INTRODUCTION

This paper reports new data from a deep well drilled in 1967 by Shell Oil Company in the Ouachita structural belt in Hill County, Texas. Shell Oil Company No. 1 Ethel W. Barrett was located 920 ft from the northeast line and 1,320 ft from the southeast line, Stephen Greenwell Survey, A-337, Hill County, Texas, approximately 65 mi south of Dallas, Texas. This test is 25 mi east and back of the frontal Ouachita thrust as mapped by Flawn et al. (1961; Fig. 1).

Reconnaissance stacked seismic shooting suggested the presence of a major structural feature in this area from reflections believed to be associated with pre-Jurassic strata. Subsequent detailed seismic work delineated a large (80 mi long and 10 mi wide) northeast-trending, doubly plunging anticlinal complex cresting at an estimated depth of 13,000 ft. A characteristic seismic section across this structure is shown in Figure 2. A sequence of seismic reflections was observed which appears similar to that associated with shallow-water Cambro-Ordovician carbonate rocks in the Strawn basin 35 mi west. Velocity values determined by refraction studies are within the range of those associated with Cambro-Ordovician carbonates. Consequently, it was postulated that the reflections in this area were emanating from uplifted lower Paleozoic shelf carbonates underlying metamorphosed overthrusts of Ouachita facies rock. Shell No. 1 Barrett with a proposed total depth of 20,000 ft, was drilled to test this concept. This test was spudded July 7, 1967, and abandoned January 30, 1968, at a total depth of 20,310 ft.

STRATIGRAPHIC SUMMARY AND CORRELATION

Cretaceous and possible thin Jurassic strata are present in this test from the surface to 3,904 ft; phyllites, slates, and quartzites are in the interval from 3,904 to 13,595 ft. A massive 6,060-ft marble section is present between 13,595 ft and 19,655 ft; the top is believed to be associated with the primary seismic reflector on this structure. A medium-gray biotite-rich quartzite, 125 ft thick, underlies the marble unit down to a depth of 19,780 ft. This quartzite in turn overlies altered medium to coarsely crystalline quartz diorite, which is present to total depth (20,310 ft). A simplified lithologic log is shown in Figure 3.

The exact age of the metamorphosed strata is unknown. Metamorphism has destroyed most of the fossil record, but some unidentified inarticulate brachiopods were recovered in the upper phyllite section and crinoid columnals are abundant between 13,911 and 15,739 ft. Formations penetrated below 3,904 ft are assumed to be Paleozoic; the massive marble section between 13,595 and 19,655 ft may represent the Devonian through Ordovician (Ellenburger) or Cambrian carbonate section similar to that in West Texas. The altered quartz diorite could be Precambrian.

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Fig. 1. Index map of East Texas, showing location of Shell Oil Company No. 1 Ethel W. Barrett.

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Fig. 2. Seismic reflection profile and geologic interpretation showing structure drilled by Shell Oil Company No. 1 Ethel W. Barrett.

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Fig. 2. Continued. See caption on page 2010.

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Fig. 3. Simplified lithologic log of Shell Oil Company No. 1 Ethel W. Barrett with location of cores and isotopic-age date determinations.

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As the exact age of these strata is unknown, the four major stratigraphic subdivisions below 3,904 ft are described simply as Formations A, B, C, and D. A description of these formations follows. Petrographic descriptions of thin sections from cores are also included.

Formation A--Phyllite 3,904-13,595 Ft

3,904-3,970 ft Weathered phyllite: red and green, gray, abundant quartz veins, dead oil present.

3,970-13,595 ft Unweathered graphitic phyllite, sericitic slate, quartz veins, and metaquartzite. Inarticulate brachiopods were found between 5,580 ft and 5,930 ft, and at 7,200 ft, indicating marine origin at least for these intervals. In general, there is progressive increase in metamorphism from top to bottom. Entire section is highly fractured and sheared, with massive emplacement of quartz vein material. Some fractures are open, for water entered the hole at 7,350 ft (15,000 ppm chlorides) and at 7,632 ft (35,000 ppm). No core was recovered from Formation A. However, large samples were recovered by use of junk basket.

Formation B--Marble 13,595-19,655 Ft

This unit is dominantly calcite and dolomite marble. Dolomite marble is fractured, but most fractures appear to be healed. Calcite marble is highly sheared and contorted, and appears to have flowed rather than fractured. Upper part of unit has thin phyllite and quartzite interbeds. Sub-units are described below as members.

13,595-13,620 ft, Member 1:
Dolomite marble and calcite marble: light gray with graphitic inclusions.

13,620-13,808 ft, Member 2:
Phyllite: medium gray, sericitic with minor calcite marble interbeds.

13,808-14,700 ft, Member 3:
Calcite marble: light to medium gray, dense, with muscovite and secondary quartz; a few beds of light-gray, calcareous dolomite marble; green phyllite, schist, and abundant quartz veins in upper 300 ft. Interval between 13,911 and 13,921 ft was cored. Core is calcite marble and shows flow structures indicating strong compressional deformation in horizontal plane.

13,914 ft Calcite marble: generally coarsely crystalline (600µ) with abundant laminations of muscovite mica parallel with bedding. Calcite crystals are strongly deformed and recrystallized, and show elongation and orientation. Glide twinning planes are abundant and suggest movement and flow of the rock. Quartz crystals are abundant. They are irregularly shaped, show only slight straining, show no mutual interpenetration at contacts, have generally smooth boundaries with calcite, and are probably hydrothermal in origin. Muscovite is present in laminations as elongate flakes commonly associated with quartz crystals and formed after the quartz event. Scattered pyrite crystals are present, and in laminations black graphite? is present in small quantities.

13,916½ ft Calcite marble: as above, with muscotive in trace amounts and numerous small quartz crystals, some in planes parallel with bedding. No visible porosity.

13,917 ft Calcite marble: as above, coarsely crystalline (600µ) with abundant tiny inclusions of quartz. Calcite crystals exhibit considerable glide twinning. Quartz grains occur as laminations; crystals are slightly strained, but show no pressure-solution relations and appear hydrothermal in origin. Scattered well-developed euhedral-subhedral crystals of plagioclase containing calcite inclusions are present and most likely are autochthonous. No intercrystalline or fracture porosity is present.

13,919 ft Calcite marble: medium-coarsely crystalline (400-600µ), with abundant anhedral quartz crystals and very abundant muscovite mica in laminations. There are elongation and orientation of calcite crystals. Quartz has smooth surfaces embaying into calcite indicating replacement of calcite by quartz. Muscovite is well developed into large plates along laminations, particularly in areas where quartz grains are present.

14,700-15,480 ft, Member 4:
Calcitic dolomite marble: white to light gray, with interlayers of calcite marble. Interval between 15,008 and 15,018 ft was cored (Fig. 4).

15,009 ft Calcitic dolomite marble: with flow structures showing recumbent folds and shears parallel with axes of folds (dipping 35° from horizontal). Crystals of dolomite and calcite are mixed and interlocking, suggesting that rock was porous dolomite plugged with sparry calcite before deformation. Recrystallization has resulted in elongation and orientation of crystals, particularly of calcite. Large calcite crystals plugging vugs have been recrystallized by deformation and exhibit good glide twinning. Well-developed muscovite plates are present along shear planes which cut across sedimentary laminations. Quartz has been introduced along shear planes and is also scattered throughout rock. Euhedral pyrite crystals occur in small amounts. Small quantities of dark material along s dimentary laminations are most likely fine-grained graphite.

15,012½ ft Calcitic dolomite marble: as above, showing lineation of carbonate crystals. Vugs were filled with calcite, then quartz before deformation. Muscovite is abundant. There is no visible porosity.

15,015½ ft Calcitic dolomite marble: as above.

15,017½ ft Calcitic dolomite marble: shows tongues of recrystallized limestone and limy dolomite interfingering and contorted. Lineation of crystals is strong. Recrystallization of calcite has resulted in growth of coarse calcite crystals with abundant glide twinning. Muscovite is present in small quantities. Autochthonous quartz crystals exhibiting small amount of straining are scattered throughout.

15,480-17,610 ft, Member 5:
Calcite and dolomite marble: gray to white, dense, interbedded. Interval between 15,723 and 15,739 ft was cored. Core shows two sets of intersecting fractures, the first dipping approximately 45° and the later set dipping 75° from horizontal. Flow structure is common in calcite marble, less so in dolomite marble.

15,725-15,726 ft Dolomite marble: finely crystalline tight mosaic, with stylolites. Rock has been fractured by two sets of intersecting fractures, both post-dating stylolite and recrystallization periods. First set dips approximately 45° from horizontal and is series of microfaults with thrust displacements of less than 1 in. Second (later) set dips about 75° from horizontal.

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Both sets of fractures are lined with large dolomite crystals and completely healed by calcite. Finely crystalline pyrite occurs along stylolitic boundaries and scattered throughout dolomite matrix. Subhedral to anhedral quartz in small amounts replaces dolomite of matrix, but is not found in fracture fill and therefore precedes shearing event. There is no visible porosity.

15,729 ft Calcite marble: with numerous dolomitic laminations. Rock has been much recrystallized with calcite crystals somewhat elongated and tightly interlocking. Dolomite laminations are discontinuous and show considerable replacement of crystals by calcite. Quartz has been introduced as anhedral crystals along irregular and discontinuous veining subparallel with laminations. Crystals exhibit some slight strain

Click to view image in GIF format. Fig. 4. [Grey Scale] Photograph of core from interval between 15,008 and 15,018 ft. Core is calcitic dolomite marble with interlayers of calcite marble.

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and replace calcite. Finely crystalline pyrite is common, disseminated throughout rock in wispy discontinuous occurrences with few larger single crystals. Tremolite is well developed and common as small prismatic crystals. This mineral is commonly found in silicated metamorphic limestones. No porosity is visible.

15,730 ft Dolomitic calcite marble: exhibits good flow structures and elongated, tightly interlocked calcite crystals with laminations of dolomite crystals. Crystals are oriented at angle to laminations. Abundant vein quartz grew as small tightly interlocked crystals, replacing calcite and generally parallel with laminations. Quartz is strained and shows some flow structures, but does not have appearance of being sedimentary. Tremolite is present in significant amounts, and finely crystalline pyrite is disseminated throughout as wispy laminations parallel with elongation of calcite crystals. There is no visible porosity.

17,610-19,440 ft, Member 6:
Dolomite marble: white, fine to medium crystalline, dense. Some cores have laminae suggestive of tidal-flat depositional environment. Intervals from 17,766 to 17,776 ft and 18,982 to 19,001 ft were cored. Core from 17,766 to 17,776 ft is highly fractured, but fractures have been healed with dolomite cement. Possible ghosts of sedimentary structures were noted. Core from 18,982 to 19,001 ft does not exhibit typical flow structures noted in previous cores.

17,766 ft Dolomite marble: highly metamorphosed, calcareous with few quartz crystals. Dolomite crystals are anhedral, varied in size, and form tightly interlocking mosaic. Extinction is irregular and imperfect. Inclusions in optical discontinuity are abundant. Calcite crystals occur as irregular small areas, probably originally filling pore space but now replacing dolomite crystals in part. There is no lineation or twinning of these small calcite crystals. Twinning is abundant in larger calcite crystals which fill large vugs. Absence of lineation and twinning of small calcite crystals implies rock was completely annealed. Nondetrital anhedral quartz crystals are present but show no straining. Many inclusions in crystals give murky look to rock. Under reflected light, ghosts of unident fied sedimentary structures are visible. No visible porosity is present.

17,768 ft Dolomite marble: as above, with fewer small calcite crystals and with larger twinned calcite crystals in poorly defined veins. Faint fractures healed with clear dolomite crystals are abundant.

18,982 ft Dolomite marble: varied crystal size. Small anhedral calcite crystals, untwinned, are scattered throughout and probably represent early pore-filling calcite. Structure of original dolomite crystals is still preserved showing little distortion or strain, but there commonly is reduction in size of about 30 percent by solution at point contacts and redeposition in pore spaces. There are abundant ghost structures which suggest that rock was originally crinoidal hash or possibly oolitic limestone. These structures have not been stretched or distorted. Rock does not have typical flow structures seen in cores from higher in section. No visible porosity is present.

18,986 ft Dolomite marble: brecciated and highly metamorphosed, consisting predominantly of areas of coarse crystals with wavy extinction and areas of finer crystalline dolomite in tight interlocking mosaic flowing between larger fragments of coarse dolomite. Muscovite is present in small amounts and is replaced in part by later vein quartz. Calcite is final mineralizing event, which replaces dolomite and quartz. Larger crystals exhibit good twinning. Luminescence reveals that rock was vuggy dolomite lined with coarse dolomite crystals prior to deformation. Metamorphism has almost totally obliterated original structures. No visible porosity is present except in microfractures, which are abundant.

18,988½ ft Dolomite marble: as above, showing abundant ghost structures. Pattern of rock looks like a solution breccia with pore spaces filled by coarse dolomite crystals with undulating extinction, followed by a quartz event with crystals showing strain, and a final calcite event with crystals exhibiting bent twinning planes. Matrix rock consists of smaller dolomite crystals tightly interlocked with abundant graphitic material present. Small calcite crystals are present, in part replacing dolomite and probably plugging earlier pore space. Twinning is common. Pyrite is present in trace amounts. No visible porosity is present except in microfractures, which are abundant.

19,440-19,655 ft, Member 7:
Calcite marble: light gray to white, interbedded with light-gray to white, finely crystalline metaquartzite with biotite and phlogopite.

Formation C--Metaquartzite 19,655-19,780 Ft

This unit is medium-gray metaquartzite with abundant biotite. Interval between 19,701 and 19,706 ft was cored.

19,701 ft Metaquartzite: shows mosaic of slightly elongated anhedral quartz grains ranging from 0.25 to 0.5 mm, with biotite, orthoclase, chlorite, microcline, and pyrite. Biotite and chlorite are at grain boundaries of quartz.

Formation D--Quartz Diorite 19,780 Ft-TD

This unit is altered medium to coarsely crystalline quartz diorite. Interval between 20,307 and 20,310 ft was cored.

20,310 ft Quartz diorite: mineral constituents include orthoclase, plagioclase, biotite, quartz, muscovite, epidote, apatite, sphene, pyrite, and sparse zircon. Plagioclase is severely saussuritized. Good residual hornblende cleavage is evident in biotite. Except for small amounts of residual plagioclase and trace zircons, all minerals in rock are alteration products, possibly hydrothermal or deuteric.

AGE DATING

Paleontologic data are not sufficient to determine the ages of the units, but isotopic age dates determined by Shell Development Company provide minimum ages of metamorphism and therefore a minimum age of deposition. Dates of metamorphism in the phyllite are Pennsylvanian, with a Permian age metamorphic event calculated in a brecciated section at 9,904 ft. A Late Devonian potassium-argon date was obtained near the top of the massive

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marble, and a Permian date was also determined in the quartzite interval overlying the basal quartz diorite. The altered diorite has Late Mississippian to Early Pennsylvanian dates of recrystallization (Table 1).

Isotopic age dates suggest three major orogenic events. The oldest event is Late Devonian. The structure could have been formed during this event by vertical uplift or by initial thrusting in the geosyncline, or possibly the date could record deposition of the rock. Late Mississippian and Pennsylvanian ages probably record the time of emplacement of the thrust sheets of the frontal and interior zones of the Ouachita structural belt. The Permian ages may record a thermal event, possibly related to a cooling of the section by epeirogenic uplift of the whole geosynclinal area. The Early Devonian age recorded at the top of the phyllite section may be detrital and related to provenance rather than a local thermal event.

STRUCTURE

An analysis of the stratigraphy and structure prior to drilling Barrett No. 1 suggested that carbonates should be present between 14,000 and 20,000 ft beneath overthrust Ouachita facies rocks. It was assumed that the carbonate strata would not be altered by metamorphism, but that the Ouachita shales above these strata would be metamorphosed. The depth to the carbonate event from seismic data was within the range predicted, and except for the metamorphism in the carbonates, the predicted stratigraphy was generally correct.

Several interpretations have been postulated for the probable origin of this structure.

1. It could have formed by alpino-type compressional decollement folding and low-angle thrusting in a structural style similar to that indicated by Bally et al. (1966) in the southern Canadian Rocky Mountains, Gwinn (1964) in the central Appalachians, and Arbenz (1968) for the Potato Hills in the Ouachita Mountains.

2. It could have formed as a germano-type block-faulted uplift involving the crystalline basement and the overlying sedimentary cover, which was later caught in thrusting from the geosyncline. It would be a semi-rooted basement block with a strong component of later thrusting.

3. It could have formed as an external miogeanticlinal ridge (after Aubouin, 1965), later caught in thrusting from the eugeosyncline.

4. It could have formed as an autochthonous ruptured basement fold or uplift caused by close compression of a geosyncline in situ (Tomlinson, 1959, p. 7).

5. It could have formed by the emplacement of a plutonic body, with concomitant uplift and metamorphism, which was overridden by later Ouachita thrust sheets.

The data from Barrett No. 1 are not sufficient to determine which of these interpretations, if any, is correct. Our preferred interpretation is that of a para-autochthonous or semi-rooted basement block involved in thrusting from the geosyncline, but probably moved laterally only a minor distance. Because cratonic facies and the underlying Precambrian crystalline basement are involved in the deformation, basement block faulting is indicated. The extensive shear and flowage features in the cratonic carbonate rocks and the general asymmetry of the anticlinal structure toward the continent and its linear continuity paralleling the orogenic belt suggest that the structure is at least partly involved in the compressional system.

DRILLING EXPERIENCE

The drilling operation was highly successful with very few mechanical difficulties. Twenty-two-inch surface casing was set at 310 ft, and water drilling continued to a depth of 4,424 ft, where 16-in. intermediate casing was set. The hole was drilled with air from 4,424 to 16,168 ft, and water was again used as the drilling medium from 16,168 ft to total depth. A string of 10 3/4-in. casing was set at 15,027 ft prior to converting to water drilling.

Table 1. Age Determinations from Shell No. 1 Barrett

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HYDROCARBON SHOWS

Methane shows on the hot-wire mud logger were prevalent in the interval 13,400 to 13,800 ft, and a 20-ft flare burned for about 30 minutes during a trip at 13,911 ft. We assume most of this gas came from near the top of formation B, member 1. Hot-wire methane shows were recorded between 19,080 and 19,440 ft, and an increase in salinity occurred in this same interval. The well began flowing salt water during a trip at 19,701 ft, and chlorides increased to 42,000 ppm. An open-hole drillstem test between 15,027 ft and 18,662 ft yielded 53 bbl of drilling fluid (34,500 ppm cl) with no trace of hydrocarbons. A maximum bottomhole temperature of 294° F was recorded.

CONCLUSIONS

Some of the conclusions that can be drawn from the data obtained in this well are as follows: (1) foreland facies deposition in the lower and possibly middle Paleozoic occurred at least 25 mi east of the present position of the edge of the frontal zone of the Ouachita structural belt; (2) several periods of tectonic activity are indicated, ranging in age from Devonian to Pennsylvanian, with a possible thermal event such as cooling in the Permian; (3) the objective carbonates at this position in the Ouachita tectonic belt are metamorphosed to green-schist facies, and essentially all reservoir rock potential, if originally present, is destroyed; and (4) there is a striking similarity between the marble sequence in this well and similar rocks in several wells in Kinney and Val Verde Coun ies, Texas, particularly the Werblow No. 1 Newton and Bunn No. 1 Horn in Val Verde County.

From an exploration economic point of view, several conclusions can be drawn: (1) seismograph is a usable tool for exploration within the Ouachita structural system; (2) deep drilling in the interior zone of the Ouachita structural belt is feasible; and (3) hydrocarbon shows and open fractures are indicated in the metamorphosed rocks.

The Ouachita tectonic belt has had a complex tectonic history involving successive periods of uplift and overthrusting, coupled with dynamic, hydrothermal, and possibly regional metamorphism. It is hoped that the data from the Barrett well have advanced our state of knowledge in unraveling the complex history of this structural trend.