uAbstract

uGeologic framework

uLog & core correlation

uPetrography

uCore porosity & permeability

uDepositional environments

   uSetting

   uPorosity volume map

uConclusions

uAcknowledgments

uReferences

uAbstract

uGeologic framework

uLog & core correlation

uPetrography

uCore porosity & permeability

uDepositional environments

   uSetting

   uPorosity volume map

uConclusions

uAcknowledgments

uReferences

uAbstract

uGeologic framework

uLog & core correlation

uPetrography

uCore porosity & permeability

uDepositional environments

   uSetting

   uPorosity volume map

uConclusions

uAcknowledgments

uReferences

uAbstract

uGeologic framework

uLog & core correlation

uPetrography

uCore porosity & permeability

uDepositional environments

   uSetting

   uPorosity volume map

uConclusions

uAcknowledgments

uReferences

uAbstract

uGeologic framework

uLog & core correlation

uPetrography

uCore porosity & permeability

uDepositional environments

   uSetting

   uPorosity volume map

uConclusions

uAcknowledgments

uReferences

Geologic Framework (Figures 1.1-1.3)

	Figure 1.1. Map showing Upper Guadalupian (Queen-Seven Rivers-Yates) production and depositional facies of west Texas and southeastern New Mexico (after Ward et al., 1986).
	Figure 1.2. Map of Seven Rivers production in Grayburg Jackson Pool showing outline of abolished Fren Seven Rivers Pool (Seven Rivers oil and gas wells denoted) and two cored wells of interest. Also shown are the axis of Artesia-Vacuum Abo reef trend and the approximate location of the Seven Rivers evaporite-carbonate transition (after Sheldon (1954).
	Figure 1.3. Type log demonstrating lithostratigraphic nomenclature for Grayburg Jackson Pool. Informal Seven Rivers Formation reservoir zones “Fren A” and “Fren B” are noted. GR = gamma ray; NPHI = neutron porosity.

 Grayburg Jackson Pool (study area) is one of several similar fields overlying the crest of the Wolfcampian Artesia-Vacuum Abo reef trend. Figure 1.1 is a map showing Upper Guadalupian (Queen-Seven Rivers-Yates) production and depositional facies of west Texas and southeastern New Mexico.  

The axis of Artesia-Vacuum Abo reef trend, which lies north of the Seven Rivers evaporite-carbonate transition, extends through the study area (Figure 1.2) (Sheldon, 1954). Lithostratigraphic nomenclature for Grayburg Jackson Pool and log features of the interval containing the reservoirs are shown on the type log in Figure 1.3.

Well Log and Core Correlation (Figure 2.1) 

Figure 2.1. Stratigraphic cross-section showing correlation between the Jackson B 35 and Gissler B 29 wells. Stratigraphic datum = top of lower Seven Rivers Formation. “Potential oil pay zones” are based on log and core characteristics (i.e., neutron log porosity > 10%; core fluorescence). Present gas/oil contact is approximately 1830 feet above sea level.

 “Potential oil pay zones” are based on log and core characteristics (i.e,. neutron log porosity > 10%; core fluorescence). Present gas/oil contact elevation is approximately 1830 feet above sea level. Figure 2.1 shows the correlation between the two wells from which cores were examined as a part of this study.

Petrography (Figures 2.2-2.16) 

Various facies types in the upper Seven Rivers Formation are described with the photomicrographs shown in Figures 2.2-2.16. They range from grainstone to boundstone. The carbonate units have been dolomitized. Grainstones are illustrated in Figures 2.4, 2.10, 2.12, and 2.15, and packstone is shown in Figures 2.2, 2.5, 2.6, 2.7, 2.8, 2.9, 2.16. Boundstone is represented by Figures 2.3, 2.11, and 2.13. Mudstone and anhydrite and shale are illustrated in Figures 2.10, 2.13, and 2.14.

	Figure 2.2. Dolomitized bioclastic oolitic or pisolitic packstone. Gissler B #29, 1857.1 feet depth, LA = 1.85 mm.
	Figure 2.3. Dolomitized algally laminated boundstone. Porosity is highest adjacent to wispy pressure solution seams; possibly due to a thin horizon of leached bioclastic material. Jackson B #35, 1884.9 feet depth, LA = 2.54 mm.
	Figure 2.4. Dolomitized grapestone grainstone. Note micritic envelopes that have been filled by anhydrite. Jackson B #35, 1895.8 feet depth, LA = 2.54 mm.
	Figure 2.5. Peloidal bioclastic packstone. Secondary porosity in this view is both interparticle and moldic. Jackson B #35, 1895.9 feet depth.
	Figure 2.6. Peloidal bioclastic packstone. This close-up view from the same sample as in Figure 2.5 is of secondary moldic and intercrystalline porosity. Jackson B #35, 1895.9 feet depth, LA = 0.38 mm.
	Figure 2.7. Intraclastic packstone/grainstone. Secondary porosity is both interparticle and intercrystalline; Gissler B #29, 1897.3 feet depth, LA = 2.54 mm.
	Figure 2.8. Dolomitized bioclastic peloidal packstone. Note abundant intercrystalline and interparticle porosity in this section. Replacement anhydrite plugs some porosity. Gissler B #29, 1899.1 feet depth, LA = 2.54 mm.
	Figure 2.9. Sandy oolitic (?), bioclastic peloidal packstone. Porosity in this very finely crystalline dolomite is intercrystalline and interparticle. Note the wispy pressure solution seam. Gissler B #29, 1911.4 feet depth, LA = 2.2 mm.
	Figure. 2.10. Anhydrite filled relict porosity in pisolitic bioclastic grapestone grainstone. The original isopachous cements fringing the pore have been dolomitized along with the rest of the carbonates in this section. Left - plane light; Right - crossed polarizers. Gissler B #29, 1911.4 feet depth, LA = 0.38 mm.
	Figure 2.11. Bioclastic peloidal boundstone. Fracture porosity is open adjacent to dolomite, but closed adjacent to anhydrite. Gissler B #29, 1915.9 feet depth, LA = 2.54 mm.
	Figure 2.12. Sandy pisolitic bioclastic grapestone grainstone. Compaction has destroyed much of the interparticle porosity in this section. Gissler B #29, 1922.4 feet depth, LA = 2.2 mm.
	Figure 2.13. Anhydrite filling fenestral porosity in algal laminated mudstone/boundstone facies. Gissler B #29, 1944.7 feet depth, LA = 2.54 mm.
	Figure 2.14. Sandy shale and nodular anhydrite. A sabkha deposit. Jackson B #35, sidewall core at 2021.5 feet depth, LA = 2.54 mm.
	Figure 2.15. Dolomitized grainstone demonstrating secondary intercrystalline porosity. Jackson B #35, sidewall core at 2042 feet depth, LA = 2.2 mm.
	Figure 2.16. Bioclastic peloidal dolomitized packstone. Note how wispy pressure solution fabrics disrupt porosity distribution. Jackson B #35, sidewall core at 2078 feet depth, LA = 2.2 mm.

Core Porosity and Permeability (Figure 3.1)

Figure 3.1. Plot of porosity vs. permeability for dolomite core plugs and sidewall cores from Jackson B #35 and Gissler B #29 wells. Only samples with permeability equal or greater than 0.01 md are included. Due to low permeability, “pay” porosity threshold is approximately 10%.

 The boundstone is characterized by low permeability even though the porosity, in some cases, may be as much as 12-14% (Figure 3.1). The grainstones/packstones have higher permeability, and in general they show a correlation between porosity and permeability. Due to low permeability, “pay” porosity threshold is approximately 10%.

Depositional Environments (Figures 3.2-3.3)

	Figure 3.2. Schematic cross-section showing regional depositional relationships of Upper Permian strata along the western margin of the Delaware Basin and the general location of the Fren/Grayburg Jackson Seven Rivers Pool (modified from Esteban and Pray, 1983).
	Figure 3.3. Isopach map of upper Seven Rivers dolomite “net porosity volume” (porosity x feet of thickness). Pore volumes were estimated from gamma ray/neutron logs of various vintages (1950’s to 1990’s). A conservative method of estimating pore volume was utilized that ignored porosity less than 4% and tended to focus on the probable grainstone/packstone higher-energy facies.

Setting 

The predominance of thick units of massive to nodular chicken-wire anhydrite interbedded with algally laminated carbonates indicates that evaporative, supratidal, coastal sabkha conditions dominated this area (Figure 3.2). Thin, porous tidal-channel carbonate units represent intermittent episodes of higher stands of base level, such that intertidal depositional conditions shifted temporarily landward.

Implications from Porosity Volume Map 

The shaded area in Figure 3.3 is maximum pore volume (phi x ft >3). Although this figure is a “net pore volume” map for the entire upper Seven Rivers, it illustrates distinct shore-perpendicular (northwest-oriented) linear trends interpreted to be indicative of tidal channels. This would suggest that the depositional environment for porous units was probably intertidal.

Conclusions 

1) Upper Seven Rivers Formation carbonates span a range of depositional environments from tidal channels to intertidal algal mats. The evaporites are interpreted to be supratidal sabkha deposits.

2) The pinchout of dolomite units into sabkha sulfate evaporites, combined with drape over the underlying Artesia-Vacuum Abo reef, provided excellent stratigraphic and structural conditions for trapping petroleum.

3) Shoreline-perpendicular packstone/grainstone tidal channel facies were extensively dolomitized, creating secondary porosity that may have been preserved by early oil migration or by later dissolution of gypsum/anhydrite.

4) Although porosity is potentially as high as 30% in the tidal-channel facies, permeabilities are relatively low and the thin porous units are preferentially oriented, making the Seven Rivers a challenging reservoir for effective secondary recovery.

Acknowledgments 

We wish to thank Burnett Oil Co. Inc., Fort Worth, Texas, for providing the cores and thin sections studied.

References 

Brister, B. S., and Ulmer-Scholle, D., 2000, Interpretation of depositional environments of upper Seven Rivers Formation from core and well logs, Grayburg Jackson Pool, Eddy

County, New Mexico in West Texas Geological Society Publication #00-109, p. 65-72.

Esteban, M., and L.C. Pray, 1983, Pisoids and pisolite facies (Permian), Guadalupe Mountains, New Mexico and West Texas, in Coated grains: Springer-Verlag, p. 503-537.

Sheldon, V.P., 1954, Oil production from the Guadalupe Series in Eddy County, New Mexico, in New Mexico Geological Society: Guidebook, 5^th Field Conference, p. 150-159.

Ward, Robert F., Christopher G. St. C. Kendall, and Paul 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.

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