Abstract

The relative inactivity of hydrocarbon exploration in the western Delaware basin and along the eastern Diablo Platform can be traced to an historical precedence, incomplete knowledge of the effects of tectonic orogenies throughout the region, and a perceived absence of commercial hydrocarbons in the area.

The earliest statements regarding the “poor” potential of the Diablo Plateau can be attributed to Beede (1918), who stated that “At best, while it may be possible that a large field may be found on the plateau yet, on account of the apparent scarcity bituminous shales to furnish the oil, and more especially the apparent absence of porous strata to serve as reservoirs for oil and gas, the likelihood of it is remote”. We now know that carbonates can be highly enriched in kerogenic material and can act as their own source and trap. Beede’s study, limited to outcrops, did not include observations on the subsurface Woodford/Percha Shale, possibly the premier source rock of the Permian basin. These strata are present over much of the Diablo Plateau in thicknesses greater than that encountered in the productive Midland basin (Comer, 1991). Similarly, permeability is of more direct importance to production because the most porous reservoir strata will not produce unless adequate permeability exists, or is created. Subsurface interpretation of permeability from outcrop can be inherently misleading, as weathering processes, both chemical and mechanical, can actively alter permeability values.

Though the region has been affected by a number of tectonic orogenies, the effect of these orogenies on possible petroleum reservoirs has not been well addressed. Evidence of pre-Woodford tectonism, though presently sparse, exists on the Diablo Platform and in the Delaware basin. Recent (1993) reprocessing of seismic profile DBGS-114 (Swift et al., 1994; Erdlac et al., 1994) at the University of Texas of the Permian Basin, reveals eastward-directed thrust faults, with offset of about 175 ft, displacing strata no younger than Woodford. The magnitude of this movement is substantiated by well control along the Diablo Platform margin. The section penetrated in the Exxon No. 1 French Ranch indicates similar (150-200 ft) pre-Woodford movement that has taken place along at least one episodically-active and long-lived fault having a total throw in excess of 2500 ft. The crest of the resulting pre-Woodford structure was stripped of potential reservoir-quality Devonian strata, that should be encountered along the flanks of the feature. Additional regional and counter-regional, up-to-the-basin, high-angle reverse faults across the basin disrupt Ellenburger through Woodford strata, offering excellent opportunities for hydrocarbon entrapment at fault barriers laterally sealed with Woodford and/or Barnett Shale.

Perhaps the most noteworthy of the areas affected by pre-Woodford faulting is the Elsinore Highland, a major, but as yet poorly defined structural positive, lying east of Elsinore field. This paleo-feature, covering several hundred square miles, lies in the axis of the traditional Tobosa basin. Wells across Elsinore field demonstrate structural inversion by having encountered more than 220 ft of Silurian and Devonian section below the Woodford. The limited well control immediately to the east, including wells in the GMW and Sierra Madera fields, encounter the Fusselman, or a thin veneer of Devonian overlaying Fusselman, directly below Woodford. Truncation of Silurian and Devonian strata along the margin of the Elsinore Highland may form significant traps.

Post-Mississippian tectonic deformation of the Diablo Platform-Delaware basin region, rather than being isolated, must be understood as part of a series of northerly-trending anticlinoriums (Bend Arch, Central Basin Platform, Diablo Platform) and synclinoriums (Fort Worth basin, Midland basin, Delaware basin) originating during Ouachita deformation along well-established, long-lived zones of weakness. The northerly trend of this sinusoidal orogen strongly suggests an east-west orientation of compression. The width of the anticlinoriums and synclinoriums varies from central to west Texas, with the Bend Arch and the Midland basin being broad, and the Central Basin Platform, Delaware basin, and Diablo Platform displaying a narrower width. The Fort Worth basin does not conform to this westward narrowing of width, probably because it is partially overthrusted by the Ouachita frontal zone. This sinusoidal anticlinorium-synclinorium development is consistent with regional east-west compression acting on a thick plate, constrained from moving at its western end, but with a series of buckles formed by the westward translation of that part of the plate located east of the constrained margin.

A northern boundary of this deforming plate may exist along the Abilene Minimum (Erdlac, 1984, 1986). The Abilene Minimum is a gravity and magnetic low of over 300 mi length representing the Grenville front, a Precambrian terrane boundary, in Texas. The Abilene Minimum is a significant, but little-understood structural element. It displays evidence of post-Mississippian right-lateral, strike-slip movement as it slices through the Central Basin Platform and enters the northern Delaware basin at the Winkler-Lea County boundary (Erdlac, 1984, 1986). Local elements that correspond with this feature have been noted as the cataclastic zone of Flawn (1956), the active Monahans seismic zone indicated by Davis et al. (1989), and the Andrews Shear Zone (Gardiner, 1990). The minimum trends westward through the Rustler Hills sulfur zone of the Diablo Platform region (Hentz, 1989), and possibly forms the northern boundary of the radon-poor portion of the Central Basin Platform (DiChristina, 1993). Thermal maturation values noted by Barker and Pawlewicz (1987) indicate an observable difference of mean random vitrinite reflectance values across the general trend of the Abilene Minimum. Values for wells northwest of the trend reach a noticeably higher level of maturation than those southeast of the trend. The Abilene Minimum may genetically relate the northern Delaware basin, the northern Midland basin, and the Knox-Baylor basin in ways not previously noted.

Buckling continued and the plate ruptured along anticlinorium-synclinorium slopes as monoclines and reverse-fault deformation. On-going deformation resulted in local development of additional folds and faults, creating a complex of structures within each anticlinorium and synclinorium. One of the anticlinorium-synclinorium slope ruptures is represented by the Diablo Platform border fault (Swift et al., 1994; Erdlac et al., 1994), displaying an estimated minimum of 2500 ft of reverse slip at the Ellenburger. An undrilled footwall structure displays a minimum of 500 ft of reversal at the Woodford level. Significant thickening of Permo-Pennsylvanian clastics off the eastern flank of this structure is demonstrated with seismic (Swift et al., 1994; Erdlac et al., 1994). At least three major Wolfcamp clastic wedges overlay the footwall anticline, those clastics extending out into the basin. The seis-stratigraphic interpretation, substantiated with well control from San Martine (Wolfcamp) field 15 mi north of the seismic line, assists in timing displacement along the border fault. It also assists in identifying a significant potential trapping mechanism for hydrocarbons that may extend along the length of the Diablo Platform margin. Similar reservoir strata are presently being actively pursued on trend with the Pakenham (Wolfcamp) field in Terrell county, along the basinward flank of the Brown-Basset anticline, in the northern Val Verde basin. Basinward of the Diablo Platform, the San Martine-like clastic wedges have the potential to develop into stratigraphic traps similar to the Pennsylvanian (actually Wolfcamp) gas sand pays of the Sonora and Ozona gas play.

The effects of more recent deformation processes, including Laramide and Basin and Range structures, and Tertiary magmatism, on hydrocarbon generation and entrapment along the Diablo Platform-Delaware basin margin are presently little studied and, therefore, primarily conjectural. These processes have been cited as strong negative, inhibiting hydrocarbon exploration in the area. The absence of conclusive studies to either confirm or deny the presence of hydrocarbons in the area does not diminish the prospective nature of the region, but rather enhances it.

Unpublished data from Erdlac documents tectonic stylolites of probable Laramide age within Cretaceous rocks exposed along Highway 67, 22 mi southwest of Fort Stockton, and within 1 mi of the Oates (Rustler) field. The proximity of Laramide tectonic stylolites to Oates strongly confirms that the field, reportedly drilled on a Cretaceous surface anticline, has been affected, if not created, by Laramide tectonism. The Oates field trends about N60°W, generally on trend with other fields such as Perry Bass, Manzanita, Kimberlin, Pikes Peak, and Sierra Madera. Other fields are located south and southwest of the presumed front of Laramide deformation, including Oates SW, Oates NE, Elsinore, GMW, and Bitterweed. These fields would definitely be within Laramide effects. Other fields, including Hershey, Hershey West, Barilla, Weinacht, Balmorhea, and Casey Draw, lay along a generally parallel trend, north and east of the generalized Oates trend. Additional Laramide deformation is suspected in the form of thrust faults along Highway 1053, near the Pecos River where remnants of the Triassic Dockum group are exposed in a road cut. Proximity of these faults to producing fields, (T. E. Bar, Tucker, McKee, Abell) again raises the question of Laramide effects on hydrocarbon production. How far into the Delaware basin Laramide deformation can be traced and the effect that the deformation had on these fields and other potential reservoirs has not been determined.

The Rustler Springs sulfur district, in the northern part of the western Delaware basin, is a world-class sulfur deposit. One of the major deposits in this mineral district, the Pennzoil deposit, has estimated ultimate reserves of more than 60 million long tons of sulfur. That deposit is attributed to breakdown of anhydrite and gypsum, in the presence of hydrocarbons and sulfate-reducing bacteria, to calcium carbonate and hydrogen sulfide. Hydrogen sulfide, in the presence of oxygen, breaks down further into native sulfur and water. Ultimately, the equivalent of one molecule of methane is “destroyed” to deposit one atom of sulfur. At 100% efficient conversion, the Pennzoil sulfur deposit represents from 215-250 MMBO, or 1.3 trillion cubic feet of gas destroyed in the conversion process. If a conversion factor of only 25% is applied, the resultant ancestral oil field would have exceeded more than one billion barrels of oil. The Kaufman-Arps method of “undiscovered reserves” estimates is based on the substantiated assumption that field-size distribution for a specific basin, sub-basin, or in general, a portion of a basin, will plot as a straight line on log-normal paper. Thus, an area that contains a billion barrel oil field may quite reasonably be expected to hold a significant number of smaller fields. It may be anticipated that a substantial number of these fields are waiting discovery.