Figure Captions

Fig. 1. Location of Ruston field in Lincoln Parish, North Louisiana.

Fig. 2. Natural gas fields in Lincoln Parish, Louisiana. The Ruston field is in the center of the parish, several miles north of the city of Ruston. (From Louisiana Geological Survey, 1980.)

Fig. 3. Ruston field outline, Lincoln Parish, Louisiana.

Fig. 4. Chronostatigraphic section of North Louisiana. (From Shreveport Geological Society, 1987.) Note: The Schuler Formation of the Cotton Valley Group has only recently been assigned to the Lower Cretaceous; most authors prior to the 1980s assigned it to the Upper Jurassic. Also note that outcropping beds of the Eocene Claiborne Group (including the Cook Mountain Formation) that lie immediately above the Wilcox Group are not shown.

Fig. 5. Surface structure map (1931) on top of Minden beds (Eocene). Contour interval, 10 ft. (From Walker, 1953.)

Fig. 6. Same as Figure 5 with surface drainage pattern superimposed. Contour interval, 10 ft.

Fig. 7. Reflection seismograph map (1934) showing contours on the base of the Ferry Lake Anhydrite. Contour interval, 50 ft. (From Walker, 1953.)

Fig. 8. Surface structure on top of Minden beds compared with seismic structure on base of the Ferry Lake Anhydrite. Contour interval, 10 ft. (From Walker, 1953.)

Fig. 9. Reflection seismograph map (1936) showing contours on top of the James Limestone. Contour interval, 50 ft. (From Walker, 1953.)

Fig. 10. Subsurface map on top of the James Limestone defined by drilling as of 1953. Contour interval, 25 ft. (from Walker, 1953.)

Fig. 11. Reflection seismograph map (1944) showing contours on the Bodcaw sandstone in the Cotton Valley Group. Contour interval, 50 ft. (From Walker, 1953.)

Fig. 12. Observe gravity map (1947) of the Ruston field. Contour interval, 0.2 mgal. (From Walker, 1953.)

Fig. 13. (Animated)Early geologic of Gulf Coast area, Cambrian through Triassic and Louann Salt deposition. Cross section from southwestern Louisiana through eastern Texas and into southeastern Oklahoma. (From Burgess, 1976.)

Fig. 14. (Animated) Later geologic evolution of Gulf Coast area, Jurassic through Tertiary (see Figure 13). (From Burgees, 1976.)

Fig. 15. Diagrammatic map outlining major structural features of the southern Atlantic and northern Gulf Coastal province. 1, Ruston field. 2, Mexia-Talco trend. 3, State-Line trend. (From Murray, 1961.)

Fig. 16. Seismic section across the Ruston field. Note the normal fault starting beneath the Cotton Valley "B" limestone and extending into the Louann Salt. (Compliments of Geosource.)

Fig. 17. Burial-history plot of the Ruston field based on the Arkla Gas Company No. 2 Mathews, Sec. 29, T19N, R2W, Lincoln Parish, Louisiana. (Plot was prepared by Renae Wilkinson of Platte River Associates, Denver, Colorado.)

Fig. 18. (Animation incl. figs. 20-23, 25-28) Isopach map from the base of the Ferry Lake Anhydrite to the base of the Cotton Valley "B" limestone. Contour interval, 50 ft.

Fig. 19. Structural cross section A-A’ of the Cotton Valley producing intervals in the Ruston field. Datum, -8972 ft.

Fig. 20. (Animation incl. figs. 20-23, 25-28) Structure map (1989) on top of the McFearin/Davis "H" sandstone with McFearin/Davis producing wells circled. Contour interval, 100 ft.

Fig. 21. (Animation incl. figs. 20-23, 25-28) Structure map (1989) on top of the Cotton Valley Vaughn sandstone. Vaughn sandstone producing wells are circled. Because there are two separate Vaughn sandstones, two water levels are shown. See text for explanation. Contour interval, 50 ft.

Fig. 22. (Animation incl. figs. 20-23, 25-28) Structure map of the Cotton Valley Bodcaw sandstone with Bodcaw producing wells circled. The original and current gas-water contacts are shown. Contour interval, 50 ft.

Fig. 23. (Animation incl. figs. 20-23, 25-28) Structure map (1989) on the base of the Cotton Valley "B" limestone showing "C" sandstone producing wells (triangles) and "D" sandstone producing wells (circles). Updip pinch-outs of the upper, middle, and lower "D" sandstones are also shown. Contour interval, 100 ft.

Fig. 24. Structural cross section B-B’ showing the upper, middle, and lower Hosston producing intervals in the Ruston field. Datum, -6534 ft.

Fig. 25. (Animation incl. figs. 20-23, 25-28) Structure map (1989) on top of the lower Hosston with lower Hosston producing wells circled. Contour interval, 50 ft.

Fig. 26. (Animation incl. figs. 20-23, 25-28) Structure map (1989) on top of the middle Hosston with middle Hosston producing wells circled. Contour interval, 25 ft.

Fig. 27. (Animation incl. figs. 20-23, 25-28) Structure map (1989) on top of the upper Hosston with upper Hosston producing wells circled. Contour interval, 50 ft.

Fig. 28. (Animation incl. figs. 20-23, 25-28) Structure map (1989) on top of the James sandstone showing James producing wells (circles) and James gas-storage wells (triangles). Contour interval, 10 ft.

Table 1. Discovery wells for all producing horizons in the Ruston gas field, Lincoln Parish, Louisiana.

EXPLORATION CONCEPTS

Ruston field was discovered as a result of surface exploration in a fairway established by prior exploration starting in the Shreveport, Louisiana, area and proceeding eastward. Geologic mapping of a surface structure north of Ruston (but south of the field) stimulated interest in the locality that led to geophysical surveys and ultimately to drilling and discovery.

By the 1940s, geologists recognized the strandline nature of the Cotton Valley sandstones and began to use this information to explore for additional structural and stratigraphic traps. The Hosston sandstones were recognized as having a fluvial-deltaic origin and they, too, became prime targets for exploration, even though many geologists were wary of these sands in the early days of ArkLa Tex (adjoining areas of Arkansas, Louisiana, Texas) exploration. It was not uncommon for geologists to refer to the Hosston as "Tragic Peak" in place of its East Texas name, Travis Peak.

Most of the important Cotton Valley and Hosston reserves in North Louisiana had been found by the early 1960s, and a paper by Pate (1963) presented the results of Cotton Valley exploration to that time. Maps and sections from his paper showed the updip limits of the Vaughn, Bodcaw, and "D" sandstones and their relationship to regional structures in North Louisiana.

Hosston and Cotton Valley exploration since the mid-1960s has consisted mostly of: (1) extensions of developed fields; (2) new stratigraphic plays such as the updip, less permeable extensions of the Davis

sandstones in North Louisiana; (3) development of "tight" Cotton Valley sandstones in northwest Louisiana and East Texas; and (4) search for additional stratigraphic and structural traps in East Texas, North Louisiana, Mississippi, and Alabama from a variety of formations. Seismic exploration has played only a minor role in this exploration because most of the new reserves being found are in stratigraphic traps associated with known structures. Many of the newer discoveries in both the Hosston and Cotton valley have been in low porosity-low permeability reservoirs, making massive sand fracturing treatments necessary for commercial production.

In recent years more wells in this trend have been drilled into or through the Upper Jurassic Smackover Formation in search of hydrocarbon reserves in oolitic limestones (Smackover "B" limestone) and the "C" and "Gray" sandstones.

Only a small number of pre-Jurassic tests have been drilled in the ArkLaTex region, and only two of these were in North Louisiana. The CVOC No. 1 Hunt-Hope in Sec. 24, T21N, R10W, Cotton Valley field, Webster Parish, Louisiana, drilled through 3589 ft (1095 m) of Louann Salt and 5086 ft (1551 m) of Eagle Mills (Triassic) red silts and shales before reaching a total depth of 20,395 ft (6220 m). The Union Producing Company No. A-1 Tensas Delta in Sec. 8, T22N, R4E, Morehouse Parish, Louisiana, reached a total depth of 10,475 ft (3195 m) in the Morehouse Formation of probable upper Paleozoic age (Berryhill et al., 1968). The lithology of the Morehouse consists principally of interbedded gray shales, siltstones, and carbonates. The prospects for additional drilling below the Louann Salt are inhibited by the difficulty in obtaining seismic data below the salt, paucity of lithologic information, and high cost of drilling deep tests.

LOCATION

The Ruston gas field lies just north of Ruston in Lincoln Parish, Louisiana, 70 mi (113 km) east of Shreveport and 35 mi (56 km) west of Monroe in north-central Louisiana (Fig. 1 and Fig. 2). The field covers a surface area of 64.5 mi2 (sections) or approximately 41,280 ac (167 km2) (Fig. 3). It lies on the north flank of the Gulf Coast geosyncline within a Hosston-Cotton Valley producing trend extending from Caddo and DeSoto Parishes on the west to Ouachita and Caldwell Parishes on the east (Fig. 1). Stratigraphic pinch-out or a combination of pinch-out and structural closure are the principal trapping mechanisms within this Lower Cretaceous-Upper Jurassic trend. Ruston field contains both structural and stratigraphic traps.

Ultimate hydrocarbon recovery from the Ruston field is estimated to be about 1,600,000 bbl of oil, 1,732,000 bbl of condensate, and 614 bcf (17.2 billion m3) of natural gas, primarily from reservoirs in the Hosston and Cotton Valley stratigraphic units. Production to January 1987 totaled 1,516,701 bbl of oil, 1,252,000 bbl of condensate, and 544 bcf (15.2 billion m3) of natural gas.

Important papers on Ruston field and associated the trend in which it occurs include Breedlove et al. (1953), Kamb (1945), Shreveport Geological Society (1946), Walker (1953) and Wisdom (1968).

HISTORY( Table 4)

Pre-Discovery

The earliest fields with Hosston or Cotton Valley production discovered in this Hosston- Cotton Valley trend were Caddo-Pine Island (1905), Homer (1919), Cotton Valley (1922), Dixie (1929), Sugar Creek (1930), and Lisbon (1936) (Wheeler, 1963). During the 1930s and 1940s, exploration advanced rapidly eastward.

Discovery

The official discovery well for the Ruston field is the Arkansas Louisiana Gas Company No. 1 W. F. Jiles located 50 ft (15 m) south and 83 ft (25 m) east of the center of Sec. 19, T19N, R2W, in Lincoln Parish, Louisiana (Wisdom, 1968) (Fig. 3). "The Jiles well was completed in October 1943 from the Jiles sandstone of the Lower Cretaceous Hosston Formation (approximately 350 ft below the top of the Hosston)" (Wisdom, 1968) (Fig. 4); the total depth of the well was 5985 ft (1824 m). Initial production from the well was 45 MMcf (1.26 million m3) of gas per day (Shreveport Geological Society, 1946).

This followed the drilling of Arkansas Louisiana Gas Company No. 1 Causey in the NE NW of Sec. 32, T19N, R2W, and its deepening in 1937 by Lide and Greer. The Causey well, drilled halfway between the surface structure and the original seismic structure (Fig. 3, Fig. 5 and.6, Fig. 7 and 8, Fig. 9), was "completed" through perforations from 5,316 to 5,322 ft (1620 to 1622 m) for 32,729,000 cubic feet (916,412 m3) of gas and a considerable amount of salt water per day" (Walker, 1953). The producing sand was the Causey sandstone member of the Pine Island Formation (Fig. 4), about 50 ft (15 m) above the top of the Sligo Formation. Wisdom (1968) reported that "No connections were made to the Lide well and, after several unsuccessful attempts to shut off the salt water, it was plugged on November 6, 1942."

The 1936 seismic survey (Fig. 9), started prior to the drilling of the No. 1 Causey, was more sophisticated than the 1934 survey and showed a subsurface structure of the top of the James Limestone very similar to the subsurface map made of the James in 1953 (Fig. 10). One more seismic survey was made in 1944 (Fig. 11), and its results were little changed from those in the 1936 survey (Walker, 1953).

The final geophysical survey in the area by Arkansas Louisiana Gas Company was a gravity meter survey made in 1947 (Fig. 12), the results of which are remarkably similar to the seismic results (Walker, 1953). The negative anomaly indicates the presence of a salt uplift beneath the field.

By 1946, three other gas-producing zones had been discovered in the field: the Causey sandstone, the Pettet Limestone, and the James sandstone. Cotton Valley production was not found until 1948, in the Crescent Drilling Company, No. 1 C. M. Mathews in the northeast quarter of Sec. 29, T19N, R2W. The "D" sandstone member of the Cotton Valley Group tested 6.7 MMcf (187,600 m3) of gas per day, with some condensate (Walker, 1953). However, the scout ticket on this well records an initial potential of only 56 bbl of oil per day.

A total of 16 horizons have proved productive in the Ruston field since it was discovered in 1943. Twelve of these, which include the Rodessa, James, middle Hosston, lower Hosston, Cotton Valley "C," Bodcaw, Vaughn, Price, McCrary, McFearin (Davis), Feazel, and Smackover "Gray" sandstone, were discovered after 1948. Many of the horizons such as upper, middle, and lower Hosston have numerous subhorizons or zones that are grouped together for convenience in mapping and simplicity in nomenclature (see Reservoir section).

Table 1 lists names, locations, discovery dates, producing horizons and their geologic ages, perforated intervals, and initial potentials for all discovery wells in the Ruston field.

Post-Discovery

Fracturing techniques that were developed in the late 1950s are used in the field to enhance the production significantly. Petrophysical logging in the early development of the field consisted of an electric log and microlog, but logging programs during later development utilized induction, sonic, gamma ray, and other types of logs for better formation evaluation.

All Smackover, Cotton Valley, and Hosston production is natural gas with minor amounts of condensate, developed on 640 ac (259 ha) well spacing. Upon completion, the producing interval is perforated, usually followed by stimulation and fracturing of most of the lower Hosston and Cotton Valley sandstones. CO2 fracturing is performed on most thin sandstones and hydraulic fracturing on thicker sandstones.

DISCOVERY METHOD

The Ruston field serves as a good example of a systematic search for new hydrocarbon reserves in a developing region, utilizing a combination of sedimentary trend mapping, surface geologic mapping, and seismic and gravity surveys. These methods led to the discovery of more than 600 bcf (17 billion m3) of natural gas and several million barrels of oil and condensate from 16 major horizons and numerous additional subhorizons. Based on these figures, and using the relationship of 6000 ft3 of gas being equivalent to 1 bbl of oil, Ruston has ultimate recoverable reserves of approximately 100 million bbl oil equivalent.

It is not likely that current exploration methods would be any better able to discover the Ruston field than those used in the 1930s and 1940s. It is a classic domal feature produced by a deep-seated salt uplift, detectable by seismology and gravity methods. The only physiographic evidence of the existence of the surface structure mapped by A.E. Oldham (in 1930) 3 mi (4.8 km) south-southeast of the subsurface structure in 1931 consists of an inconspicuous knoll surrounded by a radial drainage pattern (Fig. 6 ). There are no surface faults in the immediate vicinity; there have been no oil seeps reported in the literature.

STRUCTURE (Table 5)

Tectonic History

The Ruston field is in the North Louisiana portion of the Gulf Coast basin or geosyncline. The Gulf Coast region is classified by Bally and Snelson (1980) as basin type 1143, or an Atlantic-type passive margin (shelf, slope, and rise) that straddles continental and oceanic crust, overlying an earlier back-arc basin. It is classified by Klemme (1971) as basin type IIC c/IV. The IIC portion defines it as a Continental Multicycle basin, crustal collision zone-plate margin closed. The c/IV defines it as a Delta basin, Tertiary to Recent.

Many models have been proposed for the development of the Ouachita belt and the subsequent development of the Gulf Coast geosyncline. Some models, such as that of Burgess (1976) and of (Houseknecht, 1983). The evolution of the Gulf Coast area according to Burgess (1976) is summarized in Figure 13 and Figure 14.

Regional Structure

Ruston field is situated in central Lincoln Parish, on the northeast flank of the North Louisiana Salt basin and southwest of the Monroe uplift. West of the North Louisiana Salt basin is the Sabine uplift, which straddles the Texas-Louisiana boundary. South Arkansas has several graben trends; one is the eastward extension of the Mexia-Talco trend of Texas, and the other is the State-Line trend along the Louisiana-Arkansas border. These and other major structures in the vicinity are illustrated in Figure 15. Salt movement was wholly or partially responsible for many of the petroleum-bearing structures in North Louisiana and South Arkansas, as it was for Ruston field.

Local Structure

The Ruston field is a salt uplift as shown by a strong gravity minimum (Fig. 12) and the nearly circular pattern exhibited by structural contour maps (e.g., Fig. 10). There is no indication of salt piercement from drilling data or the seismic cross section (Fig. 16), hence the structure was probably formed as a salt pillow. The beginning of growth of the uplift is uncertain but likely started in the Upper Jurassic when sediment thickness over the Louann Salt (Middle Jurassic) was sufficient to cause salt movement upward. The movement continued into the Lower Cretaceous based on thickness data, depositional trends, and petroleum maturation data (see burial-history plot, Fig. 17).

An isopach map from the base of the Ferry Lake Anhydrite to the base of the Cotton Valley "B" limestone (Fig. 18) shows about 200 ft of thinning. Growth apparently continued to the end of the Lower Cretaceous, whereupon erosion created a major angular unconformity prior to the deposition of Upper Cretaceous rocks.

Minor faults with very small displacements have been reported in the Ruston field, but they do not show up on the structure maps. However, a fault is interpreted on the seismic cross section (Fig. 16) starting beneath the Cotton Valley "B" limestone and extending into the Louann Salt.

STRATIGRAPHY (Table 6)

The Ruston field is underlain by Tertiary and older sedimentary deposits (Fig. 4). Not shown in Figure 4 are the outcropping beds of the Claiborne Group that lie immediately above the Wilcox. A number of wells in South Arkansas and North Louisiana have also penetrated the upper part of the Morehouse Formation of Paleozoic age.

The deepest formation reached by drilling at Ruston is the Louann Salt of Middle Jurassic age (Bathonian), a unit that was very important in forming many productive ridges and domes in the North Louisiana basin. This is followed by the Norphlet Formation, also Middle Jurassic (Callovian), which is composed mostly of thin, red, sandy shales. It also contains a red conglomerate; red, pink, gray, and white sandstone; and red and gray siltstone. At the Cotton Valley field in Webster Parish, the formation consists of dark gray dolomite and a white sandstone with frosted quartz grains (Berryhill et al., 1968).

The Smackover Formation, Late Jurassic (Oxfordian) in age, is about 1500 ft (457 m) thick and was deposited as a result of a major marine transgression throughout the North Louisiana-South Arkansas region. The lower part is dense, basinal limestone with interbedded argillaceous limestone and shale beds (Berryhill et al., 1968). The middle Smackover is dark-gray to tan argillaceous limestone, intercalated with calcareous sandstones in the northern part of the basin. The two most important of these are called the Smackover "C" sandstone and the "Gray" sandstone. These sandstones appear to be deep-water fan deposits (Judice and Mazzullo, 1982).

The upper Smackover contains many east-west-trending oolite bars that may be fairly extensive or very local. The bars range from very shaly units with low permeability to very well sorted oolitic limestone with excellent porosity and permeability. The oolite bars, with good porosity and permeability, formed across Middle to Late Jurassic "highs."

The Haynesville Formation of Late Jurassic (Kimmeridgian) age overlies the Smackover and consists of anhydrite at its base, grading upward into sandstone, limestone, red shale, and siltstone. In extreme northern Louisiana and southern Arkansas, the anhydrite becomes thicker and contains interbedded red shale, forming the Buckner Formation. Much of the Haynesville represents a regression or a return to marine, marginal marine, and fluvial deposition.

The Cotton Valley Group conformably overlies the Haynesville except on some local structures where the contact appears to be unconformable. It is subdivided into two formations, the Bossier and the Schuler, which, until recently, were both considered Late Jurassic in age. However, the Bossier is now placed in the Upper Jurassic (Tithonian) and the Schuler in the Lower Cretaceous (Berriasian) by many authors (Fig. 4). The entire Cotton Valley interval represents a major regression that was interrupted by minor transgressions. Blanket sands were deposited across subsiding deltaic complexes with other thick deltaic and shelf sand complexes (Thomas and Mann, 1966; Eversull, 1985).

The Cotton Valley contains a high percentage of quartz sand delivered to the shelf by wave-dominated delta systems located in the vicinity of the Sabine uplift on the northwest and near the Monroe uplift on the northeast (Eversull, 1985). The sands were reworked by marine processes forming blanket sands updip from a massive depocenter named the Terryville (Mann and Thomas, 1964; Thomas and Mann, 1966). Numerous blanket sands are present in the upper Cotton Valley, the better known ones being the Vaughn, Bodcaw, and "D" sandstones. These sandstones merge northward into the Hico Shale, providing excellent updip stratigraphic pinch-outs across the basin (Thomas and Mann, 1966, Fig.6 ).

The upper 300 to 400 ft (91 to 122 m) of the Cotton Valley, called the Knowles Limestone, consists of interbedded argillaceous limestones and gray shales.

The Hosston Formation is Early Cretaceous (Hauterivian-Barremian) in age and varies in thickness from 2000 to 4000 ft (610-1219 m) in north Louisiana. The Hosston is predominantly a redbed facies in the updip area of the basin and grades southward (basinward) into fluvial, deltaic, and shelf sandstones, dark marine shales, and shelf carbonates.

The Sligo Formation is also Early Cretaceous in age and reflects a transgression and a return to marine conditions in the basin. The Sligo consists of gray to brown argillaceous limestones and some interbedded sandstones. Oolitic bank and reef facies, known among oilmen as the Pettet "Lime," also occur within the formation.

The Pine Island shale and the James Limestone are Early Cretaceous (Aptian) in age. The Pine Island is a gray, marine shale with a local sandstone unit known as the Causey sandstone which, at one time, produced hydrocarbons in several wells in the Ruston field. The James Limestone is composed of an oolitic to sandy gray limestone with an interbedded fine-grained sandstone. Reef buildups occur in the James in some areas in the downdip part of the basin.

The Rodessa Formation is Early Cretaceous (Aptian) in age and consists of an oolitic to a coquinoid limestone, calcareous shale, anhydrite stringers, and sandstone (Berryhill et al., 1968). The Rodessa has been subdivided into five members, which are, from oldest to youngest, Young, Dees, Gloyd, Hill, and Kilpatrick. All of these consist of limestone with shale interbeds except the Hill, which is a quartz sandstone.

TRAPS (Table 5 and Table 6)

There are more than 40 sandstone or limestone zones producing natural gas and oil from seven formations in the Ruston field: the Smackover Formation; the Schuler Formation of the Cotton Valley Group; the Hosston Formation; the Sligo Formation; the Pine Island Formation; the James Limestone; and the Rodessa Formation. Trapping mechanisms for these zones are summarized below.

The Smackover "Gray" sandstone produced minor quantities of gas at Ruston from one well drilled just south of the crest of the Ruston dome. It is likely a domal trap, but it has been penetrated in only a few wells and structural conditions are uncertain.

The Cotton Valley contains numerous structural and stratigraphic traps at Ruston, making it the most prolific gas-bearing interval. Structural cross section A-A’ (Fig. 19) shows the relationship of the various Cotton Valley producing sandstones in the Ruston field.

The lowest of the Cotton Valley producing units is called the McFearin-Davis member, which has a total of nine different productive sandstones. The McFearin sandstone or McFearin "H" sandstone (Arkla Gas terminology) produces from a closed domal trap (Fig. 20). All other traps in the McFearin-Davis interval are stratigraphic pinch-outs across the Ruston dome.

The Cotton Valley Vaughn consists of two sandstones that yield gas on the crest of the Ruston dome. The sandstones have two distinctly different gas-water levels owing to the porosity and permeability differences in the two units and the small area in which they are joined (Fig. 21).

The Cotton Valley Bodcaw sandstone in the Ruston field forms a structural trap (Fig. 22) at the crest of the dome with the original gas-water level at a subsea depth of 8642 ft (2634 m). The upper seal on the trap is a thick, impermeable marine shale.

The Cotton Valley "D" sandstones (Fig. 23) are the most prolific gas-distillate producers at Ruston field, both in terms of productive acreage and ultimate recoveries. The "D" sandstones at the Ruston field form roughly east-west stratigraphic pinch-outs across the south flank of the Ruston field (Fig.19, Fig. 23). Upper and lateral seals are interbedded marine limestones and shales.

The Cotton Valley "C" sandstone reservoir (Fig. 23) is a stratigraphic trap located in the extreme southwest part of Ruston field. It is a discontinuous, isolated sandstone, usually less than 10 ft (3 m) thick, crossing a southwest-plunging nose extending off the crest of the field. The upper and lateral seal of the "C" sandstone is the impermeable Cotton Valley "B" or Knowles Limestone.

The Hosston Formation is subdivided into upper, middle, and lower units (Fig. 24). Hosston sandstones were deposited as fluvial-deltaic channel sands that are gas-productive where they cross the Ruston dome (Fig. 25-27). All of the sandstones have downdip gas-water levels, and almost all upper and lateral seals are shales or impermeable sandstones. The dome appears to have been growing throughout Hosston time as indicated by interval thinning on the crest of the structure.

The lower part of the Sligo Formation contains a small, isolated bar of porous oolitic limestone ("Pettet Lime") on the crest of the dome, forming a limited reservoir. Away from the crest it grades into a dense, nonporous limestone. Production from the Sligo has been approximately one-third oil and two-thirds natural gas. No water level is present in the reservoir.

The Causey sandstone of the Pine Island Formation is present in most wells in the field, but porosity is developed in only a few wells on the crest and slightly downdip from the crest of the dome. Production has been predominantly natural gas.

The James sandstone forms a structural trap at Ruston (Fig. 28) with sandstone present over most of the structure and shale providing the upper seal on the reservoir. A gas-water level is present around the flank of the structure at approximately 4545 ft (1385 m) subsea. The James is now a gas-storage reservoir. The Gloyd limestone of the Rodessa Formation has yielded a very minor amount of natural gas from a domal trap.

RESERVOIRS (Table 7 and Table 8)

Ruston field has produced or is presently producing hydrocarbons from approximately 44 sandstone or limestone zones in the Smackover, Cotton Valley, Hosston, Sligo, Pine Island, James, and Rodessa. Productive horizons as of 1989 are in the interval between 4500 and 12,100 ft (1371-3688 m) deep.

The Smackover Formation had produced 300 MMCFG from the "Gray" sandstone as of 1987. The "Gray" sandstone has porosities in the 7% to 9% range and permeabilities probably less than 1 md. It averages 30 to 45 ft (9-14 m) thick and is drilled in 640 ac (259 ha) units. The lower Smackover shales are rich enough in organics to make them prime candidates as source rocks for Smackover gas.

The Cotton Valley Group has 19 producing zones: McFearin-Davis, nine; Feazel, one; McCrary, one; Price, one; Vaughn, two; Bodcaw, one; "D" sandstone, three; and "C" sandstone, one.

The nine sandstone units in the McFearin-Davis produce gas and condensate from 41 wells (Fig. 20) and have wide ranges of thicknesses, porosities, and permeabilities. Each unit is separated by shale breaks. The rocks are fine- to medium-grained, light brownish gray to brownish gray quartz wackes with a clay matrix of 6% to 10%. The primary intergranular porosity has been significantly reduced by a syntaxial quartz overgrowth cement. Other cement includes an intergranular and replacement dolomite averaging 7%, calcite averaging 1%, and a trace of pyrite. There are some oversized pores, but most are at least partially filled with vermicular clay and carbonate.

The McFearin-Davis "H" sandstone has produced gas and condensate from ten wells on the crest of the structure. Total production from 1959 to 1 January 1987 has been approximately 31 BCFG plus 181,000 bbl of condensate. Porosities measured from productive zones range from 9% to 13%. The rock has a fairly uniform thickness on-structure, averaging between 12 and 15 ft (3.65-4.6 m). The original reservoir pressure was 4200 psi (28,959 kPa), and pressure in 1987 was 1400 psi (9653 kPa). Associated condensate has a gravity of 67° to 68° API and a gas to condensate ratio of 46,785:1. The original size of this unit was 5425 ac (2197 ha), with an estimated ultimate recovery of 37.8 BCFG and 260,000 bbl of condensate.

The remaining McFearin-Davis sandstone producing units are similar, but on the average the porosities and permeabilities are lower. The sandstones are usually 6 to 10 ft (2-3 m) thick and require fracture stimulation to produce hydrocarbons. All McFearin-Davis wells are drilled in 640 ac (259 ha) units.

The Feazel and McCrary sandstones are defined separately from the McFearin-Davis by the Louisiana Conservation Commission. The porosities range from 12% to 16%. Permeabilities are unknown. The sandstones average 12 ft (3.65 m) thick, with reservoir pressures about 4400 psi (30,338 kPa). These sandstones have yielded 13.6 BCFG plus 43,000 bbl of condensate from the Feazel and 12 BCFG and more than 200,000 bbl of condensate from the McCrary between 1951 and 1987.

The Vaughn sandstone produces from an average depth of 8875 ft (2705 m) from 13 wells for a total production of 97.5 BCFG plus 86,000 bbl of condensate between 1949 and 1987. It is subdivided into the upper and lower Vaughn, with a combined average thickness of 35 ft (10.7 m). The original reservoir size of the Vaughn was 4935 ac (1999 ha), with an estimated ultimate recovery of 122 BCFG. Average porosities are 12% to 15%. The gas to condensate ratio is approximately 32,857:1 with a gravity of 62° API. The Vaughn is drilled on a 640 ac (259 ha) spacing.

The Bodcaw is a significant Cotton Valley natural gas reservoir with a cumulative production of 81 BCFG and 62,500 bbl of condensate from eight wells between 1949 and 1987. The Bodcaw sandstone averages 40 ft thick where it is productive. The typical sandstone is a fine-grained quartz arenite to quartz wacke with an average clay matrix of 5%, with traces of fossil fragments, amphiboles, muscovite, tourmaline, zircon, rutile, glauconite, and opaques. The sands are cemented primarily with syntaxial quartz overgrowths, reducing the intergranular porosity to an average of 15%. Additional cement includes 4% calcite, 1% pyrite, and less than 1% dolomite, siderite, barite, and vermicular clay. Calcite bivalve shells are commonly corroded and sometimes partially replaced by quartz. Secondary porosity is minor with some honeycombed-feldspar grains and occasional clay shrinkage.

The Bodcaw sandstone reservoir is largely depleted with an ultimate recovery of only 82 BCFG expected. It is a pressure-depletion drive reservoir that has had a pressure drop from its original 4100 psi (28,270 kPa) in 1949 to 500 psi (3448 kPa) in 1987. The average porosity is 15%, and the average water saturation is 28%. Condensate is 57° API. The original reservoir covered 3562 ac (1442 ha), with a gas column of almost 200 ft (61 m) above an original water level at 8642 ft (2634 m) subsea.

The Cotton Valley "D" sandstones have produced from 22 wells for an average cumulative production of 12 BCFG per well between 1948 and 1987. The "D" sandstone is actually three distinct sandstone units that shale out updip across the south flank of the structure. Each interval averages 15 ft (4.6 m) thick with the middle and lower sandstones converging downdip into a single interval 20 to 30 ft (6-9 m) thick in the south part of the field. The rocks are fine- to medium-grained, medium gray to brownish gray or olive black quartz wackes and quartz arenites with a clay matrix comprising between 2% and 28% of the rock. The porosity varies from 8% to 20% in the producing wells. Authigenetic dolomite (3%), calcite (1%), and lesser amounts of vermicular clay, pyrite, siderite, and barite are also present. Primary intergranular porosity is the major type followed by secondary porosity shown by oversized pores, some corroded grains, and honeycombed feldspars.

The Cotton Valley "D" sandstones are the most prolific gas producers in the Ruston field, yielding 195 BCFG and more than 600,000 bbl of condensate from their discovery in 1948 to the beginning of 1987. The gas is high in methane, has a moderate amount of ethane, and small amounts of propane, CO₂, and other gases (Table 2). The upper and middle sandstones are the most productive, pinching out about 2 mi (3.2 km) updip from the lower sandstone. Permeabilities average 200 md and porosities average 16%.

The Cotton Valley "C" sandstone has yielded natural gas from ten wells in the southwest part of the field for a cumulative production of 13.5 BCFG and 229,000 bbl of condensate and a gas to condensate ratio of 43,000:1. Production spans the period from 1961 to the end of 1987. The average thickness of the sandstone is 12 ft (3.65 m), and the sandstone has an average of 16% to 17% porosity and 600 md permeability. The original reservoir pressure was 5250 psi (36,198 kPa) at 9550 ft (2911 m), slightly overpressured for this area.

The Hosston Formation has produced from 21 sandstone units in 24 wells in the Ruston field (Figures 25-27). Some of the wells, especially on or very near the crest of the structure, have produced from multiple pay zones in all three intervals. The lower Hosston produces only very near the crest of the structure.

Seven sandstones are productive in the lower Hosston, the majority yielding natural gas and several yielding oil with gravities in the 40° API range. Other than in one upper Hosston sandstone and in the lower Sligo, they provide the only oil production in Ruston field. The best of the lower Hosston sandstones, known as the "B" sandstone, yields natural gas with an estimated ultimate potential of 16.6 BCFG.

The middle Hosston has yielded natural gas from six zones for a cumulative production of 46.1 BCFG between 1955 and 1987. The sands are distributary channel deposits varying in thickness from 5 to 30 ft (1.5-9 m) at an average drilling depth of 6800 ft (2073 m). The sandstones have porosities averaging 18%. Permeabilities are unknown. Bottom hole pressures average 3100 psi (21,375 kPa).

The upper Hosston has yielded a cumulative production of 15 BCFG and 40,000 bbl of condensate from seven sandstones since its discovery in 1943 to the beginning of 1987. The sandstones are 5 to 40 ft (1.52-12.2 m) thick and have an average drilling depth of 5450 ft (1661 m). Porosities are usually in the 15-20% range and permeabilities are in the 10-200 md range. Bottom hole pressures range from 2500 to 2700 psi (17,238-18,617 kPa).

The lower Sligo, or Pettet Limestone Member, has produced from two wells on the crest of the Ruston structure for a total of 146,000 bbl of oil and 1.4 BCFG from 1944 to 1987. The productive zone is a gray, porous, oolitic limestone that contains vertical fractures in cores. The porosity averages 18% and the permeability averages 500 md. The productive limestone is a small oolite bar found at a drilling depth of 5450 ft (1661 m) on the crest of the structure. The base of the oolitic limestone marks the transition into the Hosston sand-shale sequence.

The Causey sandstone of the Pine Island Formation is a fine- to medium-grained sandstone that has produced a small amount of natural gas (no record of the actual amount could be found) from two wells near the crest of the Ruston structure. The porosity ranges from 15% to 18%. Permeability values are not available.

The James sandstone is a light gray to brownish gray, fine- to very fine grained quartz wacke with interstitial clay comprising between 9% and 34% of the rock. Cumulative production from this horizon totaled 19 BCFG from six wells from 1948 to the time it was converted to a gas-storage unit in 1969. The average drilling depth to the formerly productive zone is 4700 ft (1433 m), and it averages 15 ft (4.6 m) thick. The porosities range from 23% to 25% in the producing wells. The original reservoir pressure was 1984 psi (13,680 kPa), and the water level was originally at 4525 ft (1379 m) subsea. The James sandstone gas storage unit is operated by Arkansas Louisiana Gas Company with input and output wells shown in Figure 28.

The Gloyd limestone of the Rodessa Formation has yielded a minor amount of gas (no record of the amount was found) from the Ruston field from one well. Core data are not available, but Trowbridge sample logs indicate the Gloyd is pseudo-oolitic to oolitic, fossiliferous limestone.

Table 2. Gas analysis, Cotton Valley "D" sandstone, Arkansas Louisiana Gas No. 1 V. V. Williamson, Sec. 36, T19N, R3W. (From Wisdom, 1968, Table II.)

SOURCE ROCKS

According to personnel of Arkla Exploration Company there is no geochemical information available for determining the identity of source rocks in the Ruston field. However, it is likely that hydrocarbons in each producing horizon were generated internally, i.e., Smackover gas probably came from Smackover shales, Cotton Valley gas from Cotton Valley shales, etc. It can be speculated that migration into the various traps started when the rocks became heated enough to generate hydrocarbons, which would be about 120 Ma for the Smackover, 110 Ma for the Cotton Valley, and about 50 Ma for the Hosston and Sligo, based on the burial history of the Ruston field (Fig.17).

No information was available on total organic carbon, kerogen type, maturation data (vitrinite reflectance or paleotemperature data from pollen coloration) from any of the Ruston producing horizons.

References:

Bally, A. W., and S. Snelson, 1980, Realms of subsidence: Canadian Society of Petroleum Geologists Memoir 6, p. 9-94.

Berryhill, R. A., W. L. Champion, A. A. Meyerhoff, G. C. Sigler, and others, 1968, Stratigraphy and selected gas-field studies of North Louisiana--stratigraphy, in Natural gases of North America: American

Association of Petroleum Geologists Memoir 9, v. 1, p. 1099-1137.

Breedlove, R. L., J. P. Jones, and A. M. Jackson, 1953, Ruston field, Lincoln Parish, Louisiana, in Shreveport Geological Society Reference Report v. 3, n. 2, p. 88-94.

Burgess, W. J., 1976, Geologic evolution of the Mid-Continent and Gulf Coast areas--a plate tectonics view: Gulf Coast Association of Geological Societies Transactions, v. 26, p. 132-143.

Eversull, L. G., 1985, Depositional systems and distribution of Cotton Valley blanket sandstones in northern Louisiana: Gulf Coast Association of Geological Societies Transactions, v. 35, p. 49-57.

Houseknecht, D. W., ed., 1983, Tectonic-sedimentary evolution of the Arkoma basin and guidebook to deltaic facies, Hartshorne Sandstone: SEPM Midcontinent Section, v. 1.

Judice, P. C., and S. J. Mazzullo, 1982, Gray sandstones (Jurassic) in Terryville field, Louisiana--basinal deposition and exploration model (abs.): American Association of Petroleum Geologists Bulletin, v. 66, p. 1432.

Kamb, H. R., 1945, Ruston gas field, Lincoln Parish, Louisiana: American Association of Petroleum Geologists Bulletin, v. 29, n. 2, p. 226-227.

Klemme, H. D., 1971, What giants and their basins have in common: Oil and Gas Journal v. 69, n. 9, 10, 11: pt. I, p. 85-90; pt. II, p. 103-110; pt. III, p. 96-100.

Louisiana Geological Survey, 1980, Parish atlas of Louisiana oil and gas fields: Louisiana Geological Survey, Folio series 4, 64 p.

Mann, C. J., and W. A. Thomas, 1964, Cotton Valley Group (Jurassic) nomenclature, Louisiana and Arkansas: Gulf Coast Association of Geological Societies Transactions, v. 14, p. 143-152.

Murray, G. E., 1961, Geology of the Atlantic and Gulf Coastal Province of North America: New York, Harper and Bros., 692 p.

Pate, B. F., 1963, Significant North Louisiana Cotton Valley stratigraphic traps: Gulf Coast Association of Geological Societies Transactions, v. 13, p. 177-183.

Shreveport Geological Society, 1946, Ruston field, Lincoln Parish, Louisiana: Shreveport Geological Society Reference Report, v. 1, p. 183-186.

Shreveport Geological Society, 1987, Report on selected oil and gas fields, Ark-La-Tex and Mississippi: Shreveport Geological Society Reference Volume VII, 153 p.

Thomas, W. A., and C. J. Mann, 1966, Late Jurassic depositional environments, Louisiana and Arkansas: American Association of Petroleum Geologists Bulletin, v. 50, p. 178-182.

Walker, J. R., 1953, Exploration history (prior to the Cotton Valley discovery) of the Ruston field, Lincoln Parish, Louisiana: Geophysics, v. 19, n. 1, p. 124-138; reprinted in Geophysical Case Histories, v. 2, p. 341-355, 1954.

Wheeler, C. T., Jr., 1963, Producing horizons of North Louisiana oil and gas fields, in Shreveport Geological Society Reference Report, v. 5, p. 1-8.

Wisdom, C. F., 1968, Ruston field, Lincoln Parish, Louisiana, in Natural gases of North America, American Association of Petroleum Geologists Memoir 9, p. 1138-1142.

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