Henry W. DeJong
Consulting Geologist
Littleton, Colo.
Sunit K. Addy
ARCO International Oil & Gas Co.
Plano, Tex.
Trans-Pecos Texas has intrigued and baffled the oil industry since the discovery of oil in the Permian basin.
Although production has been established along the southwest flank of the Delaware basin, exploratory efforts have proved fruitless in attempts to extend production to the southwest. Activity usually increases as higher-priced oil provides more risk money and declines as drilling dollars are scarcer.
This regional analysis, covering about 48,000 sq miles, attempts to make an unbiased appraisal of the potential of this highly complex area and place any exploratory merit in relationship to other areas failing within the authors' ranges of experience.
The project area extends southeastward from El Paso County to Brewster County and includes the Chihuahua trough, Marfa basin, Diablo platform, Ouachita-Marathon thrust belt, and the southwest flank of the Delaware basin.
The Delaware basin area serves to provide structural and stratigraphical continuity with a productive area.
The authors' data base includes gravity, magnetic, and seismic data together with all available well information and hundreds of published articles and papers.
TRANS-PECOS AREA
An index map and a province index superimposed on a Bouguer gravity map show the project area, the major uplifts and basins, and the various mountain ranges (Figs. 1, 2).
Gravity data, along with other geological and geophysical information, enable the authors to subdivide the area into various provinces.
Northern basins, such as the Tularosa, Orogrande, Salt Flat graben, Valentine basin, and areas in southeastern El Paso and southwestern Hudspeth counties such as the Hueco bolson, show gravity minima.
The Marfa basin, with a thick volcanic cover, fails to show a distinct gravity low. Increasing gravity values within the Marathon thrust belt are possibly due to increased thicknesses of the denser geosynclinal facies rocks of the Marathon thrust to the southeast.
Structural highs within the southeast Delaware basin probably account for the gravity maxima in that area. The Chihuahua tectonic belt with several bolson-fill areas shows a gravity lineament with highs and lows.
In the highly productive northwest portion of the Delaware basin, the gravity data indicate several highs.
The province index map is based on regional structural and Bouguer gravity data from both the public domain and proprietary sources.
Within the region, the gravity values vary as much as 100 milligals. Areas with less than -10 milligals are considered to have low gravity, and conversely those areas with anomalies greater than -10 milligals as areas of high gravity.
The values are not absolute and are to be used in a comparative sense.
Magnetic data from various industry sources fail to delineate province boundaries except in a limited way. The calculated basement depth is in general too low in comparison to actual basement depth.
Difficulties of interpretation of magnetic data in many areas result from thick volcanic cover, igneous sills, and dikes, and complex structural patterns. Results of the authors' gravity and magnetic investigation confirm, in general, the conclusions of other authors.
STRATIGRAPHY
The stratigraphic succession for this large area is complex in detail, but viewing it from the remoteness of a regional investigation the authors have condensed it to the section shown on the stratigraphic column (Fig. 3).
The many, and at times rapid, facies changes combined with the tectonic history produced many lithologic variations. Further, the great number of investigators who have tended to work relatively small, isolated areas provided a large array of names.
Note that most of the units shown in Fig. 3 are productive in the Delaware basin.
The construction of regional cross-sections permitted the authors to tie surface and well data while at the same time presenting a sense of the structural complexity involved throughout the area and allowing the authors to carry consistent units throughout the project area. The authors tied seismic data to these cross-sections and vice-versa.
The authors basically mapped rock systems, e.g. Permian, Cretaceous, and so on, and isopached some formational units when readily carried or if they are of unusual importance. Well data and the large number of measured and described surface sections (Fig. 4) gave the thickness and environmental control sought.
All parameters studied in the area are affected in one or more ways by their presence, and their areal extent was kept in mind as the work progressed. They represent the foundation for many of the negative factors present in Trans-Pecos Texas.
FORMATION ISOPACHS
Eleven isopach maps from Cambrian through Cretaceous (omitting the Triassic) constructed from well and outcrop data gave the authors the present areal extent of the individual units and formed the basis for their interpretation of the geologic history.
Published outcrop data and sample logs, supplemented by extensive literature review established time and depositional environment relationships.
Each unit received an environmental description/classification that resulted in its designation as reservoir, source, or seal.
Numerous outcrop sections and sample logs provided the lithologies the authors required to assign a depositional environment to the unit.
Shows and production for the individual units are shown on each map. Figs. 5 through 15, the 11 isopachs, also show a type well or surface section for each unit.
TECTONIC HISTORY
The geologic history and tectonics of Trans-Pecos Texas are highly complex.
Within the area are sedimentary basins and structural arches, bolsons, intrusive igneous bodies, volcanic piles, horsts, grabens, rifts, diapiric emplacements, and plutons.
To these are added compressional faults, tensional faults, wrench faults with alternating direction of horizontal displacement, faults with as much as to 16,000 ft of vertical displacement, simple folding, complex folding, two overthrust belts, basin and range tectonics, hot water springs, and mining districts.
The area has been tectonically active throughout its entire geologic history and is seismically active today. By contrast, the hydrocarbon rich Permian basin has remained essentially undisturbed for the last 230 million years or since the end of Permian time.
Viewed in terms of global tectonics, Trans-Pecos Texas has been influenced by compressional and shear stresses generated by plate collisions in Precambrian and Permo-Pennsylvanian time and compressional stresses due to plate movements in Devonian-Mississippian and Laramide times.
Superimposed on this is an overprint of tertiary basin and range tensional tectonics.
PRECAMBRIAN
During Precambrian the Trans-Pecos area was the site of the Van Horn orogenic belt, believed to be the result of a collision between a plate moving from the southwest against the stable North American craton.
Lineaments in Arizona document left-lateral displacement for at least 4 miles in Arizona with the lineament believed by some workers to extend southeastward into Texas.
Shear zones and fault patterns that developed at this time have probably influenced or controlled subsequent patterns.
CAMBRIAN-MID-DEVONIAN
The early Paleozoic sedimentation resulted from widespread invasion of epeiric seas during a period when the plates were pulling apart and the Trans-Pecos area was located on a slowly subsiding plate margin.
Cambrian Bliss sandstone was deposited in the marginal marine environment as seas advanced onto Precambrian terrane. The Bliss was followed in succession by shelf carbonate sequences represented by the El Paso-Ellenburger, Simpson, Montoya, Fusselman, Upper Silurian, and Lower and Middle Devonian rocks.
These shallow seas were regionally widespread, extending for the most part across the southern and midcontinent parts of the present U.S.
Periods of widespread subaerial exposure resulted in much dolomitization of the original limestone. During Simpson time, epeiric uplift to the west restricted Simpson deposition to the eastern part of the Trans-Pecos.
A differential basining of the widespread shelf produced the Tobosa basin in the present Permian basin area, which included the eastern part of the Trans-Pecos.
From late Silurian to middle Devonian time, uplift in Arizona and New Mexico caused deep scouring that bevelled or removed entirely all previously deposited Devonian and fate Silurian carbonates in the Trans-Pecos.
DEVONIAN-MISSISSIPPIAN
The late Devonian Woodford/Percha black, organic-rich shale was deposited on the erosional surface with sediment derived from an Arizona-New Mexico provenance.
This was followed by widespread deposition of Mississippian marine carbonates and shales.
During this period there was regional compressive force exerted from the west that may have rejuvenated left-lateral shear along northwest-southeast trending faults.
PERMO-PENNSYLVANIAN
The Permo-Pennsylvanian was a major tectonic period for the Trans-Pecos area.
In the eastern part, after early Pennsylvanian continental shelf deposition that ranged from dark, deepwater micrites to light, shallow-water biomicrites, the collision of the African and American plates produced the Marathon overthrust belt, which bounds the area of interest on the southeast.
A foreland basin trend developed in front of the thrust plate advancing from the southeast, and sedimentation in the region changed from shelf carbonates to basin flysch deposits that were derived from highlands on the thrust plate during the Upper Pennsylvanian and Wolfcamp.
In response to the compressional force from the southeast a right-lateral sense of movement developed along pre-existing linear features, and right-lateral displacements are believed to have occurred in a northwest trend during each uplift and their subsequent alignments.
In any event, the period was one of enormous vertical displacements of as much as 10,000 ft. The Marfa basin graben developed and accumulated about 12,000 ft of Permo-Penn flysch sediment.
In the western part of the Trans-Pecos area, a northwest trending trough developed in the Pedregosa basin, which received 4,000-5,000 ft of basinal limestones and shales.
These grade laterally northeastward in reef and back-reef facies, shallow marine limestone and finally a shoreline facies on the flanks of the Florida archipelago located west of El Paso.
The area of the Hueco bolson also seems to have been structurally low, at least in the vicinity of the Humble No. 1 State "DW" well, which encountered 5,000 ft of Wolfcamp rocks.
Portions of the Diablo uplift trend were emergent and influenced sedimentation around them. The Orogrande basin sagged and filled with Permian sediments.
TRIASSIC-JURASSIC
The Trans-Pecos was mostly emergent during Triassic time. The Triassic Bissitt formation and Dockum group represent continental deposits in parts of Brewster and Pecos counties.
The Chihuahua trough began to subside in Jurassic time, and some carbonate and evaporate deposition reached northward across the Mexican border into Texas in the vicinity of the Malone Mountains in Hudspeth county.
The remainder of the Trans-Pecos area was emergent during the Jurassic.
CRETACEOUS
The Chihuahua trough developed fully during Cretaceous time, and a carbonate section as much as 20,000 ft thick accumulated there.
A veneer of shallow marine and continental deposits spread over the rest of the Trans-Pecos except in the Marfa-Glass mountains area, where a topographic high excluded all but uppermost Cretaceous sedimentation.
LARAMIDE
Compressional forces from the southwest resumed during the Laramide orogeny.
Northwest-trending linear features including faults in Texas are believed by some to have reverted to a left-lateral fault zone, and the Cretaceous overthrust belt rose out of the Chihuahua trough onto the southern perimeter of the Trans-Pecos.
TERTIARY
Volcanic extrusive and intrusive activity took place in the Oligocene at about the same time and in conjunction with basin and range tectonics, which imprinted a regime of tensional features over everything that proceeded.
Volcanic piles developed in the Davis mountains area and at various other localities, generally in the eastern part of the Trans-Pecos. Rift-like trenches developed in locations now occupied by the Hueco-Presidio-Redfern bolson trend along the Chihuahua trough hingeline, and by the Salt Flat graben.
The Santiago-Del Norte mountain thrust was modified by normal faulting and there was probably readjustment along many of the existing zones of weakness.
RECENT SEISMICITY
The frequency and magnitude of the present seismicity in the Trans-Pecos area is revealed by a 4 year earthquake monitoring program, 1977 through 1980, conducted by the University of Texas in the Van Horn region.
The program recorded 63 earthquakes with epicenters within the Trans-Pecos. Magnitudes ranged from 0.7 to 3.6 and averaged 1.8.
A 1931 earthquake with its epicenter located south of Van Horn in the Valentine basin had a 6.4 magnitude. An analysis of first-motion data suggests that the earthquake was caused by motion on a right-lateral strike-slip fault striking North 59 West and dipping 70 northeast within the region investigated.
This area is the most active seismically, and the activity is believed to represent an active fault zone hidden beneath alluvium in the Valentine basin.
STRUCTURAL PROBLEMS
Structural problems abound in such a large area as indicated on Fig. 2.
The authors' gravity data coupled with surface and well information suggests a rift continuity between Salt Flat graben, Valentine basin, and Marfa basin, but they have insufficient evidence to tie the three segments together.
The structural complexity of Trans-Pecos defies ready analysis, and efforts toward understanding to date have probably only scratched the surface.
ANALYSIS
From the standpoint of structural complexity and tectonic history, the most striking difference between the prolific Delaware basin and the Trans-Pecos is the fact that the Delaware basin has been quiescent for about 230 million years, or since Permian time, whereas the Trans-Pecos area has been tectonically active throughout its geologic history, and continues to be tectonically active today.
The entire area is intensely and complexly faulted and fractured. There may never have been large hydrocarbon accumulations because of this, but if there were, they have probably since been destroyed by leaky fracture systems.
The hydrocarbons would have leaked upward and been dissipated at pre-Cretaceous erosional surfaces.
Meteoric water has invaded the more porous or more brittle and fracture-prone parts of the stratigraphic section and filled structural highs as hydrocarbons escaped.
The area was uplifted repeatedly in subregional areas during the time frame extending from late Mississippian through post-Cretaceous and causing erosion and nondeposition.
During the long period of time fairly large areas were subjected to meteoric waters.
Such recharge areas have been identified from the isopach maps.
The fracture systems are too permeable to be overcome by hydrodynamic pressures.
Oil shows in Cretaceous, which is generally regarded as thermally immature, may have resulted from residual leaking from below or from local in-situ retorting in the Cretaceous itself by volcanic activity.
Unique conditions may have preserved some of the traps, but they are probably small.
MODERN MOVEMENT
Analysis of repeated leveling measurements often reveals contemporary vertical movement in unstable areas. The National Geodetic Survey ran leveling lines across the Hueco mountains, Diablo plateau, Salt Flat graben, Guadalupe Mountains, and into the Delaware basin in 1934 and partly reran the lines in 1943 and 1958.
The results of these surveys indicate that the total relative uplift measured reaches about 19 cm plus or minus in the eastern part of the Diablo plateau, with the regions around El Paso Tex., and Carlsbad, N.M., showing minimal uplift.
The data indicate that uplift appears to be occurring more or less continuously, although not at a constant rate in all areas.
The contemporary arching of the Diablo plateau-Salt Flat graben area is consistent with other geophysical and geological data.
The gravity profile along the traverse also indicates a Bouguer high coinciding with the arching. Quaternary fault scarps in the Salt Flat graben, tilting of the alluvial plains on the western slopes of the Guadalupe Mountains (bajada surfaces), entrenched consequent streams on such slopes, recent seismicity in general, etc., are among other evidence pointing to recent vertical movements, probably along numerous fault planes.
Additional leveling lines along a short route between Van Horn and the Guadalupe Mountains also show recent uplift. Here the relative uplift between the Salt Flat graben and Van Horn was as much as 10 cm within a distance of 60 km.
A similar larger vertical movement was observed between Sierra Blanca, 19 km northwest of Van Horn, Tex., and a site 25 miles north of Sierra Blanca.
Two other areas in which leveling measurements were made along the Rio Grande uplift also show substantial vertical movement. The areas around Socorro and the Espaola basin in New Mexico have recorded recent magmatic activity and are located on a high heat flow trend.
It is suggested that intra-crystal magmatic activity is the cause of uplift along the Rio Grande rift in New Mexico.
Although no recent magmatism has been observed on the Diablo plateau, a similar explanation can be suggested for vertical movement caused by a deep-seated magma chamber or by incipient adjustment caused by the existence of one in the recent past.
In any case, the vertical movement in the Diablo plateau area indicates possible recent reactivation of faults. This, in turn, is detrimental to any hydrocarbon accumulation in this area since available seismic data show that most structural traps are fault-associated.
Incidentally, numerous wells in this area had shows, but none has reported any production; on the contrary, most had fresh water, for which the faults may have been an easy migration channel. END PART 1 OF 2
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
