South Eubank field, Kansas: Deeper development using 3D seismic

Scott L. Montgomery
Petroleum Consultant
Seattle
Ernie Morrison
Consulting Geologist
Derby, Kan.

Giant Hugoton gas field is well known as an area of abundant gas reserves in Pennsylvanian-Permian reservoirs. Yet oil and gas production has long been established in Mississippian-aged horizons as well, which appear to be underdeveloped in many areas of southwestern Kansas and northern Oklahoma. A number of major and independent companies have recently targeted these horizons for increased drilling, and the success of several of these efforts is notable.

In particular, a recent drilling project focused on Chester (Upper Mississippian) sandstones in Haskell County, Kan., has resulted in over 20 new producers and a greatly improved understanding of reservoir quality, geometry, and distribution. Located in South Eubank field, this work was guided by a combination of 3D seismic data and new core information. Drilling success was predicated on the ability of such data to delineate a narrow, sandstone-filled paleo-valley system within which reservoirs are concentrated.

The development of Chester production in South Eubank field is considered significant in that it suggests similar drilling efforts in other fields located along the relevant paleo-valley system have good potential for uncovering new reserves.

Geologic setting

The study area is located within the Hugoton embayment (Fig. 1 [104,153 bytes]), a shallow northern extension of the Anadarko basin. The basin and embayment are predominantly the result of Late Paleozoic subsidence and surrounding uplift associated with the Ouachita collisional event, whose early influence is recorded in a regional unconformity at the base of the Pennsylvanian section.

To the east, the Hugoton embayment is bounded by the Central Kansas uplift, from which all Mississippian strata were eroded. The pattern of subcrop beneath the basal Pennsylvanian (Morrowan) unconformity shows progressively younger Mississippian units to the southwest.¹ The youngest of these units, included within the Chester Group, are confined to the southwestern corner of Kansas (Fig. 2 [57,755 bytes]). Current-day structure in this area consists of NW-SE oriented normal faults that penetrate the Mississippian and gentle closures in overlying Pennsylvanian and Permian strata.²

Upper Mississippian stratigraphy in the study area includes the St. Louis, Ste. Genevieve, and Chester Group intervals (Fig. 3 [36,402 bytes]). Chester strata, consisting of sandstones, siltstones, carbonates, and shales, lie between the overlying Morrowan unconformity and a regional disconformity at the top of Ste. Genevieve carbonates. These strata thicken to the southwest, from a zero edge to as much as 500 ft. In South Eubank field, the Chester interval varies from 100-300 ft thick.

The Ste. Genevieve disconformity has associated with it an incised paleo-valley system trending north-south some 50 miles or more through Haskell and Seward counties, Kan., into Beaver County, Okla. In places, this paleo-valley cuts through the Ste. Genevieve into the top of the St. Louis. Channel fill in the Eubank field area consists of siltstone, shale, minor limestone, and, more abundantly and significantly, very fine- to fine-grained sandstones that comprise important oil reservoirs.

Analysis of the facies, sedimentology, and petrography of these sandstones suggests that they were deposited by estuarine flooding of a pre-existing fluvial system during post-Ste. Genevieve transgression. Chester-age strata exhibit local isopach maxima of up to 400 ft within the paleo-valley system.

Eubank, S. Eubank fields

The total Eubank field complex consists of four producing areas: North Eubank, Eubank, East Eubank, and South Eubank pools, covering a total of about 27 sq miles.

Initial discovery was made in 1958 at the White Eagle 1 Eubank, in 28-28s-34w, which flowed 502 b/d of 37° gravity oil from Lower Chester sandstones at a measured depth of approximately 5,245 ft.³ The well was drilled on the basis of a closure mapped in Permian strata. Earlier development of giant Hugoton gas field resulted in two or three shallow wells per square mile in this area, allowing for good control at the Permian level.

During the next several years, follow-up drilling at Eubank established production from six other intervals, including the Ste. Genevieve, Morrow, Cherokee, Marmaton, and Lansing-Kansas City units. A second important accumulation, located southeast of Eubank field, was opened in 1960: Victory field also produced from Mississippian-Pennsylvanian reservoirs and confirmed the Chester as a primary producing horizon in western Haskell County, Kan.

Continued drilling at Eubank field indicated a complex pattern of reservoir development in the Chester. Good sand development appeared to be restricted to paleo-lows, with the highest quality reservoir sandstones located within an incised paleo-valley system (Fig. 4 [114,516 bytes]). This valley system was identified and outlined on a regional scale from log data by the 1970s⁴ but proved difficult to map precisely due to narrowness and lack of good seismic reflectors at the appropriate stratigraphic levels. In a few cases, where superior well control existed, it was possible to delineate the limits of the valley more accurately (Fig. 4). In the 1980s, Mesa Petroleum conducted an extensive 2D seismic survey across the field. Resulting data were able to image major structures but not the paleo-valley and thus were of little help in evaluating future Chester potential.

Renewed subsurface exploration in the mid-1990s by Hugoton Energy Corp. sought to expand Chester and Morrow production in the South Eubank field area, where drilling had not been as extensive as to the north. This activity led to the drilling of the Clawson 1-9, in 9-29s-34w, in 1994, a significant discovery within the paleo-valley system.

On the basis of several subsequent wells, the company decided to conduct a 3D seismic survey over a 26 sq mile area for the purpose of better defining the incised channel. The shoot was designed with a bin size of 110 x 82.⁵, with the 110-y axis oriented N-S. This design was intended to take advantage of the known structural orientation in this area and to best image the narrow valley.

Most wells drilled during and after the shoot was performed had an acoustic log run in them for tie-back to the 3D data. This allowed for the type of detailed isopach and structure mapping (Fig. 5 [319,323 bytes], Fig. 6 [286,951 bytes], and Fig. 7).

3D seismic data

Fig. 5 is an isochron map derived from the 3D data set, corresponding to the interval between top of Morrow and base of the Chester (Ste. Genevieve disconformity). The data clearly delineate the paleo-valley system, which is less than 1,000 ft wide in many locations and appears remarkably linear in the north-south direction. In addition, the isochron data suggest that significant changes in channel-fill thickness typify the valley system along strike. Several maxima, associated with areas of maximum channel width, are separated by areas of "thinning" where the channel is most narrow.

That these changes may be related to variations in the relative depth of channel incision is suggested by the time-structure map (Fig. 6), which approximately corresponds to the Ste. Genevieve disconformity. Differences of up to 100 ft are observed in this surface, for example, between the low (yellow) in the northeast quarter of Sec. 16 (top center of map) and the shallower channel to the south, in Sec. 21. Comparison between Figs. 5 and 6 also indicates the existence of several side channels that occur at points where the main valley is deepest. These side channels enter the main channel at right angles, suggesting significant relief along the valley wall or structural control, or both.

Also evident on Figs. 5 and 6 is a major NW-SE trending fault (shown in red), downthrown on the west. Displacement along this fault measured at the Ste. Genevieve disconformity varies from 80-150 ft, increasing to the south. The upthrown block, within which the paleo-valley occurs, is tilted to the northeast.

Fig. 5 indicates significant thickening of the combined Morrow-Chester interval across the fault, suggesting it was undergoing displacement during deposition of these units. A map of the overlying interval encompassing the Kansas City to Morrow section (Fig. 7) implies continued, low-level influence of this fault on thickness patterns. However, it is likely that such influence is due to differential compaction, since no significant offset occurs in reflectors above the Morrow.

Drilling results

As a direct result of the interpreted 3D data, Hugoton Energy drilled 20 new producers within the paleo-valley system alone, 14 in succession, with another 13 successful wells beyond the valley limits.

Drilling confirmed that wells within the incised channel have a significantly thickened lower Chester succession, consisting of two or more sandstone packages that do not necessarily correlate between different isopach maxima. The lowermost sands that overlie the Ste. Genevieve disconformity tend to be water productive, while overlying packages are oil-bearing.

Estimated ultimate primary recovery for new producers averages 100,000 bbl based on a drainage area of 40 acres and 15% recovery. Significant waterflood potential (up to 40% total recovery) is believed to exist in the better quality channel-fill sandstones.

Two wells were cored, the Clawson 2-9, in 9-29s-34w, and Black 4-3, in 3-30s-34w, located, respectively, in the northern and southern portions of the valley system at South Eubank field. Description and analysis of recovered samples indicated massive, wavy-bedded, cross-laminated, and shale-laminated sandstones with porosities of 8-13.7% and permeabilities of 0.39-141 md.

Interpreted facies included tidal flat, estuarine channel, and sand wave/ tidal bar sands, with reservoir quality at a maximum in the higher energy channel and sand wave facies.5 These sands are mostly fine-grained, well-sorted quartz arenites and sublitharenites with moderate amounts of quartz overgrowth and calcite cements and some evidence of fracturing.

The 3D seismic and core data were also used to locate prospects in areas adjacent to the paleo-valley. These areas included structurally low areas, e.g., between the valley system and the mentioned fault, where estuarine flooding and sand-depositing tidal flat deposition were presumably concentrated. Roughly half of the wells drilled in these locations were successful. The Chester sandstone interval is considerably thinner and the sands are more clay-rich, but wells are still capable of economical production.

Conclusion

The recent drilling program by Hugoton Energy significantly expanded the limits of South Eubank field by adding some 30 new producers with combined reserves of over 2.1 million bbl. Success was based on a combination of 3D seismic data interpretation, which accurately identified a paleo-valley system, and subsurface mapping based on existing and new core information.

Experience at South Eubank might be used as a guide for other development projects in western Kansas and northern Oklahoma. This would especially apply to producing areas located along the relevant paleo-valley system, such as Cutter, Silverman, and Archer fields.

References

Goebel, E.D., Mississippian rocks of western Kansas, AAPG Bull., Vol. 52, No. 9, 1968, pp. 1,732-78.
Rascoe, B., Jr., and Adler, F.J., Permo-Carboniferous hydrocarbon accumulations, Mid-Continent U.S., AAPG Bull., Vol. 67, 1983, pp. 979-1,001.
Fugitt, L.B., and Wilkinson, R.D., Eubank field: Kansas Oil and Gas Fields, Vol. II: Western Kansas; Kansas Geological Survey, 1959, pp. 13-20.
Severy, C.L., Subsurface stratigraphy of the Chesteran Series, Southwest Kansas, unpub. MS thesis, University of Colorado, Boulder, 1975, 61 p.
Mongtomery, S.L., and Morrison, E., South Eubank field, Haskell County, Kan.: A case of field re-development using subsurface mapping and 3D seismic data, AAPG Bull., in press.

The Authors

Scott L. Montgomery is a Seattle petroleum consultant and author. He is lead author of the "E&P Notes" series in the AAPG Bulletin and the quarterly monograph series "Petroleum Frontiers" published by Petroleum Information/Dwights LLC. His current research interests include frontier plays and field re-development in North America. He holds a BA degree in English from Knox College and an MS degree in geological sciences from Cornell University. E-mail: [email protected]
Ernie Morrison worked for Exploration Logging in 1975-76 as a mud logger and later was with Coastal Corp., Hugoton Energy Corp., and Chesapeake Energy Corp. He is primarily an exploration geologist with expertise in western Kansas and has taught geology at the community college level. He holds BS and MS degrees in geology from West Texas State University.