Upper Permian (Guadalupian) Facies and Their Association with Hydrocarbons--Permian Basin, West Texas and New Mexico

Frank B. Conselman (2)

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Ward et al (1986) presented a comprehensive but wide-ranging paper on hydrocarbons in Guadalupian facies in the Permian basin of west Texas and New Mexico. To the extent that it represents modern geologic thinking on this subject, it creates several problems for people like myself, who were actively engaged in the development of the area discussed during its boom period half a century ago. Some of these questions may be matters of terminology and nomenclature, but there still appears to be a corpus of disagreement that deserves discussion.

My chief reservation has to do with the general characterization of the reefoid petroleum reservoir in Lea County, New Mexico, and its southward extension, facies variations and all, into Winkler County, Texas. I visualize it as relatively monolithic, without practical internal division into Tansill, Seven Rivers, and Queen equivalents. I consider it also as a more or less laterally continuous body, with few interruptions, including, from north to south, such giant fields as Monument, Eunice, Cooper, Jal, and their Winkler County counterparts. In short, the producing Guadalupian dolomite reservoirs of the eastern side of the Delaware basin are considered rather closely analogous to the exposed western horn of the regional crescent, along the Guadalupe Mountains front, where subsurface facies may be readily recognized.

Throughout an area of considerably more than 100 mi² (260 km²), the producing Capitan reef reservoir displayed a remarkable uniformity of fluid distribution and stratification. The gas-oil interface was close to 195-200 ft (60-61 m) subsea; the oil-water interface was at or slightly above 300 ft (91 m) subsea.(FOOTNOTE 3) Production casing was set, and total depth determined on that basis. Given the elevation of the drill site, fluid occurrence in the reef was closely predictable. This sort of fluid stratification is possible only where lateral and vertical permeabilities are widespread and relatively uniform, with facies variation immaterial. Fracturing does not appear to have been a significant factor.

The subsurface Capitan reef is cavernous, and suggestive of the exposed Guadalupe Mountains reef at the surface. Lost circulation in the Lea County fields was common, and in some instances resulted in dangerous and even fatal blowouts and fires. We depended primarily on examination of samples (generally poor) for our subsurface interpretations. Our first instrumental logging program was disastrous; our first perforating gun is still stuck in the hole in which it was fired 50 years ago. We lacked such terms as "grapestone grainstones" or even "bioturbated dolowackestones," but at least we did better than our gallant allies, the engineers, who used, and still use, the term "formation" for any and all kinds of subsurface lithology, and apparently achieve gratifying technical fulfillment y this means.

Well penetrations generally stopped safely above the respected 300-ft (91-m) subsea level. However, a well we drilled (Gypsy-Gulf 1 Cooper) in the Cooper field of southern Lea County is remembered as having drilled, cored, and tested four distinct oil-water boundaries, interpreted as representing four separate permeability streaks within the reef section. Source of the fluids was considered to be the basin to the west.

More than 1 billion bbl of oil has been produced from the Capitan reef reservoir, and it was undoubtedly massively invaded by basinal "sulfur" water as the gas cap was dissipated and the oil column depleted. It distresses me to read that the reef porosity was commonly "water saturated" (Ward et al, p. 239) or, worse yet, "during the 1940s the Capitan reef was proven to be a water [sic] aquifer" (Ward et al, p. 246). It would seem that a hydrocarbon reservoir with several hundred feet of producing thickness and a supergiant production history would deserve a little more respect than accorded by Ward et al. Incidentally, the watersupply wells credited (Ward et al, p. 246) as commonly producing 25,000 BWPD (3,180 m³ of water/day) were matched commonly by the original oil wells which, under the requirement of physical potentials as a basis for prorated

FOOTNOTE 3. This information is based on personal experience. To my knowledge, it has not been published previously.

allowables, were acidized, if necessary, to produce daily petroleum figures at least that high.

As regards lateral relations between reefoid and backreef facies, I particularly recall two instances involving the eastern limits of Capitan production. We (Gypsy-Gulf) drilled, cored, and drill-stem tested several hundred feet of bleak and barren back-reef dolomite and argillaceous dolomite in a dismally dry hole on a Hatchett lease immediately east of Monument townsite and only a few locations from production--as effective a barrier to migration as could be imagined. Also, I recall the remarkable discovery of sweet, green, paraffin-base, light-gravity oil in sandstone with metabentonite laminae, in the Langlie-Mattix area of Lea County, a very short distance east of the crestal Cooper field, with its usual dark, sour, mid-gravity crude. Doubtless the intervening spatial and stratig aphic relations have been worked out in the years since these events--events that left little doubt that the Capitan reef had an impermeable eastern boundary on the Central Basin platform.

I therefore question the classification of the trap at Eunice-Monument field as primarily structural (Ward et al, their Table 2, p. 253). It is no more structural than other trend fields like Cooper, Jal, Kermit, and Hendrick; the latter two are listed (Ward et al, their Table 3, p. 258) as stratigraphic. Incidental structure in the overlying beds may be ascribed to draping caused by differential compaction rather than lateral dynamics.

Ward et al attribute first discovery of oil in the Permian basin to Mitchell County on the Eastern shelf in 1921 (Ward et al, 1986, p. 239; Myres, 1973, p. 139). The Westbrook field of Mitchell County is generally credited with being the first important production in the Permian basin, but if the Eastern shelf be included, this does not hold. Minor oil production from the Toyah area of the Delaware basin dates from 1903 (Myres, 1973, p. 95), but production on the Eastern shelf goes back to 1879 in Brown County (Havins, 1958, p. 114). Disregarding the axial Bend arch discoveries at Ranger and Desdemona, oil had been found and produced with various degrees of excitement in Stephens, Brown, Coleman, and Taylor Counties by 1916-1917. By 1918, the Breckenridge boom in Stephens County was w ll under way (Rundell, 1977, p. 173), followed shortly by the Pioneer discovery in Eastland County. In 1921, when Westbrook field was in its initial stages, Breckenridge field produced over 31 million bbl of oil (Rundell, 1977, p. 173).

Whether or not Westbrook field belongs on the Eastern shelf, Post field (Ward et al, p. 257), in Garza County, definitely does not. Geographically, it lies on the northern or northeastern rim of the Midland basin; stratigraphically, the San Andres grades into regional evaporites and clastics to the east and northeast, this gradation providing the updip trapping mechanism here and for nearby San Andres fields in the same relative position. The San Andres of the Post field does not crop out as a dolomite or anhydritic dolomite anywhere on the Eastern shelf, and its equivalents have been erosionally removed from the shelf area east of the outcrop of the San Angelo sandstone at their base.

With respect to this same Eastern shelf, the columnar section given for "Eastern shelf and outcrop" (Ward et al, their Figure 3, p. 242) is of doubtful value. It consists of the names of three thin marker zones in the "upper Guadalupian" and three Oklahoma names for the "lower". For a section as widely known by as many people for as many years as that of the Eastern shelf, this treatment seems a little cavalier.

Obviously, Ward et al (1986) made a major contribution to the knowledge of the region, and I find much more to praise than to challenge. I am particularly grateful to them for having made dozens of references to "dolomite" without once resorting to that etymological abomination, "dolostone." If "dolostone" comes, can "gypstone" and "anhystone" be far behind?

References:

Havins, T. R., 1958, Something about Brown: published by the author, p. 114.

Myres, S. D., 1973, The Permian basin: Abell-Hanger Foundation, p. 95.

Rundell, W., Jr., 1977, Early Texas oil--a photographic history, 1866-1936: College Station, Texas, A&M University Press, 260 p.

Ward R. F., C. G. St. C. Kendall, and P. M. Harris, 1986, Upper Permian (Guadalupian) facies and their association with hydrocarbons--Permian basin, West Texas and New Mexico: AAPG Bulletin, v. 70, p. 239-262.

Acknowledgments:

(2) Consultant, 34 Lakeshore Drive East, Ransom Canyon, Texas 79366.

AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.