware and Midland basins. No such facies were penetrated by the Barrett well, and the "reef" feature in this location may turn out to be a seismic artifact.

However, other wells along the northern shelf area, such as the Pure Oil 1 Lane and Gulf Oil ¹Hubbard, have encountered thick carbonate facies4 dated as both Wolfcampian and Leonardian-Guadalupian, with reported dead oil shows.

The productive interval in the 1-480 Barrett consists of interbedded sandstones, dolomites, and limestones suggestive of shallow marine to restricted depositional environments. Produced fluid from this interval includes oil, as stated, and fresh water. Testing of the other zones indicates that produced water becomes fresher with depth. There is some speculation that the high percentage of kaolinite in the productive sandstone zone may be related to fresh water chemical effects.

Logs indicate porosities above 20%, but there are concerns about relative permeability, given the nature of the formation fluid. The accumulation at this location may therefore have a strong hydrodynamic component. This should be seen as opening up new exploration potential for the basin.

References

Luff, G.C., A brief overview and oil and gas potential of the Marfa basin, West Texas, in Marathon-Marfa Region of West Texas, Permian Basin Section, SEPM, Symposium Guidebook, 1981, pp. 111-131.
DeJong, H.W., and Addy, S.K., Only relatively small production seen in basins of far West Texas, Part 1, OGJ, Jan. 20, 1992, pp. 59-65.
DeJong, H.W., and Addy, S.K., Broad view indicates hydrocarbon potential low in far West Texas, Part 2, OGJ, Jan. 27, 1992, p. 97-102.
Ammon, W.L., Geology and plate tectonic history of the Marfa basin, Presidio County, Tex., in Marathon-Marfa Region of West Texas, Permian Basin Section, SEPM, Symposium Guidebook, 1981, pp. 75-101.

The Author

Scott L. Montgomery is a petroleum consultant and author residing in Seattle. He is the lead author of the "E&P Notes" series in the AAPG Bulletin and the quarterly monograph series "Petroleum Frontiers" published by Petroleum Information/Dwights LLC. His current research interests include frontier plays and field redevelopment in North America. He holds a BA degree in English from Knox College and an MS in geological sciences from Cornell University.