TRANS-PECOS TEXAS - 2 [CONCLUSION] BROAD VIEW INDICATES HYDROCARBON POTENTIAL LOW IN FAR WEST TEXAS

Henry W. DeJong
Consulting Geologist
Littleton, Colo.

Sunit K. Addy
ARCO International Oil & Gas Co.
Plano, Tex.

This is the second of two parts of a discussion of the geology and oil and gas potential of the basins of far Southwest Texas.

THERMAL MATURATION

Maturation values are available for six wells in the Marathon area, Brewster County, for three wells in the Marfa basin, Brewster and Presidio counties, and for one well in the Presidio bolson (Fig. 16).

The cross-section suggests a greater degree of maturity in the vicinity of the Marathon uplift where the oil window comes to the surface. Maturity decreases towards both ends of the section.

The top of the gas window reaches 13,000 ft on the Delaware basin or northeast end with the top of the oil window plunging in a southerly direction to about 8,500 ft in the Presidio bolson at the southwest extension of the cross-section.

All maturity values were determined by vitrinite reflectance analyses except for the Sun 1 McElroy well, which recovered an Ellenburger core bleeding oil.

There appears to be a complete range of thermal conditions that would pose no problem for timely generation depending on the particular area and depth.

A similar conclusion may be derived from the geohistory modeling of both the deep and shallow parts of Marfa basin based on Exxon 1 Kennedy and Sun 1 McElroy.

For example, the oil-prone Devonian Woodford shale in the deep Marfa basin area entered the oil window about 270 million years ago immediately after the formation of the basin and the gas window around 250 million years before present.

Woodford rocks became overmature approximately 175 million years ago.

All Ordovician to Pennsylvanian rocks are overmature in the deep portion of Marfa basin. Some of the Permian rocks, however, are within the oil and gas windows.

In the shallower part of Marfa basin, modeling the Sun well indicates that the Ordovician to Pennsylvanian rocks lay within the oil and gas windows at present.

THERMAL WATER

Thermal waters occur throughout Trans-Pecos Texas, extending from extreme northwestern El Paso County southeastward to the Big Bend area with the largest number of occurrences in Presidio County and adjoining Mexico (Fig. 17).

Thermal waters are defined as those waters with a surface temperature about 15 higher than the annual mean temperature of the region.

In southwest Texas the mean annual temperature is about 71 F.

The classification of thermal water in wells is perhaps a little more arbitrary; however a geothermal gradient greater than about 1.8 F./100 ft has been used in the literature.

Three possible sources for thermal water occurrence have been suggested. They are Groundwater circulation to depths of at least 3,000 ft; an abnormally high thermal gradient caused by blockage of vertical heat flow by low conductivity layers; and the presence of relatively shallow magma bodies. The last is considered to be least likely.

In general, the authors favor the groundwater circulation model and the negative influence on the presence and accumulation of hydrocarbons in the Trans-Pecos.

In areas where these circulation conduits exist, the formations that might have contained oil or gas are likely to have been invaded by fresh water, and evidence for this is seen in several areas.

The greatest concentration of fresh water in wells and springs occurs in those areas that have had severe structural movement.

The Mobil 1 Adams in Brewster County, total depth 10,600 ft, 127 parts per million, offers evidence that freshwater mi,ration has taken place even in the Marathon overthrust area, where deep wells recovered fresh water from Ordovician Ellenburger.

The authors find most thermal springs along the Rio Grande rift, although some occur in the bolson fills and others in the Jeff Davis County volcanics.

All of the alternative mechanisms cited above may work simultaneously, but certainly the strongest effect in the Chihuahua trough can be attributed to thermal springs.

CARBON DIOXIDE

Carbon dioxide is a significant constituent of the Ellenburger and Siluro-Devonian reservoirs in the Delaware and Val Verde basins.

Within the study area there is a marked increase in carbon dioxide in a southwesterly direction, that is, toward the Diablo platform, the Marathon uplift, and the Ouachita thrust belt.

The occurrence of carbon dioxide in natural gas, while little understood, appears to have several possible origins:

Alteration of carbonates by igneous intrusion.
Action of acids on carbonates and bicarbonates in igneous, metamorphic, and sedimentary rocks.
Metamorphism of limestones or related rocks.
Introduction of meteoric water heavily charged with carbon dioxide by solution of carbonates.
Carbon dioxide associated with volcanics.
It appears that its origin in the Ellenburger and Siluro-Devonian is related to:
Strong tectonic events that uplifted the Diablo platform, Marathon-Ouachita regions, etc., in Trans-Pecos Texas. These events produced fault and fracture Patterns conducive to migration of carbon dioxide rich waters, and
Igneous activity on a massive scale in Tertiary time. Carbon dioxide is less soluble in a high temperature regimen, and carbon dioxide from such a source would migrate to structurally high areas.

In any event, the lower Paleozoic section in the project area is carbon dioxide prone, no matter what its origin. Both the Ellenburger and Siluro-Devonian become more heavily charged, approaching 100% in a southwesterly direction from the Delaware basin.

SOURCE ROCKS

Source rocks are present in one formation or another throughout the area, e.g. the Cretaceous and Jurassic sections in Chihuahua trough, and the Woodford in Marfa basin.

However, deposition of relatively pure clastics such as in the Permian in Marfa basin lack source rocks. The general conclusion is that source rock is available.

The discussion of the various stratigraphic horizons given earlier highlights the source-reservoir-seal potential for each formation.

SEISMIC REVIEW

The authors have examined a total of 1,004 miles of common depth point seismic data in Trans-Pecos Texas acquired and processed in the 1960s and 1970s.

The data quality ranged from very poor to fair. These data are concentrated in four areas: Salt Flat graben, southern Chihuahua trough, Marfa basin, and Diablo platform.

The Salt Flat graben appears to have been flushed by fresh water.

The Southern Chihuahua trough characteristics include immature sediments in the Permian section and possible fresh water in the lower Paleozoics, and while the Marfa basin has not been completely condemned due to the potential for small stratigraphic traps in the Permo-Penn section, there has been serious fresh water induction in the lower Paleozoics.

The Diablo platform, in turn, appears to have been severely invaded by fresh water during Pennsylvanian time.

In summary, wherever water recovery data exist, in almost all cases the reports show that test wells flowed fresh water.

The authors believe that hydrocarbons were possibly once generated and accumulated in various structures in these four areas, as indicated by hydrocarbon shows and traces of dead oil, but were later flushed out by fresh water or escaped to the surface.

Small stratigraphic traps with hydrocarbons may still exist.

SALT FLAT GRABEN

Two 1975 lines, one along the axis of the graben and the other across it, gave a total 60 miles of coverage.

This CDP data was of poor quality except for fair data in some parts. The authors interpreted Upper Guadalupe, Silurian, and Precambriam horizons; the interpretations are tentative due to poor data quality, a severe static problem, and meager subsurface data.

Fig. 18 shows a formline time structure map on top of the Silurian possibly the Fusselman, which has a distinct appearance and can be followed with some confidence. The authors find indications for the presence of deep structures in the graben involving basement and possible reef growth in the Permian section.

The sudden loss of coherent reflections in zones with draping of sediments suggest the possibility of reef development.

The 1 Davis, location B in Fig. 18, appears to have been drilled in a thick Permian section where a reef has been interpreted. The suspected reef drillstem tested 6,397-6,560 ft and recovered mud, possibly a result of low porosity.

Texaco 1 Culberson, location A, was drilled on what seems to be the most promising structure in the graben, a broad horst with rollover and possible closure on a fault.

The well tested a full string of brackish (1,155 ppm) water in Ellenburger at 4,883-4,942 ft. The authors have no indication of water recoveries on the other wells shown in Fig. 18.

S. CHIHUAHUA TROUGH

A reasonable amount of seismic data (143 miles) of fair to good quality existed for this area (Figs. 19, 20).

This is a time structure map of Base Woodford. A northeast-southwest line across the area is shown (Fig. 20).

The regional strike is northwest-southeast, and there are large thrust-bounded structures formed during the Laramide orogeny. Subsurface information comes from seven major tests.

Fairway 1 Birdsall was an Ellenburger test northeast of the hingeline on a rollover feature and was plugged and abandoned with no testing.

West and Cockburn 1 Presidio State, 2 miles northeast of the hingeline, was an 8,774 ft Ellenburger test that recovered a show of oil in Ellenburger and 1,600 ft of fresh water from the same formation. The authors' seismic review shows a rollover structure at the location.

Wesley West 1 Presidio Trust State, 4 miles southwest of the hingeline and within the Chihuahua trough, bottomed at 8,002 ft in Wolfcamp as a test on a fault trap.

The same structure was the site of Linderman Bros. 1 Bledsoe, a 3,515 ft Cretaceous test that was P&A. The 2 Bledsoe, located slightly off structure, was abandoned in Silurian at TD 12,499 ft with no recoveries reported.

ARCO 1 Presidio State bottomed at 8,500 ft in Permian. The well encountered favorable porosity with no shows of hydrocarbons.

At 7,000 ft, soluble organic matter shows a high value that has been interpreted as due to possible migration of heavy hydrocarbons from a deeper horizon.

Geochemical analysis of ARCO 1 Presidio State has shown that all of the Cretaceous and most of the Permian section has low total organic carbon and immature sediments. There are several other shallow Permian and Cretaceous structures in the area, but source rock quality and maturity remain as problems.

Hazelwood & Lasly 1 Davis, a 2,200 ft Cretaceous bolson test in the deeper part of the basin, apparently had no shows. However, there are springs in the area.

The lower Paleozoic rocks, however, show potential. The Base Woodford structure indicates several areas as leads that are mostly fault related traps.

The Wesley West 1 Bledsoe, the 12,499 ft dry Silurian test, was located somewhat off the prime location for a test of the lower Paleozoics, but the well did test the Cretaceous and Permian at a good structural position.

The most damaging evidence against a Lower Paleozoic tests in this area comes from West 1 Presidio, which flowed 1,600 ft of fresh water on test of Ellenburger. However, the Ellenburger also had traces of oil giving rise to the possibility that the environment was conducive to oil generation in the lower Paleozoic beds that were later flushed by fresh water.

Inasmuch as the lower horizons in some areas may not have been exposed to fresh water flushing, some potential in these deep horizons remains.

However, there is small likelihood of upside potential, and considering the lack of success in Trans-Pecos Texas as a whole, any deep test would have a very low chance of success. The widespread occurrence of thermal springs along the Rio Grande do not enhance the area.

MARFA BASIN

The authors had a substantial amount of data available from the 1960s and 1970s in this area (Fig. 20), although most of it is of poor quality due to the older vintage and the presence of about 3,000 ft of volcanic cover at the surface.

A Silurian time structure map represents the major lower Paleozoic structural grain in the basin (Fig. 21).

Marfa basin can be broadly divided into three areas: the northern shallow Marfa, north of the Walnut Draw fault; the Central Marfa deep bounded by Chalk Draw and Walnut Draw faults in the south and north, respectively; and the southern shallow Marfa, south of the Chalk Draw fault.

The Central Marfa deep is an asymmetric graben bounded on the south by the Chalk Draw fault with approximately 10,000 ft of displacement down to the north. The Walnut Draw fault makes up the northern boundary with a cumulative displacement of roughly 6,000 ft down to the south.

The regional dip within the graben is to the south with the deepest part of the graben immediately north of the Chalk Draw fault.

Exxon 1 Kennedy, drilled in the deeper part of the graben, encountered the Ellenburger at 18,610 ft.

Within the graben there are several faults, mostly northwest-southeast trending. All faults are of normal extension type. Within the graben itself several rollovers, some mapped with closure, were found.

In the northern shallow Marfa and southern shallow Marfa several structural highs were observed. However, the sediment column is much thinner in these areas than within the graben.

Pan Am 1 Moody south of Chalk Draw fault records the Ellenburger top at 8,140 ft; the Ellenburger was topped in the Beer 1 York, north of Walnut Draw fault, at 6,509 ft. The dip in the Northern Shallow Marfa is more or less rolling without any regional pattern, whereas, that in the Southern Shallow Marfa appears to be southwest.

FRESH WATER RECOVERIES

Some 23 wells, all dry, have been drilled in Marfa basin with fresh water recoveries that came from the Precambrian, Bliss, El Paso-Ellenburger, Simpson, Montoya, Siluro-Devonian, and the Wolfcamp.

Two wells reported brackish water from the Permian, and one had sulfur water in the Ellenburger.

The Exxon 1 Kennedy had water recovered from the Ellenburger, Simpson, Devonian, and Silurian and the Upper and Lower Permian with salinities ranging between 12,000 ppm Cl and 30,000 ppm Cl.

The Marfa basin area was a part of an epeiric sea during the period Cambrian to Mississippian.

During this time, this sea extended over much of West Texas, and the lower Paleozoic rocks, some of which are prolific oil and gas reservoirs in the adjacent Delaware basin, were deposited on a shelf platform.

At the same time the rocks also suffered periodic epeiric uplift and tectonic activities that resulted in broad open folding and possible normal faulting. The Mississippian was followed by uplift and erosion with the advent of Ouachita orogeny.

This orogeny, which began in late Pennsylvanian and ended in Permian, was caused by the collision of the North American plate with the African-South American plate. During the time, the Marfa, Delaware, and Val Verde basins were formed in front of the Marathon-Ouachita thrust belt.

BASIN DEVELOPMENT

Marfa basin (Deep Marfa in Fig. 20) was actually developed as a subsiding graben between Chalk Draw fault (10,000 ft throw down to the north) and Walnut Draw fault (or fault zone, 6,000 ft throw down to the south).

A thick sequence of Permo-Penn, particularly Wolfcamp rocks was deposited in the basin, which progressively tilted to the south during deposition.

At the end of the Paleozoic the entire region was uplifted including the Deep Marfa as well as the Northern and Southern Marfa (Fig. 21). A major part of the area probably remained exposed to erosion through Triassic and Jurassic periods.

Following this episode, the area was covered by an epeiric Cretaceous sea that intermittently deposited shallow water carbonate, sandstone, and shale. The early Tertiary was another period of uplift and erosion that was later followed by explosive volcanic activity during middle and late Tertiary.

This resulted in a blanket of volcanic rocks, 3,000 ft on the average, over much of the area along with intrusives of the same age. The Laramide orogeny in the Upper Cretaceous and early Tertiary and the middle Tertiary Basin and Range faulting did not affect the area to any great extent.

Mapping of Silurian in the northern shallow Marfa revealed many structural highs even with limited seismic data. Of the indicated 17 structural highs five have been drilled; thus the area is favorable for hydrocarbon accumulation from the structural standpoint.

HEAT, WATER PROBLEMS

Two questions arise, however: the thermal maturity of the sediments and possible fresh-water flushing.

A well 17 miles northeast of Well E in Fig. 21 indicated mature rocks in the Simpson and Woodford sections. It can be assumed that the thermal conditions of the lower Paleozoics in the Shallow Marfa area were capable of hydrocarbon generation.

Fresh water is a serious problem here. Of the five wells drilled, two had successful drillstem tests with recovery of salt water.

Coupling the authors' knowledge of the stratigraphic record and the geologic history, it appears that the area was uplifted and eroded in late Mississippian and Pennsylvanian time and consequent exposure to meteoric waters.

Any hydrocarbons generated during early Paleozoic time partly escaped during this period of erosion, and the remainder that may have accumulated in the structural traps were probably flushed out by fresh water. There would have been substantial fault leakage, also.

The Silurian structure map (Fig. 21) shows southern regional dip in the Deep Marfa basin, but there are several local folds and faults that serve as structural leads. Of the 21 leads, 10 have been drilled.

Reservoir and source rocks are present, but the interval 12,000-19,000 ft in Exxon 1 Kennedy indicates an overmature section and flushing by fresh water. Other wells have overmature sections, but ARCO 1 Mitchell has strata falling within the oil and gas generation window.

INTRUSIVES, FLUSHING

The high thermal maturation demonstrated in some wells may be due to the presence of igneous intrusives nearby that may have served as conduits to the Tertiary volcanic flows observed over much of the basin.

Flushing by fresh water in the Deep Marfa is a greater problem, and fresh and brackish water was recovered in five wells. Again, the time of flushing was late Mississippian and Pennsylvanian when the area was exposed to erosion and meteoric waters.

A substantial Permo-Penn section thickens to the south, with fewer structures and poorer reservoir quality. These rocks were also exposed to influx of surface waters during Triassic and Jurassic time.

The authors do not rule out the possible occurrence of small stratigraphic traps but believe there is a remote chance of finding a large accumulation due to the extensive faulting and trap destruction.

The authors have less knowledge of the area south of Chalk Draw fault but have identified four structural lead areas, and three of them were drilled dry. A test of the Ellenburger produced fresh water.

The authors can only conclude that the potential for significant hydrocarbon accumulations in the overall Marfa basin area remains slim at best.

DIABLO PLATFORM

Based on 220 miles of fair to good seismic data, the authors constructed a Mississippian structure map (Fig. 22) that indicates numerous normal faults trending roughly northwest-southeast.

Depth to Mississippian varies from 200 ms (1,600 ft) in the southern part of the area to 1,200 Ms (9,600 ft) in the northern part. In all, the authors mapped at least 22 traps or structural leads, of which 11 have been tested.

The boundary between the Diablo platform and the Chihuahua trough is marked by a normal fault, down to the southwest with substantial throw, and the authors' data crosses into the trough where thrust-faulting may be observed in the Permian.

On the platform, the authors found faults that affect the Lower Paleozoics but do not affect the Pennsylvanian rocks and others that affect the entire Paleozoic sequence.

The authors concluded that the area was subjected to faulting more than once during Paleozoic time or that some older faults were reactivated during late Paleozoic time.

The Mississippian structure maps shows several structural highs consisting of rollovers and uplifted fault blocks.

Erosion of Pennsylvanian rocks on rollovers and the truncation of Pennsylvanian events by the overlying Cretaceous suggest that the uplift took place immediately after deposition of the Pennsylvanian.

On the other hand, the horst block on which the 1 Hunt was drilled was probably pre-Pennsylvanian in age since the Pennsylvanian section in this well is thin and does not show any truncation of Pennsylvanian events.

Both pre-Pennsylvanian and Pennsylvanian faulting are evident on the platform.

The Texaco 1-3 State of Texas was drilled on an uplifted fault block with Pennsylvanian onlapping on granite.

This indicates the presence of structural highs on the platform during the lower Paleozoic and these areas were probably sites of non-deposition.

However, erosion of already formed lower Paleozoics on this structure cannot be ruled out, and these findings are in agreement with previous reports which suggest that the Diablo platform was intermittently active during the Paleozoic era but a strong uplift occurred in Pennsylvanian time, probably at the same time as the Ouachita orogeny.

IMPORTANT WELLS

There are several important tests on the platform (Fig. 22).

All of them are dry, but many had minor hydrocarbon shows.

Many also reported water recoveries, usually fresh but occasionally brackish or salty. Formations with known tests range from Cambrian through Cretaceous.

The authors have concluded that hydrocarbons were generated and trapped in structures formed during the lower and middle Paleozoics, but during the late Pennsylvanian the whole area was uplifted and exposed to erosion.

During this time, pervading meteoric water flushed the hydrocarbons from the traps. Post-Paleozoic rocks are also barren, based on drilling, but the formations are of continental type.

The most likely remaining traps, if any, would appear to be stratigraphic in nature, e.g. a reef enclosed in a dense lime, a sand lense in shale, or a similar-type feature.

PRODUCTION, SHOWS

Production in the review area is confined to the southwest flank of the Delaware basin with significant to minor reserves established in Permian, Mississippian, Devonian, Silurian, and Ordovician carbonates and cherts.

With the exception of a few small oil fields in Permian and Mississippian, the area is gas productive. Estimated reserves range from 1 bcf to 1.1 tcf.

Porosity is generally low, and fracture enhancement is apparent in several fields.

The maximum productive column known is 2,500 ft or greater in Oates, Northeast field, in Devonian cherts and dolomite.

Elsinore field has the distinction of having 27 producing wells with 16,640 proven acres.

The larger traps are northwest-southeast trending anticlines, usually with one large flanking fault. Readily available data for Delaware basin fields may be found in the literature.

The above is in sharp contrast with Trans-Pecos Texas. Several wells in the Chihuahua trough and Marfa and Marathon basins have reported live and dead oil shows, and some have had minor gas recoveries. These areas appear to have been flushed by fresh water.

This coupled with intrusive and extrusive bodies in a highly tectonically-active area make the presence of large productive traps extremely doubtful.

Drilling on the Mexican side shows no better results. Wells there encountered numerous oil and gas shows in Cretaceous rocks, many of them visibly associated with Tertiary volcanics. The shows may represent the end result of a natural form of in situ retorting.

The oil and gas shows associated with formation water recoveries suggest that the stratigraphic column in Trans-Pecos Texas, while possessing both source and reservoir beds, has been faulted and fractured to such an extent that leakage of hydrocarbons and the induction of fresh water has destroyed any significant hydrocarbon accumulations.

INDUSTRY ACTIVITY

The petroleum industry is sporadically active in Trans-Pecos Texas.

Seismic work and some drilling take place every year, but the current lack of money and tax incentives has reduced activity to a marginal effort.

All of the production thus far is located in the Delaware basin and the Marathon thrust belt, and all of the major basins and the Marathon overthrust command what little industry attention exists.

CONCLUSIONS

The authors' principle concern is trap preservation.

The preponderance of evidence suggests that, while Trans-Pecos Texas cannot be completely condemned, the geologic events in the history of the area made the preservation of hydrocarbons on an economic scale remote.

The authors believe that faulting and fresh water induction destroyed or reduced potential trap size to a present day uneconomic level.

Chance factors should be placed at a fraction of 1% ... optimistically.

Copies of bibliography are available from Sunit K. Addy, c/o ARCO International Oil & Gas Co., P.O. Box 260888, Plano, TX 75026-0888.

ACKNOWLEDGMENTS

The authors acknowledge the major contributions of George W. Whitney and Ralph Worthington, who participated in the original evaluation. The authors also thank the drafting personnel of Atlantic Richfield Co. and the company itself for its support.