Canyon sandstones form a prolific low-permeability gas play in the Val Verde Basin of southwest Texas. Exploration and development activity is at a high level, but little published information on Canyon geology is available. Although several geographically and stratigraphically distinct "Canyon" intervals (Upper Pennsylvanian-Lower Permian) are productive, our study focused on characterizing the stratigraphy, diagenesis, and natural fractures of Sonora Canyon sandstones in Sutton County and Ozona Canyon sandstones in Crockett County. These Canyon intervals are composed of hundreds to thousands of feet of thinly interbedded sandstone and mudstone, which formed in slope and basin depositional systems.Because outcrops are not present in the Sonora and Ozona intervals and seismic data were not available, well logs and cores formed the data base for this study. Sandstone distribution was mapped regionally and locally in Sutton County, where dense well control and core allowed field-scale facies characterization. Sonora sandstones lie in a wedge-shaped interval along the southwest margin of the Eastern Shelf, whereas the more tabular Ozona interval occupies a basin-floor position adjacent to the south margins of the Ozona Arch and the Central Basin Platform. Other Canyon or "Wolfcamp" sandstone intervals lie adjacent to the Ouachita orogenic belt in the south and west parts of the Val Verde Basin.Sonora and Ozona sandstones were deposited in deep-water, turbidite systems. Several depositional facies were identified in core: conglomeratic sandstone, thick- and thin-bedded turbidites, chaotic facies, and hemipelagic mudstone. These facies recur in characteristic associations to form the elements of submarine fans: slope channel, proximal channelized fan lobe, distal lobe, and lobe fringe. Individual fan lobes are a few hundred feet thick and a few miles wide, and fan channels are less than 100 ft (< 30 m) thick and less than 1 mi (< 1.6 km) wide. Channel and lobe depositional elements are complexly interbedded and laterally coalesced. Regionally, Canyon fans form multiple-sourced, strike-elongate submarine ramps and slope aprons.Most Sonora and Ozona sandstones are fine- to medium-grained litharenites. Chertand sedimentary and low-rank-metamorphic rock fragments are the predominant lithic grains. Original porosity and permeability were largely destroyed by compaction and by cementation by quartz and carbonate minerals. Pervasive diagenetic modification masked the expected relationship between depositional facies and reservoir quality. In Sonora sandstones, however, early siderite cementation preserved some intergranular porosity by inhibiting mechanical compaction and precipitation of quartz cement. Although siderite-cemented layers developed preferentially in Bouma T, turbidite divisions, predicting siderite-enhanced porosity will require more comprehensive investigations.Natural fractures in Canyon core were mapped and described to determine their attributes and orientations. Ozona and Sonora fractures are typically subvertical extension fractures that terminate at the boundaries of beds or cementation zones. Clay-filled fractures in siderite-cemented zones form the most common fracture class in Sonora sandstones but may be flow barriers. Although quartz- and carbonate-cemented fractures are less common in core, fracture porosity is preserved locally along their traces. Spacing between larger, more permeable fractures could not be observed directly but is probably comparable to the thickness of the quartz-cemented intervals that contain the fractures-several feet to tens of feet. A wide range in fracture strike was observed in oriented Sonora core, but subsurface fractures trending generally northeastward are most prone to be open because of in situ stress conditions. Complexly interacting geologic variables determine the intrinsic attributes of Canyon gas reservoirs. Although depositional environment controls reservoir shapes, dimensions, and internal compartmentalization, our findings suggest that diagenesis and natural fractures exert equal or greater influence on reservoir quality and gas productivity.
Hamlin, H. S., Clift, S. J., Sutton, S. P. Hentz, T. F., and Laubach, S. E., 1995, Canyon Sandstones--A Geologically Complex Natural Gas Play in Slope and Basin Facies, Val Verde Basin, Southwest Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 232, 74 p.