LOCATION

The Quitman field is located 3 1/2 miles northwest of the town of Quitman in the west central part of Wood County, Texas. It is approximately 15 miles north of the center of the East Texas basin and 40 miles northwest of the East Texas field.

HISTORY AND DEVELOPMENT

According to available records, the Quitman prospect attracted first interest in 1930 when the Shell Petroleum Corporation discovered a refraction high in the area which was coincident with a gravity minimum. This discovery led to detailed reflection seismograph work which confirmed earlier indications of an

Fig. 3. Quitman Oil Field, Wood County, Texas by E. R. Scott

Oversized Image :click to view

Fig. 4. Quitman Oil Field, Wood County, Texas by E. R. Scott

Oversized Image :click to view

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uplift and outlined the structure. A heavy leasing program followed, and during the latter part of 1934 the Atlatl Royalty Corporation offered spreads of leases for sale to support the drilling of a Woodbine test well. The well was drilled in June, 1935, and at a final depth of 5,040 feet had penetrated 600 feet of the Woodbine formation. No indications of oil or gas accumulations were found in the objective beds, and a slight showing of oil detected in a sand of Eagle Ford age did not appear to offer commercial possibilities.

Failure of the Atlatl well discouraged general interest in the prospect; however, the discovery of commercial oil in the lower Glen Rose formation at Rodessa in December, 1935, and the Paluxy discovery at Talco during the following year indicated deeper possibilities for the Quitman structure. Although the quality of these objectives was considered to be somewhat speculative, some holdings were kept intact or consolidated for the deeper prospects. Additional interest was created in the area in 1940 when deep commercial oil was discovered at Pittsburg approximately 30 miles northeast of Quitman.

In 1942, the Shell Oil Company, Incorporated, initiated steps toward the promotion of a deep test well at Quitman by offering a cash and acreage contribution. While negotiations were in progress, Paluxy production was discovered at Coke, about 12 miles northeast, which enhanced the prospects of this formation at Quitman and led to completion of arrangements for an exploratory test. This well was drilled by the Delta Drilling Company on the J. B. Goldsmith property at a location indicated by seismos to be near the apex of the structure. The top of the Paluxy was logged at 5,776 feet, subsea, and approximately 25 net feet of oil-saturated sand was penetrated between 5,795 and 5,825 feet, subsea. Upon completion December 1, 1942, the well recorded an initial flowing potential of 726 barr ls of oil per day through a 1/4-inch choke. Subsequent development of the field was relatively gradual in spite of the fact that extensive outsteps early in the development period proved a major accumulation in the Paluxy reservoir and confirmed the seismic interpretation of the structure. This somewhat dilatory development may be attributed to complicated subsurface conditions and pooling requirements involved in consolidating property interests in small and irregularly shaped tracts. Field rules adopted soon after discovery of the field provided for 40-acre spacing with no well located closer than 467 feet to a lease line and wells on the same lease separated by a minimum distance of 990 feet. These spacing rules were in accordance with Government regulations and resulted in orderly de elopment on a relatively uniform pattern.

Although the most intensive drilling activity occurred during the first 2 years following discovery, development of the Paluxy reservoir continued into early 1947. In this period 90 Paluxy tests, including 19 failures, were drilled.

In November 1945, oil was discovered in, and produced from, a thin sand member (Harris sand) of the lower Eagle Ford formation below 4,000 feet. The discovery well, the Shell's Atlatl Harris No. 1, pumped 61 barrels of low-gravity

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Fig. 1. PORTION OF NORTHEAST TEXAS STRUCTURAL MAP CONTOUR DATUM -- TOP GEORGETOWN

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oil per day. Because of the uncertain significance of this discovery, development has been unusually slow. To date, 10 producers have been completed in this sand and a light drilling program is being continued. The shallow-drilling campaign has resulted in the discovery of oil, apparently in commercial quantities, in the sub-Clarksville sands of the upper Eagle Ford formation, 300 feet above the Harris sand member.

STRATIGRAPHY

The normal stratigraphic sequence in the Quitman area from the Eocene-Wilcox at the surface to the top of the upper Glen Rose in the Comanche is illustrated in Figure 2. The succession of formations approaches the normal development for the East Texas basin and the stratigraphic boundaries are based on the results of investigations by Bailey, Evans, and Adkins (FOOTNOTE 3) and A. C. Wright.(FOOTNOTE 4)

In general, the formations penetrated attain an exceptional degree of uniformity throughout the field, permitting accurate electrical-log correlation. Noteworthy exceptions are the sandy Woodbine and Paluxy formations which are characterized by erratic distribution of sediments, nevertheless, the overall thickness of these formations remains fairly uniform. Even the thick Upper Cretaceous shale sections may be subdivided in remarkable detail by utilizing the amplified normal resistivity curve on electrical logs.

The strongest stratigraphic break in the section is represented by the Comanche unconformity at about 5,000 feet where part of the Buda limestone, the Maness shale,(FOOTNOTE 5) and an undetermined amount of additional sediments were removed prior to deposition of the Upper Cretaceous beds. Within the Quitman field, the Comanche unconformity is indicated by progressive east-to-west truncation of the Buda resulting in complete removal of Buda porous zone over the western part of the uplift (Fig. 5). Correlations indicate the amount of differential truncation within the field to exceed 60 feet.

The type log (Fig. 2) depicts other slight unconformities in the stratigraphic section occurring at the top of the Woodbine, top of Eagle Ford, and top of Ector. These formation breaks are based on regional information and there is little, if any, correlative evidence in support of them at Quitman.

STRUCTURE

The Quitman structure is a faulted anticline trending in a northeasterly direction over a distance of approximately seven miles. It is one of a series of structures in a belt trending northeastward across the northern slope of the

FOOTNOTE 3. T. L. Bailey, F. G. Evans, and W. S. Adkins, "Revision of Stratigraphy of Part of Cretaceous in Tyler Basin, Northeast Texas," Bull. Amer. Assoc. Petrol. Geol., Vol. 29, No. 2 (February, 1945).

FOOTNOTE 4. Personal communication.

FOOTNOTE 5. T. L. Bailey, F. G. Evans, and W. S. Adkins, op. cit., p. 176.

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Fig. 2. QUITMAN FIELD TYPE LOG

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East Texas basin. The maximum uplift occurs in the southwestern portion of the anticline which is reflected in the greatest width of the structure. A secondary closure is found at the northeast and the two culminations are separated by a shallow structural saddle. Seismic data indicate the closure on the Comanche beds to be in excess of 300 feet. The dip of the southeast flank (basinward) averages approximately 10°, whereas the gentler northwest dip apparently does not exceed 5°. The structure of the Upper Cretaceous formations is conformable with the underlying Comanche but exhibits slightly diminished closure.

The most outstanding feature of the Quitman structure is the characteristic fault pattern consisting of a complex system of intercalated grabens. With the exception of occasional shallow displacements confined to individual graben blocks, all faults strike in a northeasterly direction generally paralleling the axis of the structure and dip toward the axial plane at angles of 45°-50°. Displacements vary from a few feet to a maximum of 350 feet, and the excellent correlation permits attainment of an exceptional degree of accuracy in calculating the amount of displacement as well as the location of fault intersections.

Two cross sections of the Quitman structure, drawn transverse to the structural axis, are shown in Figures 5 and 6, and the locations of the two sections are shown by AA^prime and BB^prime on Figures 3 and 4 (pocket on hack cover). These sections illustrate the composite graben structure characterized by the absence of underlying horst blocks. Individual grabens can be traced longitudinally the entire length of the field and lose their identity or undergo modification only at intersections of converging faults. The absence of horst blocks beneath the grabens results from termination of faults having lesser displacement at the base of each graben segment. The larger fault of the fault couple forming the graben continues with depth; however, the movement below the intersection is dimini hed by an amount approximately equivalent to the throw of the smaller fault. This relationship is made possible by the low dips in the gently arched Quitman structure wherein the stratigraphic throw of the faults approximates the vertical displacement. This condition may be repeated several times along the axis of the structure where faulting is most intense with successive reductions in displacement at increased depths. As the result, the most complex faulting is found in the shallower beds with progressive simplification at increased depths evidenced by the fewer number of faults and their diminished displacements. The pattern is clearly demonstrated in Figure 6 where faults G and L evidence movements in excess of 100 feet at subsea depths of approximately 3,500 feet; however, fault G is terminated at its intersection with L and the latter shows only 20 feet of displacement at the Paluxy depth. In fact, the maximum amount of faulting at the Paluxy level on this section does not appear to be more than 20 feet.

Of the multiple faults exhibiting appreciable displacements at shallow or intermediate depths only two (faults B and P) reach the Paluxy throughout the

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field with undiminished throw. Displacement relationships of these two faults at their intersection is undetermined since the base of the graben formed by this fault couple occurs below the depth of drilling. Fault L and the bifurcating fault M form an inner graben on the Paluxy formation in the southwestern part of the anticline. As they converge toward the northeast, the base of the graben rises above the Paluxy with fault M terminating at L and cancelling most of the displacement on the latter fault. This relationship is illustrated in Figure 3 which exhibits the weak remnants of the graben structure northeast of the L-M fault intersection.

The structure of the shallow reservoirs is illustrated in Figure 4. This map, contoured on the Harris sand member of the lower Eagle Ford formation, reveals the greater complexity of faulting in the shallower beds together with the increased amount of displacement.

RESERVOIRS

PALUXY SANDS

The principal oil accumulation of the Quitman field is found in sands of the Paluxy formation at subsea depths ranging from 5,652 feet to 5,909 feet. The structure contoured on the top of the formation is shown in Figure 3. This formation averages 400 feet in thickness and appears to be conformable with the enclosing sediments. It consists mainly of an alternating series of shales and sands, accompanied by varying amounts of fine silty material. Lignite is found in places associated with the sands. The most outstanding feature of the Paluxy formation is the extremely rapid lateral variation in distribution of the sands. In general, this condition is so pronounced as to preclude accurate identification of individual sand bodies over any appreciable distance.

The net thickness of the productive sand section ranges from 2 to 96 feet and averages approximately 35 feet. There is no free gas cap and the oil-water contact at 5,909 feet, subsea, is common to all fault blocks excepting the segment on the upthrown side of fault P where the water level appears to be as high as 5,805 feet, subsea. In the highest well on the Quitman structure the sand attains a structural elevation of 5,652 feet, subsea, proving an oil column of 257 feet. As suggested by its irregular distribution, the physical characteristics of the sand are variable. The texture is fine- to medium-grained and the grains may be well sorted, exhibiting porosities as high as 27 per cent with permeabilities of 1,000-1,500 millidarcys. With increasing amounts of fine silty mate ial in the pore space of the sand, porosity and permeability decrease progressively to a point where the sand is not producible. The formation is well consolidated.

The sandy complex in the Paluxy is subdivided arbitrarily at a point midway in the formation. This is marked by the development of a fairly consistent sand body approximately 200 feet below the top of the Paluxy. However, with the exception of four high wells, this sand does not attain adequate structural

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Fig. 5. QUITMAN FIELD WOOD COUNTY, TEXAS CROSS SECTION "A-A"

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Fig. 5. Continued. See caption on page 426.

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Fig. 6. QUITMAN FIELD WOOD COUNTY. TEXAS CROSS SECTION "B-B"

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elevation to rise into the oil column; therefore, the bulk of production is obtained from sands occurring in the upper 200 feet of the formation. Within this stratigraphic interval, the net sand content varies from almost nothing to 123 feet. There is no linear pattern in the arrangement of the individual sand bodies and efforts to establish depositional trends have been somewhat fruitless. In general, however, lateral gradation from sand to shale commonly occurs in all upper Paluxy sands simultaneously, forming areas essentially devoid of producible sands. Single outstepping wells in places presage the approach toward an area in which permeable sands are absent. In four places, outstepping wells in areas of already poor sand development resulted in failure, but the fifth encountered efinite sand improvement.

The thickest Paluxy sand section is developed on the southeast flank of the structure; however, toward the southwest it grades in a short distance into shale, forming an area barren of production extending over the southwestern part of the anticline. Much of this area is high structurally but is non-productive because of the complete absence of clean sands. Thick sands also occur in the upper Paluxy over the northwestern part of the structure and continue northeast throughout the saddle area into the northeastern secondary closure; however, they disappear rapidly on the northeastern plunge of the anticline toward the Manziel field. An exploratory well drilled midway between the two fields encountered only shale in this section, suggesting that the entire intervening area may be barren of sands. However, the Manziel field, 5 miles distant, which likewise produces from Paluxy sands, also exhibits favorable sand development on the southwest (Quitman) side of the structure.

The irregular distribution of sands forming the Paluxy reservoir poses a perplexing problem in sedimentation and its importance deserves equal rank with structural considerations in planning a development program. The following sequence of events is suggested as an explanation of the present sand distribution. The general character and composition of the sediments evidence an abrupt change in sedimentary environment following deposition of the limestones and shales of the underlying Glen Rose formation. This apparently represents introduction of a widespread littoral environment (FOOTNOTE 6) with extensive marginal lowlands. Sands, transported by eolian agencies, were deposited on the marginal lowlands in dune-like masses. Oscillations of the shore line submerged the sand deposits, an the disseminating action of shallow-water currents served to modify the configuration of the original "dunes." This accounts for the present erratic distribution of sediments in the Paluxy formation in which sands appear to diminish in thickness outward in all directions from localities of maximum sand deposition. Lateral variations are relatively sharp; however, in general, shale intercalations increase in number and thickness as lean sand areas are approached. Distribution relationships offer the strongest

FOOTNOTE 6. W. H. Twenhofel, Principles of Sedimentation (1939) pp. 92-95.

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argument favoring an original windblown origin for the sands. Other factors, supporting this explanation include: (1) the almost complete absence of fossils, (2) the uniformity of grain size resulting in larger pore spaces and higher permeabilities, particularly in the lower range of porosity which differentiates these sands from water-deposited sandstone.(FOOTNOTE 7) Although the sand grains fail to exhibit physical characteristics typical of eolian deposits such as frosting and a high degree of sphericity, wind-borne sands of low sphericity are not rare.(FOOTNOTE 8)

Locally, "black sands," varying in thickness from a few inches up to 40 feet, are found in the Paluxy reservoir. Their appearance indicates some oil saturation; however, analyses prove that the organic material filling the pore space should be classified as asphalt instead of crude oil. Studies by Taliaferro and Stanfield (FOOTNOTE 9) showed that only 17 per cent of the total hydrocarbon content is crude oil. They concluded that the material can not be produced by present methods because of the high viscosity of the oil fraction and the solid or semi-solid state of the remaining fractions. Exhaustive tests have confirmed this conclusion. The contact between "black sands" and normal oil-saturated sands is very sharp and bears no relation to structural or stratigraphic factors. Although the occurrence of asphalt is sporadic, it is commonly found in the vicinity of the oil-water contact suggesting that its formation results from reaction between the salt water and hydrocarbons.

LOWER EAGLE FORD (HARRIS) SAND

This sand occurs at an average subsea depth of 3,850 feet and exhibits an over-all thickness of about 27 feet. Its average net thickness probably does not exceed 12 feet. Unlike the Paluxy, it is remarkably uniform in distribution; however, sorting is poor, resulting in relatively low permeabilities (generally less than 100 millidarcys), although the porosity may exceed 30 per cent.

As shown in Figure 4, the structure of the Harris sand is more complexly faulted than the deeper Paluxy reservoir, and the productive areas are controlled by the fault pattern. Although present data are inadequate to define the limits of accumulation, the productive areas appear to be confined to the higher parts of the upthrown fault blocks. On the northwest of the anticline, production has been proved in the upthrown segments of faults G and M, and on the southeast flank an accumulation has been found on the upthrown side of fault L. Other fault segments may contain exploitable accumulations; however, the central graben area, extending the entire length of the structure, appears to be barren. Moreover, accumulations in adjacent upthrown segments appear to e separated by non-productive areas.

FOOTNOTE 7. G. E. Archie, personal communication.

FOOTNOTE 8. W. H. Twenhofel, op. cit., p. 283.

FOOTNOTE 9. D. B. Taliaferro and K. E. Stanfield, "Asphaltic Sands Found to Occur in Oil Fields," Bull. Amer. Assoc. Petrol. Geol., Vol. 30, No. 4 (April, 1946).

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SUB-CLARKSVILLE SANDS

To date, these sands have not produced; however, they appear to be oil-saturated throughout most of the field. Two sand members are present, occurring in the upper part of the Eagle Ford formation approximately 300 feet above the Harris sand and 25 feet below the base of the Ector chalk. The combined thickness of the two sands totals about 40 feet and the net thickness varies from 2 to 20 feet. The sub-Clarksville structure is modified only slightly from the structure illustrated on the Harris sand reservoir (Fig. 4).

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