Abstract

The Delaware Basin of southeastern New Mexico and West Texas is a classic example of a petroleum-evaporite intracratonic basin. Development of this basin began during the Precambrian phase of tectonism (1300-850 Ma). During the Grenville Conver-gence/Orogeny (∼1000 Ma), high-angle faulting of basement rock created regional lines of weakness that were to control deformation later in time, even up to the present

In the Early Paleozoic, from the Cambrian to the Mississippian (610-310 Ma), during the passive margin phase, epicontinental seas repeatedly covered the area and ∼2 km of limestones, sandstones, and shales of the Bliss, Ellenburger, Simpson, Montoya, Fusselman, Wristen, Thirtyone, Woodford, and Barnett formations/groups were deposited in a slowly subsiding, areally extensive area known as the Tobosa Basin where shallow water sediments were laid down “layer-cake” fashion. This area was alternately inundated and exposed so that many of the units (in particular the Ellenburger, Fusselman, Wristen, and Thirtyone) were subjected to episodes of karstification. Later in time, the Simpson, Woodford, and Barnett shales would become source rocks for hydrocarbons, while the limestones/sandstones of the other units would become reservoirs for hydrocarbons.

The latest Mississippian, extending through the Pennsylvanian and into the Early Permian (310-265 Ma), was a time of collision of the continents Gondwana and Laurasia to form the supercontinent Pangea. Faulting split the old Tobosa Basin into three sections: the Midland Basin, Central Basin Platform, and Delaware Basin. Faults and folds became avenues or traps for hydrocarbons which were generated in Lower Paleozoic rock during this time of high heat flew. Positive areas in the Pennsylvanian (Pedernal landmass, Central Basin Platform, and Marathon region) supplied sediments to the Morrow, Atoka, Strawn, and Upper Pennsylvanian (Cisco, Canyon, Gaptank) units, which sediments also became good source and reservoir rocks for hydrocarbons.

In the Permian, during the Permian Basin phase (265-230 Ma), the Delaware Basin subsided rapidly, accumulating 3-5 km of limestones, shales, sandstones, and then evaporites in deposits of the Wolfcampian, Leonardian, Guadalupian, and Ochoan Series. During Wolfcamp time, the Hueco Limestone was deposited in the basin while the siliciclastics and carbonates of the Neal Ranch and Lenox Hills formations were deposited in the Glass Mountains. In Leonard time, the general sequence of backreef-reef-basin facies was established around the margins of the Delaware Basin. The “reefs” at this time were only “bank” or “patch” reefs, but they laid the foundation over which the massive Guadalupian reefs later grew. Leonardian strata in the Guadalupe Mountains are the Yeso, Victorio Peak, and the lower parts of the San Andres and Cutoff; in the Apache Mountains the Victorio Peak; in the Glass Mountains the Skinner Ranch, Hess, Cathedral Mountain and Road Canyon; and in the basin the organic-rich Bone Spring Limestone.

The Guadalupian was the time of the famous Capitan Reef Complex the extent of which defines the limit of the study area. The late Leonardian- early Guadalupian San Andres and Cutoff In the Guadalupe and Apache Mountains are equivalent to the lower Word in the Glass Mountains. The middle Guadalupian backreef Grayburg and Queen are equivalent to the Goat Seep reef in the Guadalupe Mountains, to the Munn and Goat Seep in the Apache Mountains, to the upper Word and Vidrio in the Glass Mountains, and to the Cherry Canyon in the basin. The upper Guadalupian backreef Seven Rivers, Yates, and Tansill are equivalent to the Capitan reef both in the Guadalupe and Apache Mountains, to the backreef Gilliam and Capitan in the Glass Mountains, and to the Bell Canyon and Altuda in the basin. The uppermost part of the Bell Canyon (uppermost Lamar) and uppermost Altuda are Ochoan (Dzhulfian), not Guadalupian, in age. Part of the Capitan “upper massive” in the Glass Mountains may be (bioepigenetic ?) Tessey Limestone.

Early Guadalupian Cutoff time was characterized by incised submarine canyons and debris flows along the basin margin in the Guadalupe Mountain area. Later in time, the Brushy Canyon onlapped the Cutoff, and the Cherry Canyon sandstone tongue partially accumulated in a submarine canyon which extended into the shelf. San Andres deposition represented a time of marine transgression, but as Grayburg time approached the sea regressed and shallower carbonates then, and afterwards, took the place of deeper-water, canyon-fill deposits. By middle to late Guadalupian time the Permian sea had shrunk to the confines of the Delaware Basin and conditions became favorable for massive sponge-algal reef growth, for shallow water backreef limestones and siliciclastics, and for thick sequences of basinal sandstone. Depositional environments included basin and basin margin facies, forereef and reef facies; outer shelf, shelf crest, and inner shelf facies; and evaporite shelf facies. The shelf crest represented an alternately emergent to submerged, peritidal environment characterized by pisolite and tepee structures. The reef facies consisted of reef and forereef members where the massive reef grew over (prograded) its own forereef debris. The Capitan is considered to be a stratigraphic reef containing a small organic component and a larger inorganic component bound together into a wave-resistant structure. It was a barrier reef in the Guadalupe, Apache and Glass Mountains, but was broken into discontinuous mound-like structures by submarine canyons on the north and east sides of the basin. Its location on the west (Salt Flats) side of the basin is unknown. Sands of the basinal Delaware Mountain Group (Brushy Canyon, Cherry Canyon, and Bell Canyon) were supplied from sources in the north and northwest (Pedernal landmass) and from the south (Marathons) On the north and east sides of the basin, sand moved from the backreef, through the reef via submarine canyons, and then into the basin along sandstone channels. While spillover, eolian, and debris-fan processes may have been minor mechanisms transporting siliciclastics into the basin, by far the dominant mechanism was turbidity and/or density currents. The sandstone channels of the Delaware Mountain Group later became reservoirs for hydrocarbons.

Ochoan time (250-230 Ma) marked an abrupt end to reef growth and the supply of siliciclastics, with the closing off of an inlet channel to the open sea. This inlet may (or may not) have been the Hovey Channel in the Glass Mountain area. The Castile Formation was most likely a shallow-water deposit (<tens of meters), rather than a deep-water deposit. The Guadalupe (and Apache?) Mountains were possibly uplifted during Castile time, and a freshwater lens was established in the reef (represented by the eogenetic spar episode). Massive anhydrite/halite units of the Castile and Salado Formations filled, and then covered over, the edges of the basin, respectively. Alternating layers of Castile anhydrite and halite were deposited in an increasingly desiccant basin, culminating in the tidal-flat evaporates (mostly halite) of the Salado. The Salado may (or may not) have transgressed over the Guadalupe and Apache Mountains. time, potash minerals of the McNutt member were deposited in the northern part of the basin.

Dolomites and evaporites of the Rustler Formation were deposited later in a mud flat to shallow-sea environment. Finally, the continental redbeds of the Dewey Lake in the latest Permian to Early Triassic formed a floodplain deposit alternating between mudflat and fluvial environments. In the reef margin the competent Capitan separated from the bedded backreef facies and became solutionally enlarged along tectonic fractures (Stage 1 fissure karst). Dolomitization of this reef rock occurred primarily in Ochoan time, probably by a reflux-brine mechanism, but some local dolomitization happened later in time.

The Mesozoic was a time of tectonic stability (stable platform phase; 230-80 Ma) when the Delaware Basin area remained positive except for the Early- and mid-Cretaceous when a seaway spread far into the continent. Mesozoic deposit/on began with the terrestrial, flood-plain, alluvial-fan deposits of the Triassic Chinle Group (Santa Rosa, Dockum formations) on the east side of the basin. The Triassic and following Jurassic represented a long episode of exposure, karsting and dissolution, both in the Capitan reef (Stage 2 spongework karst) and in basin evaporites. Mesozoic paleokarst in the basin filed with heterolithic breccias, and was later to become the site of sulfur mineralization. By the end of the Early Cretaceous an epicontinental sea had transgressed over the Glass Mountains, all the way to the slope face of the Apache Mountains, and northward to the Guadalupe Mountains-Carlsbad region, even covering the Guadalupe Mountain summit plain where sediments filled Stage 1 fissure karst (Type 2 gravel dikes). At the end of the Cretaceous, a non-marine episode began as the area became subject to uplift, tilting erosion, dissolution, and karsting.

The Cenozoic began with the Laramide phase (80-40 Ma) where, due to the convergence of the Farallon and North American plates, the entire Rocky Mountain region was uplifted. Laramide events in the Delaware Basin are uncertain but probably involved ∼1.2 km of uplift similar to the rest of the southern Rocky Mountain region. The Oligocene (40-30 Ma) is known to have been a time of transition between subduction/compression and crustal extension thinning, and it was also a time of extrusive and intrusive calcalkalic volcanism (volcanic phase). In the Barrilla Mountains, tuffs and basalts of the McCutcheon and Buck Hill volcanic series were extruded, while in the Glass Mountains and basin, intrusive bodies and dikes were emplaced. The late Oligocene marked the beginning of Basin and Range tilting and heating, which phase caused hydrocarbon generation and migration, generation of H₂S, and migration of J₂S to form sulfide deposits and caves around the basin margin. Also during the late Oligocene to Miocene, the hydrodynamic system which continues to the present was set up. This system created such features as dissolution troughs, salt anticlines, brine reservoirs, breccia pipes, karst domes and mounds, and evaporite karst.

What began in the Oligocene continued into the Miocene (25-6 Ma) during the Basin and Range phase, with the most rapid extension and block-faulting taking place in the middle to late Miocene. Faulting on the west side of the Delaware Basin caused the uplift of the Guadalupe, Apache, and Glass Mountain blocks along high-angle, north-northwest-trending normal faults: these uplifted blocks then shed sediment to the Miocene-Pliocene Ogallala Formation on the High Plains, and to the Miocene-Pleistocene Gatuqa Formation in the ancestral Pecos River Valley. Heat flow increased from the earlier volcanic phase, perhaps reaching 40 50°/km on the west side of the basin. This high geothermal gradient is believed to have set up convection cells in the reef where circulating fluids dissolved small caves (Stage 3 thermal karst), and where calcite (thermal spar) permeated rock of the entire region, from the Guadalupe to Apache to Glass Mountains and basin. This high geothermal gradient may have been responsible for hydrocarbon generation in Wolfcamp, Leonardian, and Delaware Mountain Group rocks, even up to the Ramsey member of the Bell Formation. Migration of oils from the Wolfcamp and Bone Spring, or from Lower Paleozoic reservoirs along faults, into Guadalupian shelf reservoirs may have taken place during this time.

In the Miocene and continuing into the Pliocene, the hydrologic regime, where water flowed downgradient from west to east across the basin, continued to produce dissolution features in evaporite rock. As hydrocarbons reacted with Castile anhydrite solutions, H₂S was produced which was either oxidized to native sulfur in the basin or which migrated into the reef, there to collect in structural (anticlinal) or stratigraphic (base of Yates) traps. The H₂S formed sulfides in the reduced zone, but as the mountains continued to uplift and the water table in the Capitan aquifer dropped, the H₂S became oxidized to sulfuric acid and dissolved out the large cave passages (Stage 4 sulfuric acid karst) in the Guadalupe Mountains. On the north side of the basin, where the Capitan reef was covered by evaporites, water flow in the Capitan aquifer was interrupted by sandstone channels and ascended into overlying evaporites to form breccia pipes.

By the Pliocene-Pleistocene the basic physiography of the Delaware Basin area was much as it is today. The ancestral Pecos River was the base level for the region, but the exact position of this river system and how it was hydrologically related to Guadalupe Mountain caves is uncertain. At about 600,000 ybp, the Pecos River cut into the Capitan aquifer at Carlsbad causing water to exit at Carlsbad Springs instead of the Hobbs area. This breaching may have terminated breccia pipe formation on the north side of the basin and it may have also caused the water table in Guadalupe Mountain caves (represented by the Big Room and Lower Cave levels in Carlsbad Cavern and equivalent levels in Lechuguilla Cave) to drop abruptly. The Pecos River also built up Late Quaternary alluvial terrace deposits south of Carlsbad (Blackdom, Orchard Park, and Lakewood surfaces). Caliche soils (Mescalero caliche) formed in the area as well as alluvial fans and stream gravels, wind-blown sand, and playa deposits. Basin and Range fault movement and relative uplift is still occurring in the Delaware Basin area, with the most recent (April, 1995),large magnitude (5.6) earthquake having its epicenter in the Glass Mountains.