EL DORADO: AN OLD FIELD WITH POTENTIAL

Paul J. Ramondetta
Oxy USA Inc.
Oklahoma City

Premature abandonment of productive acreage because of mechanical failure of early wells is the primary justification for redrilling a field such as El Dorado. But reservoir heterogeneities are pervasive and must be understood before attempting to recover the potential remaining reserves in such intervals as the Admire, Viola, Simpson, and Arbuckle.

OIL FIELDS

"The best place to look for oil is in an oil field." This semihumorous truism is used by those who promote redrilling of old fields as an inexpensive but effective way to increase reserves.

This growing trend has been studied and encouraged by many in the oil industry.

The foremost justification is geological heterogeneity. The El Dorado field in south central Kansas is a complex example of how this philosophy may or may not be applied.

El Dorado is a sprawling, multipay, multidome field covering 40 sq miles, located atop the Nemaha ridge in central Butler County (Figs. 1, 2, and 3). See Table 1 for stratigraphy and approximate depths to pay intervals.

Up to 15 horizons have been reported to contain hydrocarbons. El Dorado is producing 2,200 bo/d and cumulative production amounts to nearly 300 million bbl of oil since its discovery in 1915 by Cities Service Oil Co.

Developmental drilling peaked between the years 1916 and 1918 at a maximum of 116,000 bo/d in 1918.1 The field was discovered through the mapping of surface beds.

REMAINING POTENTIAL

The axis of the El Dorado structure strikes N20E (Figs. 1 and 2), parallel to the Nemaha ridge. The field is fault-bounded on the east. Structural deformation during late Devonian, and again in Mississippian, severely affected Pre-Pennsylvanian strata; they were exposed and subjected to extreme weathering and erosion. The next and last major period of deformation occurred during the Permian.

The field consists of seven fault-bounded domes of varying size. The Shumway Dome is the largest, roughly 20 sq miles. The Shumway dome is actually two fault blocks of nearly equal size, the East and West Shumway domes. Within each dome the beds strike roughly N45W, except for the West Shumway dome where they strike N20E.

BASAL PERMIAN

The Admire sand (650-ft sand) is a basal Permian stratigraphic unit. It is extremely heterogeneous, but development was essentially geometric without regard to facies relationships. Many wells were prematurely abandoned. Therefore, the Admire offers good potential for redevelopment.

Jordon and Tillman2 determined that the Admire distributary channel sands have optimum reservoir characteristics compared to other facies. The sands are fine grained, well-rounded quartz sands with calcareous cement. Beds are 14-20 ft thick with 28% porosity and over 400 md permeability.

These channels occupy roughly 1,300 acres and are oriented NE-SW, parallel to the Nemaha ridge, but cut across structure from one dome to the other.

Best development is on the crests and flanks of Chesney and Oil Hill domes. Structure has had little effect on production.

Admire oil was first noted in 1915 from the Stapleton No. 1, the original discovery well. Development proceeded rapidly in 1916, on a 2-7 acre spacing grid. Completions were open hole as with all other horizons.

Initial production varied from 40 to 80 bo/d. By 1918, average production was 20 bo/d. Initial production on the margins, away from the distributary channels, was 5-10 bo/d.

By the 1920s, the openhole intervals were cleaned out and shot with explosives. After the initial drawdown of reservoir pressure, fracture treatment was necessary to maintain profitable production. Also in the 1920s, an air-injection project was initiated. The air-drive flood may have been one of the first secondary projects ever installed.

Waterflooding started in the late 1940s and peaked in the late 1950s and early 1960s. In 1963, the Admire alone was producing over 2,300 bo/d from 375 wells or over 6 bo/d well. This 1963 production is more than the current field production of 2,200 bo/d from all horizons.

Individual well cumulatives have been in excess of 100,000 bbl of oil.

In the mid-1960s the decision was made to terminate the Admire flood and abandon nearly all producers. The following were the likely factors behind that decision:

• Many of the 375 wells were marginal and should have been abandoned, but those clustered along the distributary channels were commercial.

• The large number of marginal wells flanking the more productive fairways may have given the false impression of a nearly depleted field with limited remaining life.

• The concept of a rapidly depleting resource was reinforced by the poor market conditions of the 1960s. Low oil prices then as today cause the industry to focus on short-term economics rather than the management of long-term resources.

Only two Admire wells escaped abandonment, and these continue to produce without the benefit of nearby injection. One declined from a high of 41 bo/d in 1963 to 12 bo/d by 1970. From the end of 1969 to present, the two wells have accumulated nearly 50,000 bbl of oil.

At the shallow depth of 700 ft, they continue to have positive earnings, even at only 2 bo/d apiece. The important point, however, is that recoverable oil was left behind over much of the Admire delta complex.

In 1974, a small 51-acre pilot project was initiated to determine if more oil could be extracted from the Admire through the use of chemical agents. Unfortunately, the results were inconclusive, but the unit has accumulated over 140,000 bbl of oil and continues to operate profitably.

Eventually, with the right flood pattern conforming to geological conditions, with the right chemical mix, and the right oil price, the remaining oil within the 1,300 acres of distributary channel fairways can be extracted. Even at today's prices, new drilling followed by conventional waterflooding would be economical.

VIRGILIAN STAGE

Upper Pennsylvanian rocks are characterized by cyclothems. The cycles include marine shales and limestones alternating with nonmarine beds. Several small pay zones with limited areal extent exist in the Virgilian stage of the upper Pennsylvanian series at depths ranging from 800 to 1,500 ft.

These zones include the Burlingame limestone, White Cloud sand, and Douglas sand. Limestone units are relatively thin in the Virgilian as shales, siltstones, and fine-grained sandstones dominate.

As in the Admire, these zones have been waterflooded. With the right combination of oil prices and enhanced oil recovery (EOR) technology, these zones may make attractive targets in the future. Their distribution, however, is much more limited than the Admire and probably will never stimulate great interest.

MISSOURIAN STAGE

The Lansing and Kansas City of the Missourian stage of the upper Pennsylvanian are massive carbonate units that have been prolific producers of oil on the crests of the various domes. Primary development began in 1917, and waterflooding has continued since the late 1940s. High structural position is critical toward achieving commercial production.

The main porosity zones are relatively homogeneous, consisting of medium to coarsely crystalline fossiliferous and oolitic limestones with abundant grainstone and excellent intercrystalline and vuggy porosity. Most of the wells drilled in the 1950s and 1960s have accumulated between 30,000 and 60,000 bbl of oil.

Potential exists for reworking existing wells. Success has been achieved by running cased-hole, thermal-decay logs on old wells to identify additional zones of interest. The high chloride content of the formation waters is especially suitable for such logs.

The various Pennsylvanian cyclothems and the resulting stratification yield opportunities for new zone development within the Lansing and Kansas City. The abundance of active and inactive wells throughout the area offer ample opportunity for workovers, and little need for new drilling.

HEPLER SAND

The Hepler sand, also of the Missourian stage of the upper Pennsylvanian, is a very fine-grained green, brown, and gray dirty sandstone; its distribution is sheetlike with variable thickness and reservoir quality. It has been productive on the Chesney dome where it was waterflooded. It is present elsewhere in El Dorado but is either tight or wet.

A technological breakthrough in EOR is needed to improve the outlook for this limited pay zone. Below the Hepler sand, no significant potential exists in the Pennsylvanian.

PRE-PENNSYLVANIAN UNCONFORMITY

In Kansas the pre-Pennsylvanian unconformity is a regional surface of great significance. In El Dorado it separates rocks of the Cherokee group (middle Pennsylvanian) from much older strata ranging in age from lower Mississippian to upper Cambrian.

Aside from the obvious effect of missing strata, the unconformity is important for several reasons:

• It was and perhaps still is a major conduit for fluid migration.

• It separates two distinctly different hydraulic regimes.

• The long periods of subaerial exposure helped enhance porosity and permeability in the underlying carbonates.

• The overlying Cherokee shales provide an effective seal for the underlying reservoirs.

Pre-Pennsylvanian deposits are much more deformed than the overlying Permo-Pennsylvanian strata. Structural deformation is relatively uniform from surface beds to the pre-Pennsylvanian unconformity.

In El Dorado, Cherokee shales progressively overlap lower Mississippian carbonates, Viola, Simpson, and Arbuckle. On the crest of the West Shumway dome, the highest area in the field, the Cherokee overlaps the deeply eroded lower Arbuckle. The Precambrian granite basement is less than 200 ft below the unconformity.

LOWER MISSISSIPPIAN

A large belt of Mississippian oil production exists downdip of the northern flanks of the Shumway and Oil Hill domes. Fluid production tends to be low volume but steady. This report will not dwell on this production because it is more related to the Pierce field, north and west of El Dorado.

Rocks of Ordovician age subcrop at the pre-Pennsylvanian unconformity over most of the field. The majority of oil production has been from the Ordovician.

VIOLA

The Viola dolomite is the youngest unit of Ordovician age to subcrop at the unconformity, usually at a depth of 2,500 ft. Under optimum conditions, roughly 50 ft of Viola is preserved with 20 ft of porous sucrosic dolomite on top underlain by 30 ft of tighter more argillaceous carbonates.

Porosity of the upper pay zone can be as high as 24%. Differential erosion on the unconformity, however, causes fluctuations in Viola thickness and reservoir quality.

The Viola, as other Ordovician reservoirs in El Dorado, is driven by an extremely powerful water drive which maintains reservoir pressure above 700 psi. Production is primary and structural position critical.

Viola oil production occurs on the flanks of the El Dorado domes forming a loose apron around them, and also at localized highs where Viola porosity is breached by the pre-Pennsylvanian unconformity.

Recent drilling indicates significant opportunity for extending Viola production along the flanks of the domal highs. Reservoir analyses from modern open hole logs indicate a high degree of heterogeneity.

Localized disruptions in the reservoir have been responsible for reduced production in some wells. These disruptions are mostly related to the effects of erosion on the Viola unconformity. Oil-water contacts do vary because the reservoir is compartmentalized. In some areas the distribution of active wells is haphazard, more related to mechanical survival than reservoir potential.

SIMPSON SAND

The Simpson sand (Saint Peter), also Ordovician, is up to 70 ft thick but its thickness varies considerably. Like the Viola, it subcrops at the pre-Pennsylvanian unconformity at roughly 2,500 ft.

The Simpson is also productive along the flanks of the domes, and it has variable reservoir quality.

Individual beds of this pure quartz sand have maximum thicknesses of 20 ft. It is mature, well sorted, clean, medium-to-coarse grained, friable, well rounded, and frosted.

The sand is interbedded with thin beds of discontinuous green shale. A widespread prominent "red rock" marker bed separates the more productive upper Simpson from the lower Simpson.

Both Viola and Simpson have been mapped as sheetlike deposits. However, the recent use of open hole logs on new wells indicates heterogeneity. The porosity zones vary in thickness and quality.

Sands in the Simpson, once thought to be continuous, are actually separate. Fairways of high permeability sand flanked by lower quality sand are apparent from production data; oil-water contacts vary.

The Simpson forms a band of production around the various domes up to 2 miles wide. The current distribution of active wells is haphazard. As in the Viola it is more related to mechanical survival than geological reality.

ARBUCKLE GROUP

The Arbuckle group in the Cambrian occurs between a depth of 2,300 and 2,500 ft on the domes of El Dorado. Thickness varies from under 200 ft on the crest of the West Shumway dome to nearly 800 ft in low areas.

Dolomite is dominant. Chert, sandstone, limestone, and thin shale stringers are also present. Sedimentation is cyclic. Beds are upward shoaling and generally 10-20 ft thick.

Porosity is best developed in the coarser upper portions of the cycle. Sucrosic dolomudstones form the base of each cycle and grade upward to fossiliferous and oolitic grainstones up to 6 ft thick. Porosity in the grainstones is both vuggy and coarsely intercrystalline (15-20%) with over 100 md permeability.

Finely intercrystalline porosity is occasionally developed in the sucrosic dolomite, yielding relatively low-to-moderate permeabilities; but more often the dolomudstones are tight. Hence, much of the Arbuckle is stratified, and to a certain extent oil production is controlled by this cyclicity.

Massive dolomite is common in the lower Arbuckle and is more fractured and less influenced by stratification. Fracturing cuts across stratification and lessens its effects.

Excellent potential exists for redrilling large areas of old Arbuckle production, especially on the West Shumway dome. The following discussion applies directly to the West Shumway dome but can be applied elsewhere in El Dorado with some modification to wherever the Viola, Simpson, or Arbuckle have been productive.

The great majority of Arbuckle wells on the West Shumway dome were drilled between 1917 and 1920 on very dense 4.4-acre spacing. Completions were all open hole with uncemented casing. Penetrations varied from 10 to 65 ft. Initial potentials were very high, ranging up to 20,000 bo/d. Oil production plunged and water production soared by the early 1920s.

Large open hole intervals invited massive invasions of "bottom water." The lack of cemented casing likewise caused massive invasions of "top water."

In most cases, bottom water coned up from below rather than laterally across the reservoir, this was aggravated by the high rates at which the wells were pumped. Coning up of bottom water caused the wells to water out.

Pinnacles of coned up water existed around each well bore, and oil slowly fed the upward-moving fluid. As wells were abandoned the cones subsided allowing the oil and water to return to a layered state.

Production was further reduced by barium sulfate scale caused by the mixing of bottom and top waters. Stimulation techniques such as acidization did not exist at the time. Hapless operators saw no alternative but to plug out existing wells on a grand scale. By the 1930s, most wells had been abandoned.

Those wells which survived the plugging onslaught of the 1920s received a new lease on life in the mid-1930s when acid treatments were initiated. Acidization greatly increased production and extended the life of those wells. The acid cleaned out the open holes and stimulated the reservoir. However, this was never followed up by a comprehensive infill program to replace the wells abandoned earlier.

Today a haphazard distribution of active wells remains. A new production base is being designed to systematically extract the remaining reserves from the vast, sometimes stratified, body of fluid.

The extremely high original IP's on the West Shumway dome resulted from large cavernous porosity due to deep karstification on the Arbuckle unconformity. This area had been referred to as the area of "broken lime" or simply the "cavity area" by early drillers.

Serious lost circulation problems have plagued some recent wells. A map of original IP's reflects permeability trends, and reveals a pattern similar to modern caves where subsurface drainage and dissolution follows fracture systems.

The wells with high IP's are within fractures that have been enlarged through dissolution. Also, highly permeable collapse breccias developed in response to cavern growth. Hence, dolomite intervals with no matrix porosity can nonetheless yield large amounts of fluid.

Individual well performance can be outstanding. The Shumway No. 5 for example flowed 2.5 million bbl of oil from September 1917 to April 1918 3 and then continued to produce by pump until 1957.

The nearby Shumway No. 1 is still producing 20 bo/d and 900 bw/d with a very high fluid level.

Some of the new offsets initially produce large volumes of water-free oil. These high fluid trends strike in a rough north-south and east-west direction on the West Shumway dome, presumably parallel to the fracture system.

Kerans4 wrote about the effects of karstification on oil production from the Ellenburger, which is the stratigraphic equivalent to the Arbuckle in Texas. Although important differences exist between the two areas, his analysis does explain the compartmentalization of producing zones and the resulting heterogeneity in production.

Without a paleokarst model, production trends in El Dorado would be very difficult to explain.

Significant remaining reserves in the Arbuckle of West Shumway dome have been confirmed through recent drilling. New wells are being drilled amid the 4.4 acre array of old first-generation wells.

Electric logs indicate up to 30 ft of pay just below the unconformity, the same interval that was so densely developed in 1917. The better wells are expected to make over 50,000 bbl of oil. Good completions with sound cement jobs and modern logs are critical to achieving profitable production.

Arbuckle stratification is more significant on the other domes. First-generation wells were spaced 10 acres apart and typically had IP's of only 50-100 bo/d, despite open hole penetrations of over 50 ft.

This is in sharp contrast to the West Shumway dome where a much older Arbuckle section subcrops at the unconformity. Stratification opens up opportunities for production from thin individual low-to-moderate volume pay zones. Open 10 acre locations still exist, so the sparser drilling offers additional opportunity.

Arbuckle stratification has been responsible for increased production in other fields in Kansas such as Bemis in Ellis County and Augusta. Mullins and Ireland5 described increased potential in the Edwards field by redrilling an area where only openhole completions had existed before.

The new cased-hole wells allow operators to selectively perforate individual stratified porosity zones in the Arbuckle. The best possible cement preceded by good flushing to wash away mudcake is critical in these kinds of completions.

In cases where all pay zones are low volume with no intervening water zones, openhole completions may still be desirable. This avoids penetrating deep water horizons, thereby minimizing the risk of communication with the unwanted water through poor cement or fractures.

Unfortunately, some areas have been falsely condemned because of poor completions. Due to the large number of old oil fields in Kansas similar to El Dorado, there is substantial potential for applying these ideas statewide.

REDRILLING EL DORADO

In El Dorado, redrilling portions of the field offers significant potential for the Admire, Viola, Simpson, and Arbuckle pay zones. Geological heterogeneity has been offered as the prime justification for redrilling. Drilling density, however, in most parts of the field is sufficiently high to capture whatever heterogeneity exists. Any pod of oil that was left uptapped by the 2-10 acre spacing is too small to worry about.

Therefore, the prime justification for redrilling is premature abandonment of early wells due to mechanical and other reasons previously mentioned. Geological heterogeneity is not an important factor in justifying the redrilling, but it is vital toward planning and implementing any new drilling program.

Ultimate reserves are difficult to predict. All attempts to quantify remaining reserves in El Dorado have been futile. Volumetric analyses and decline curves have been consistently pessimistic.

Empirical approaches are better. Simply from observing the performance of recently drilled wells, a range of possibilities is obtained.

Future wells can be assumed to behave similarly.

Quantitative analysis is impeded by:

• Missing production data

• Lack of modern logs that allow reservoir analysis

• Complex geology complicated by lack of data

• A very powerful water drive

• An uneven fracture system that cuts across stratigraphy and affects the distribution of oil.

It is possible that the dynamic hydrology of the Ordovician section is actually causing migration of new oil into the old reservoirs. This explains the large cumulatives and great longevity of the field.

New wells generally experience sharp declines in their first year as water encroaches. Extrapolation of this decline to determine ultimate reserves is almost always wrong because most wells will level out, many at 5-10 bo/d and then produce at that level indefinitely just as the old wells.

There is a tendency to saddle these wells with lifespans that fit human perceptions, usually 5-10 years even though there are 73 year old wells still producing with flat declines.

The gradual depletion of a large field is not a continuous decline to zero, there is instead a series of steps and plateaus.

If productive acreage was prematurely abandoned, a given field could be poised for a significant rebound when redrilling occurs.

For broad decline curves to be valid one must assume that all that could have been done to increase production has been done. That assumption is probably never true for large complex fields like El Dorado.

There is a general lack of strategy in dealing with such fields. Most old fields have many operators, making a unified effort difficult.

The piecemeal sell-off of leases within large fields is usually the result of limited economic analysis centered on individual leases rather than the whole field.

By focusing on small pieces rather than the larger picture, strategies become fragmented, short lived, and dispensable.

Economics are oil-price sensitive. Most new 2,500-ft wells should have payout periods ranging from less than 1 year to 3 years, assuming $15-20/bbl oil prices. Such wells should cost less than $90,000 to drill and equip.

Initial rates of production from new wells can range from 5 bo/d to over 100 bo/d. Large infrastructure allows some operators to produce profitably at rates as low as 2 bo/d.

REFERENCES

1. Reeves, J.R., "El Dorado Oil Field, Butler County, Kansas," in Structure of Typical American Oil Fields 1, American Association of Petroleum Geologists, 1929, pp. 160-167.

2. Jordan, D.W., and Tillman, R.W., "Reservoir geology of the Admire 650 ft sandstone, micellar-polymer pilot area, Butler County, Kansas," Cities Service Co., G 81-25, Technical Report #93, 1981.

3. Fath, A.E., "Geology of the Eldorado Oil and Gas Field," State Geological Survey of Kansas, Bulletin 7, 1921.

4. Kerans, C., "Karst-controlled reservoir heterogeneity in Ellenburger Group carbonates of West Texas," American Association of Petroleum Geologists Bulletin, V. 72, No. 10, p. 1160-1183, 1988.

5. Mullins, C.A., and Ireland, W.C., "Additional Arbuckle reserves developed below original completion depth in stratified reservoir," Society of Petroleum Engineers of AIME, Paper No. SPE 1963, 1967.

   Copyright 1990 Oil & Gas Journal. All Rights Reserved.