KEY MORROW PREDICTIVE EXPLORATION MODEL: SOUTHWEST STOCKHOLM FIELD

Sept. 10, 1990
C. Brent Shumard Consulting Geologist Tulsa Loren E. Avis Plains Petroleum Operating Co. Denver The 1979 discovery of Southwest Stockholm field is directly attributable to the reevaluation of an untested and underpressured (1,038 psi at 5,130 ft) lower Morrow sandstone (Lower Pennsylvanian) in a well that appeared to be a dry hole drilled and abandoned 10 years earlier.
C. Brent Shumard
Consulting Geologist
Tulsa
Loren E. Avis
Plains Petroleum Operating Co.
Denver

The 1979 discovery of Southwest Stockholm field is directly attributable to the reevaluation of an untested and underpressured (1,038 psi at 5,130 ft) lower Morrow sandstone (Lower Pennsylvanian) in a well that appeared to be a dry hole drilled and abandoned 10 years earlier.

The Southwest Stockholm field provides an important predictive exploration model for Lower Pennsylvanian Morrow fluvial valley fill sandstone reservoirs in the Hugoton embayment/Las Animas arch area of the western Midcontinent.

The model clearly shows that major surface drainage was significantly influenced by structurally related paleotopography that controlled local fluvial sandstone geometry during Morrowan time.

Successful exploration for productive Morrow sandstones includes a familiarity with abnormally low pressured porous and permeable siliciclastic reservoirs. Knowledge of underpressured reservoirs is critical, because they can often obscure the productive hydrocarbon potential during drilling of a well.

FIELD SETTING

Presently, the field has nearly 90 wells and has produced 5.5 million bbl of 40 gravity oil. Original oil in place is estimated at 27 million bbl of oil.

Southwest Stockholm field is in Greeley and Wallace counties, Kan., on the southeast flank of the Las Animas arch in the northwestern part of the Hugoton embayment (Fig. 1). The Hugoton embayment is the northwest extension of the Anadarko basin in central Oklahoma that extends into the panhandles of Texas and Oklahoma, western Kansas, and southeastern Colorado. The regionally prominent Las Animas arch separates the Hugoton embayment from the Denver basin.

Southwest Stockholm field is one of several Morrow oil fields producing from a primarily fluvial sandstone trend along the central Kansas-Colorado state line. The field covers parts of 13 sections beginning at the southern end in 8-16s-41w, Cheyenne County, Colo., and extends northward to 24-15s-43w at the Kansas-Colorado state line in Wallace County, Kan. Present study of the field covers primarily the southern part, south of the north lines of 32- and 33-15s-42w.

A generalized stratigraphic column for the Hugoton embayment is applicable to the Las Animas arch area of southeastern Colorado (Fig. 2). The estimated total thickness of sedimentary rocks in the field area is about 6,200 ft. The type log for the field shows the detailed Lower Pennsylvanian stratigraphy of the Morrowan Series that rests unconformably on Upper Mississippian Ste. Genevieve limestone and is overlain disconformably by Middle Pennsylvanian Atokan strata (Fig. 3).

PREVIOUS WRITING

Avis and Boothby (1982) give a succinct review of Morrow discoveries in southeastern Colorado from 1977 to 1981, including Southwest Stockholm field. Krystinik et al. (1987) and Krystinik (1989a and 1989b) reported on a spectrum of studies that

summarized work on Morrowan valley fill reservoirs along the central stateline area.

Weimer et al. (1988) described the exploration potential of Morrowan valley fill reservoirs in southeast Colorado.

Nontechnical, informative articles dealing with the various field discoveries in the Morrow play along the central stateline area include those by Keener (1987), Knowles (1987), McCaslin (1988), Stremel (1988), Petzet (1989), Brown (1989), and High (1990).

Another paper (Shumard, in press) gives a detailed description of Southwest Stockholm field, portions of which are summarized in this paper.

FIELD DISCOVERY, DEVELOPMENT

The first well drilled in the Southwest Stockholm field area was C.R. Associates 1 Evans in April 1969 in NE NE SE 11-16s-43w, Greeley County, Kan. (Fig. 4). The wildcat was drilled in an attempt to establish oil production from Mississippian carbonates on the southeast flank of the Las Animas arch. There was no geological consideration prior to drilling the 1 Evans that Morrow sandstones would be a potential reservoir objective.

During drilling, the well penetrated three Morrow sandstones with no shows of oil or gas. These reservoirs are underpressured-relative to normal hydrostatic pressure for drill depths to the Morrow in the Hugoton embayment. It is not uncommon for these reservoirs to mask hydrocarbon shows during drilling, particularly if the sandstones have high porosity and permeability. Wire line log evaluation at the well site indicated higher than acceptable water saturations and that the well would be nonproductive of hydrocarbons.

The suite of logs run on 1 Evans consisted of gamma-ray, neutron, sonic, caliper, and resistivity curves. It is speculated that the undetected gas in the Stockholm sandstone reservoir negatively affected the neutron porosity curve, and calculations indicated an apparently high salt water saturation. Drillstem tests of the sandstones to conclusively condemn the well were not run.

During the late 1970s Texas Oil & Gas Corp. initiated an exploration program to develop low BTU gas reserves in Southeast Colorado. This effort was responsible for several Morrow oil and gas discoveries in the Las Animas arch area (Avis and Boothby, 1982). One of these discoveries was Southwest Stockholm field in March 1979. Following a reevaluation of the logs on 1 Evans, Texas Oil & Gas drilled an offset twin to the abandoned well.

The Texas Oil & Gas 1-E Evans in NE NE SE 11-16s-43w was successful and became the discovery well for Southwest Stockholm field. The well was completed for 185 b/d of oil and 1,100 Mcfd of gas from lower Morrow sandstone, informally named Stockholm sandstone.

The greatest increases in production occurred in 1985 and 1986, when development was in progress near the central area of the field. During this period of development Remuda Corp. discovered a second pay, the middle Morrow sandstone informally named Johannes sandstone, in 1985 while drilling the 1 Cheek in SE SW SW 6-16s-42w. The 1 Cheek was completed for 275 b/d of oil and 100 Mcfd of gas.

Secondary oil recovery commenced in December 1987 with limited waterflooding in 11- and 12-16s-43w at the southern end of the field. Primary and secondary recovery is estimated at more than 11 million bbl of oil, or about 40% of OOIP. Initial production rates (pumping) have ranged from nine to 369 b/d of oil with an average of nearly 177 b/d of oil.

Gas production is secondary to oil in economic importance. Produced formation water is negligible on a fieldwide basis and is not considered an economic or production problem.

Pressure and production data from the field characterize it as a high permeability, porous, solution gas drive reservoir.

FIELD, RESERVOIR STRATIGRAPHY

With reference to the type log (Fig. 3) the stratigraphy of the Morrowan Series in Southwest Stockholm field is summarized. The locations of two log cross sections displaying the fieldwide stratigraphic relationships of the depositional units are shown (Fig. 5).

The earliest Morrowan deposits in the area are basal transgressive marine clastics sometimes referred to as the Keyes sandstone. This unconformity sandstone is white to light gray, fine-grained, subangular, submicaceous, glauconitic, and occasionally shaly. Grain size may increase near the base.

The basal Morrow sandstone has an erratic distribution in the field where it occupies paleotopographic and drainage lows on the Mississippian unconformity (Fig. 6). The basal Morrow sandstone attains a maximum thickness of 44 ft and may have been deposited on a karst modified terrain on the subaerially exposed Mississippian surface. This unit is not productive in the field, and hydrocarbon shows have not been reported.

The lower Morrow limestone unconformably overlies the Ste. Genevieve limestone, except where the basal Morrow sandstone is present (Fig. 3). This burrowmottled wackestone/packstone facies is light gray to tan, slightly arenaceous, fossiliferous, chalky, and glauconitic. Although this sequence is predominately carbonate it generally contains relatively thin beds of gray to black marine shale.

The isopach map of the lower Morrow limestone is shown in Fig. 7. The top of the isopach lower Morrow limestone interval is a significant intraformational unconformity created during a relative fall in sea level during a major regression of the Morrowan sea. The unit ranges in thickness from absent to more than 100 ft and is thinnest where downcutting prior to valley fill deposition is greatest. In the central part of the field the lower Morrow limestone is not present where it has been removed by valley erosion (zero isopach values in Fig. 7). The most striking aspect of the lower Morrow limestone isopach interpretation is that it represents a paleotopographic concept revealing an incised valley and the related paleodrainage patterns.

The primary reservoir in the field is the Stockholm sandstone. This facies is a white to gray, fine, medium to very coarse grained, subangular to subrounded, frosted, friable, and commonly glauconitic quartzose sandstone. The isopach of the net pay with 10% or greater porosity is shown in Fig. 8 and contained within the narrow incised paleo-valley shown in Fig. 7.

Cross section A-A' clearly shows that the Stockholm sandstone is juxtaposed against the impermeable valley walls formed by the Lower Morrow limestone in the deeply incised area near the central part of the field. The isopach map represents a composite of all depositional facies within the unit. The sandstone valley fill deposits range in width from .25 m to .75 m and are continuous along the valley axis (Figs. 6, 8).

Average width to thickness ratio of the valley fill is about 100. Net pay thickness for the field averages 22 ft with a maximum of 64 ft. Maximum gross thickness of the Stockholm sandstone is 68 ft near the central part of the field and averages 25 ft for the entire field.

Intergranular porosities of the Stockholm sandstone are as high as 24% and average 17%. Permeabilities from conventional core analyses average 400 md and range from 50 md to 1,890 md.

The second productive reservoir in the field is the Johannes sandstone (Fig. 3). This middle Morrow predominantly clastic sequence varies in rock type and related reservoir quality.

The variation in lithology ranges from a shaly limestone and calcareous siltstone to reservoir quality gray to white, fine, medium to coarse-grained, submicaceous, calcareous and glauconitic quartzose sandstone, The gross isopach map (Fig. 10) shows a more limited extent when compared to the Stockholm sandstone.

The inset of Fig. 10 is an isopach of the net pay with 10% or greater porosity. Maximum gross and net pay thickness is 40 ft with an average net pay of 15 ft. Log derived porosities average 13% with a maximum of 22%. The thickest and productive facies has a north-south alignment situated over the west bank of the paleo-valley (Fig. 10).

Generally, the Johannes sandstone is separated from the underlying Stockholm sandstone by a gray to black shale (Fig. 3).

Cross section A-A' (Fig. 9) across the central part of the field indicates that both sandstones may be in reservoir communication. The subtle boundary between the Stockholm and Johannes sandstones in the Remuda Corp. 3 Cheek, N 1/2 SW SW 6 16s-42w, is interpreted as an erosional break (diastem) rather than uninterrupted deposition.

A dark gray to black, slightly arenaceous shale interval separates the Stockholm and Johannes sandstones from the overlying Atokan Series (Fig. 3). This shale interval averages nearly 100 ft in thickness and occasionally contains upper Morrow sandstones which have poor or limited development and are not known to have had hydrocarbon shows.

SEISMIC STRATIGRAPHY

Fig. 11 shows part of an industry speculative seismic line that runs west-east and is centered immediately north of the Trans-Western 4 Bergquist, NW NW NE 32-15s-42w, Wallace County, Kan. This section is 30 fold Vibroseis with a 10-110 Hz nonlinear sweep.

The seismic line cuts the valley nearly perpendicular to the axis.

The principal anomaly (Fig. 11) is related to the thickening of the valley fill sequence and occurs as a doublet at the lower Morrow limestone interval. The lower Morrow limestone is identified as a single peak on the west end of the section which splits into a doublet on either side of the 4 Bergquist and returns to a single peak on the east end of the section.

If the lower peak of the doublet is followed the concave shape represents the incised valley. The Stockholm sandstone is not apparent on the seismic section.

A second anomaly, which is not definitive of valley incisement, is the amplitude anomaly between the Marmaton reflector and the lower Morrow limestone (Fig. 11). This anomaly appears as a peak that has an amplitude that is about one third the amplitude of the lower Morrow limestone peak.

Valley incisement is very distinct on this seismic section. Other seismic lines across the field may not be as definitive because of depth and cross-sectional symmetry of the incisement, angle that the seismic line cuts the valley, changes in lithology and surface velocity problems above the valley system.

DEPOSITIONAL ENVIRONMENTS

A 43 ft core from the Texas Oil & Gas Corp. 2-E Evans, NW SE 11-16s-43w, near the south end of the field (Fig. 5) was described and interpreted (Fig. 12).

The cored interval (corrected to log depth) 5,154-97 ft covers five major depositional facies units within the Morrow.

The lower Morrow limestone, Units One through Three, is interpreted as shallow open marine shelf deposits. The genetically related carbonate and shale units represent elements of cyclic marine deposition in a broadly fluctuating early Morrowan (earliest Pennsylvanian) transgressive-regressive sequence. The upper contact of Unit Three is truncated (Fig. 12) and represents a sequence boundary within the Morrowan Series.

The Stockholm sandstone, Unit Four, is interpreted as a multistory fluvial valley fill sequence consisting of at least five major vertically stacked depositional events. Valley filling was by a braided, nonavulsive fluvial system characterized by flashy or rapid and extreme discharge fluctuations.

Unit Four contains thick (up to 39.6 in.) sets of high (up to 27) and low angle coarse to very coarse grained planar and trough crossbedded sandstone. Separating these sets are black shale lenses, ripple laminated black micaceous shale, and fine to very fine grained, well sorted sandstone. The coarser grained crossbeds are commonly composed of thin (4-6 cm), stacked, upward fining textural sets.

Overall, coarse to very coarse grained sandstone, the absence of a well developed fining upward texture, and the low volume of mud and silt is characteristic of Unit Four. The rippled shale laminae, fine grained sandstone and black shale lenses (clay drape or swale fill) variously contain pyritized horizontal burrows, load structures, pyrite nodules, and occasional carbonaceous plant debris.

Light brown oil staining is recognized throughout most of Unit Four. Areas that are unstained include the zones containing the finer grained sandstone and rippled shale laminations.

Unit Five (Fig. 12) is interpreted as a transgressive mud dominated low energy marine environment with tidal flat and estuarine influences. The dark gray to black nodular to fissile shale contains fine to very fine grained sandstone lenses. Pyrite and siderite(?) nodules occur within the unit. The sandstone lenses display a variety of sedimentary features including load structures, ripple laminations, and remanent flaser bedding. An estuarine influence is probable for Unit Five and is based in part on the low gradient (4 ft/mile) paleo dip oriented fluvial valley associated with the marine depositional setting.

Environment of deposition of the middle Morrow Johannes sandstone is subject to speculation. Interpretation is based on lithologies from mud log descriptions, and the mapped areal geometry of the predominantly coarse clastic facies. The Johannes sandstone may represent reworked fluvial deposits of a younger valley fill sequence through nearshore marine processes that may have redistributed the sand to form sandbars over compacting marine muds.

STRUCTURE, TRAP

The local structural setting of the field reflects its proximity to the Las Animas arch (Fig. 1). Structure contour mapping on top of the Morrowan Series (Fig. 4) shows an irregular but generalized northeast-southwest strike with structural dip to the east-southeast at approximately 50 ft/mile. The structural grain is modified by easterly trending secondary anticlinal and synclinal features. The overall irregularity of the contours over the field has resulted from differential compaction of upper Morrow shales overlying the fluvial valley fill sandstone within the deeply incised paleo-valley.

In the west central portion of the mapped area is a comparatively minor southeast plunging anticlinal feature. Stratigraphically trapped hydrocarbons in the field occur in an arcuate trend around the southeast plunge of this anticline.

The Stockholm sandstone has an easterly tilt with a maximum structural relief of about 100 ft. The eastward tilting of the reservoir body began with post-Morrowan downwarping that continued to the end of Permian and was accentuated by recurrent upward growth of the Las Animas arch culminating in Late Cretaceous (Laramide) orogeny). The incised arcuate trend of the paleo-valley shown in Fig. 7 is attributed to a deflection of the surface drainage by structurally controlled paleogeomorphological processes active during valley downcutting.

Indirect evidence of faulting is indicated by a northeast-southwest reentrant contoured in the lower one third of the map (Fig. 4). The postulated northeasterly trending fault is inferred to be downthrown to the south (down to the basin) flanking the southeast margin of the field. Surface evidence provided by U.S.G.S. topographic mapping shows a similar feature on the present land surface that is occupied by the ephemeral stream bed of the easterly draining South Ladder creek. The stream course implies structural control based on deep incisement up to 25 ft along a relatively narrow and straight course through the southern parts of 11- and 12-16s-43w and 7-16s-42w. The age of the inferred fault is considered post-Morrowan, probably Late Cretaceous (Laramide), and would not have influenced sandstone deposition.

BIBLIOGRAPHY

Avis, L. E., and D. R. Boothby, "Promising Morrow discoveries in southeastern Colorado," OGJ, Vol. 80, No. 22, 1982, pp. 163-170.

Brown, D., "Kansas Morrow," Oil & Gas Investor, Vol. 8, No. 12, 1989, pp. 22-32.

High, L., "Walking the state and narrow," MidContinent Oil World, Vol. 3, No. 12, 1990, pp. 24-26, Keener, W., "High plains promise," Oil & Gas Investor, Vol. 7, No. 1, 1989, pp. 14-24.

Knowles, B., Special report, "Morrow play taking shape on Colorado-Kansas border," in Kansas Report, Petroleum Information, Vol. 34, No. 46, 1987, pp. 10-13.

Krystinik, L. F., "Morrow formations facies geometries and reservoir quality in compound valley fills, central state line area, Colorado and Kansas" (abs.): AAPG Bull., Vol. 73, 1989a, p. 375.

- - - -, "Depositional and diagenetic controls on production in Morrow valley fills, central state line area, Colorado/Kansas" (abs.): AAPG Bull., Vol. 73, 1989b, p. 1,048.

Krystinik, L. F., D. W. Bowen, and H. G. Swanson, "Depositional systematics and exploitation of Morrow valley fill complexes in Cheyenne County, Colo." (abs.), AAPG Bull., Vol. 71, 1987, p. 579. McCaslin, J.C., "Kansas-Colorado Morrow draws interest," OGJ, Vol. 86, No. 43, 1988, p. 80.

Petzet, G. A., "Low prices cut U.S. oil drilling, but several key plays proceed," OGJ, Vol. 87, No. 17, 1989, pp. 44-54.

Shumard, C. B. (in press), Stockholm Southwest field: Hugoton embayment, U.S. Anadarko basin: AAPG Treatise of Petroleum Geology, Atlas of Oil & Gas Fields, compiled by E. A. Beaumont and N. H. Foster.

Stremel, K., "Morrow discoveries fuel drilling," MidContinent Oil World, Vol. 3, No. 1, 1988, pp. 16-17.

Weimer, R. J., S. A. Sonnenberg, and L. T. Shannon, "Production from valley fill deposits, Morrow sandstone, Southeast Colorado: new exploration challenges and rewards," (abs.), AAPG Bull., Vol. 72, 1988, p. 884.

Copyright 1990 Oil & Gas Journal. All Rights Reserved.