EXPLORATION FOR BASAL SILURIAN RESERVOIRS IN WESTERN ILLINOIS

Stephen T. Whitaker, Richard H. Howard
IBEX Geological Consultants Inc.
Champaign, Ill.

The discovery of two oil fields, Kellerville and Siloam, in shallow (600-675 ft deep) basal Silurian carbonates (Fig. 1)(49549 bytes) in 1958 and 1959 respectively, was the first new production in western Illinois since the discovery of the Devonian Hoing sandstone at Colmar-Plymouth field in 1914 (Fig. 2)(25257 bytes).

A modest oil exploration boom resulted from these discoveries, but no significant new production was established within the next few years and exploration activity diminished.

A second, and more major, drilling boom in western Illinois resulted from official recognition in 1982 of a significant oil discovery in basal Silurian rocks at Buckhorn East oil field, later Buckhorn Consolidated (Fig. 2)(25257 bytes). The area was seemingly a wildcatter's paradise: oil at depths of 600-700 ft, open farmland, and a state still eager to allow oil exploration. Within a relatively short time, numerous rigs were moving into western Illinois in the hopes of repeating the successes experienced at Buckhorn East.

Unfortunately, there was no adequate geologic model that explained the oil accumulations in western Illinois. Drillsites were chosen on the basis of stream drainage patterns, structural highs mapped on shallow Pennsylvanian coal seams, radiometrics, and even random drilling. None of these methods proved successful in locating basal Silurian reservoirs similar to those at Siloam, Kellerville, or Buckhorn.

Compounding the problems caused by the lack of a geological model was the fact that the cheap drilling and relative isolation from regulatory authority in Springfield attracted many non- qualified operators and promoters. Consequently, many holes were not logged, and many were never even permitted. locations were commonly not surveyed, making accurate documentation of well locations and surface elevations difficult to impossible. Stories abounded of farmers going out to their fields and finding holes on their land that had not been there only a day or two earlier. Worse yet, many holes were not plugged property, causing landowners to be justifiably concerned.

The lack of any measurable success in the drilling campaigns in western Illinois in the mid- to late-1980s ultimately led to virtual abandonment of the play. This abandonment may have been premature, since no serious exploration model had been used and many of the tests drilled were little more than promoters' scams. The recent development of a workable geologic model for basal Silurian hydrocarbon entrapment, however, augers well for the discovery of additional oil accumulations in western Illinois similar to that in Buckhorn Consolidated field.

PRODUCTION HISTORY

After the discovery of oil in both Kellerville and Siloam fields, development drilling proceeded at an enthusiastic pace. Some wells had initial production rates of over 200 b/d of oil. By 1966, however, production had declined to the point that both fields were abandoned after having produced 179,369 bbl of oil at Kellerville and 208,500 bbl at Siloam.

Both fields were re-drilled beginning in 1974, but it was not until the drilling boom of 1982, following the announcement of the discovery of Buckhorn Consolidated field, that re-drilling began in earnest. Presently, total production from each field is over 300,000 bbl.

Oil was discovered at what later became known as Buckhorn Consolidated field in 1980 by Big Prairie Oil Co. at its 1 Franks lease well, in NW SW SW 36-1s-4w. Production from this well went unreported, however, and therefore failed to encourage additional development. Formal discovery well status was awarded in April 1982 to the 1 Hippen well drilled by R&R Enterprises in NE SE SE 36-1s-4w. This well had reported initial production of 125 b/d from the basal Silurian dolomite. Since that time, over 200 wells have been completed as producers in the field with total production exceeding 1.7 million bbl of oil. Individual wells have produced as much as 70,000 bbl of oil.

BUCKHORN GEOLOGY

The study area is on the northwestern flank of the Illinois basin a few miles east of the crest of the Mississippi River arch, which separates the Illinois and Forest City basins (Fig. 2)(25257 bytes). The entire area was subjected to Pleistocene glaciation, which left a mantle of till, loess, sand, and gravel that locally reaches 200 ft in thickness. Isolated outcrops of Pennsylvanian and Mississippian strata can be found along stream valleys. Outcrop and Subsurface data indicate that bedrock strata dip gently eastward at average rate of about 8 ft per mile.

The Study area contains an abbreviated geologic section due to both non-deposition of strata and erosion (Fig. 1)(49549 bytes). Due to missing strata, unconformable contacts, and facies variations, many operators in the area failed to completely grasp the geologic history that affected the formation of the important reservoirs of the region. Consequently, the reasons for the generation of reservoirs at the base of the Silurian remained an enigma.

Previous work in the area had already documented that reservoirs in basal Silurian strata in western Illinois are isolated and occur where the Silurian is abnormally thick. These reservoirs have been assigned to either the Kankakee or Edgewood formations, although the Edgewood may in actuality be a dolomitic facies of the Kankakee. Semantics aside, the reasons for the association of dolomite porosity and abnormal thickness of Silurian strata were not understood. Predicting reservoir development and future drillsites seemed virtually impossible.

Structural maps on the top of the Silurian provide little insight to reservoir development. At Buckhorn field, the reservoir's lack of structural expression perplexed developers (Fig. 3)(35164 bytes). Although it was clear that structure was not a primary trapping mechanism, operators had little other mapping technique to use to promote drilling.

The first significant study of oil and gas potential in western Illinois was published by Howard1 as a result of the Kellerville and Siloam discoveries. This study was mainly a documentation of the geology and fields in the region and did not put forth a theory that explained the occurrences of reservoirs in basal Silurian strata.

A SECOND THEORY

Harper, Pilkington, and Beach2 3 authored the first published theory on Kankakee porosity development in Buckhorn Consolidated field and provided the first geologic model for improving exploration for Silurian reservoirs in western Illinois. The theory speculates that localized dolomitization developed preferentially beneath remnant topographic highs on the eroded Kankakee surface (Fig. 4)(50872 bytes).

The theory assumed that the top of the Ordovician Maquoketa shale was relatively flat and that Silurian strata were deposited uniformly over this surface. A lengthy period of erosion during mid-Devonian time dissected the exposed Silurian strata leaving only remnants of the Lower Silurian Kankakee formation. Dolomitization was most pronounced under the remnant highs.

There are two major problems with this theory. First of all, Devonian strata, which unconformably overlie the Kankakee formation, do not thin over these supposed remnant highs as would be expected. In fact, Devonian strata conform almost perfectly to the assumed topographic expressions as mapped using the Harper, Pilkington, and Beach theory (Fig. 4)(50872 bytes). Secondly, there is no adequate explanation for why dolomitization would be more pronounced at the base of the remnant Silurian highs. These problems suggest that the Harper et al. theory is flawed.

ALTERNATE THEORY

Whitaker developed an alternative to the Harper and Beach theory for Buckhorn-type reservoir development in 1987 that was to be included as part of a geologic study of western Illinois already being prepared by J. E. Crockett and B. J. Seyler4 at the Illinois State Geological Survey. This study has never been published in its entirety. A portion of the study was presented in Frontiers quarterly5 and was also presented as a field study by Crockett, Seyler, and Whitaker.4

Whitaker's theory asserts that valleys were cut into the surface of the Maquoketa shale prior to deposition of Silurian strata (Fig. 5)(48382 bytes). A subsequent relative rise in sea level inundated the landscape and resulted in the deposition of Silurian carbonates on the Maquoketa shale. Meteoric waters migrated downward through the Silurian deposits, becoming concentrated in the topographically lowest points where the underlying Maquoketa shales formed an impermeable valley floor. The valleys on the Maquoketa surface, therefore, were the loci for dolomitization of the lowest Silurian sediments.

Subsequent deposition of Silurian strata, and perhaps Lower Devonian strata, was eventually terminated by a regional fall in sea level at the end of early Devonian time. An extended period of exposure and erosion commenced that lasted roughly 40 million years, during which time any Lower Devonian rocks that had been present, and a significant portion of Silurian rocks, were removed. The eroded surface was finally covered by the sea once again late in Middle Devonian time resulting in the deposition of the Cedar Valley limestone followed by the organic-rich New Albany shale.

SUPPORT FOR NEW THEORY

Valley incision on the Maquoketa surface prior to deposition of Silurian strata has been documented by isopach maps and outcrop work. Outcrops of the Maquoketa shale indicate that the uppermost portions of this unit were deposited in very shallow water6 indicative of a marine regression. In northeastern Illinois, Buschbach6 noted that erosion had occurred on the exposed Maquoketa surface prior to deposition of any Silurian strata and that this erosion had produced valleys up to 100 ft deep on the Maquoketa surface.

It appears likely that minor uplift and/or a sea level drop at the close of Ordovician time exposed the Maquoketa surface along the basin flanks including western Illinois. This exposure allowed the incision of a pattern of shallow valleys in the soft shales of the Maquoketa.

Early in Silurian time, advancing seas inundated these valleys and deposited shallow-water carbonates throughout much of the midwest. Dolomitization of the basal Silurian rocks is most pronounced within these valley-fill sediments (Fig. 5)(48382 bytes).

The distinct gamma ray marker in the Devonian New Albany shale, noted as the "Laura zone" in Fig. 5 (48382 bytes), is of relatively consistent thickness throughout the area denoting a highly organic-rich zone that was presumably deposited with minimal topographic relief. Consequently, this marker provides a horizon ideally suited for isopach mapping.

An isopach from the base of the Laura zone to the top of the eroded Silurian surface (Fig. 6)(35519 bytes) shows little variance in thickness. This observation suggests that the there was little topographic relief on the Silurian surface after the 40 million years that the area was exposed to erosion prior to deposition of Devonian strata. This conclusion supports the fact that mapping on the Silurian surface has provided no insight to basal Silurian porosity development.

An isopach map from the Laura zone to the top of the Maquoketa surface, however, clearly reveals a pronounced thick trend which indicates a topographic low on the Maquoketa (Fig. 7)(39955 bytes). This low trend is indicative of an incised valley on the Maquoketa surface.

Porosity in basal Silurian carbonate, and associated hydrocarbon accumulation, coincides with the Maquoketa valley (Fig. 8)(34405 bytes). In general, the best porosity is found in the central portions of the valleys where dolomitization had been most pronounced.

APPLICATIONS OF THEORY

Additional Buckhorn-type fields will be found by locating paleovalleys on the Maquoketa surface that are filled with Silurian carbonates. Predicting and detecting these paleovalleys and the associated basal Silurian reservoirs is a difficult task, however, given the quality and quantity of control in western Illinois.

Where well control allows it, geologic mapping may indicate proximity to paleovalleys on the Maquoketa surface by anomalous thicks on the Laura zone to Maquoketa isopach. This map, however, would require the use of wireline logs utilizing gamma ray and porosity curves which most operators have refused to run. An alternate method is to map anomalous thin areas on an isopach of the Maquoketa shale, an interval that is commonly noted on drilling time logs and on sample tops. Few wells have been drilled deep enough, however, to encounter the entire section of Maquoketa.

It is conceivable that high-resolution seismic data could be used to detect thickening in the Silurian strata and corresponding depressions in the Maquoketa surface or to possibly detect porosity in basal Silurian carbonate. It is not economically feasible, however, to randomly acquire large amounts of seismic data in the hopes of encountering one of these basal Silurian reservoirs.

Recent exploration efforts in western Illinois have provided another potential indicator of paleovalley locations on the Maquoketa surface. Nyvatex Oil Corp. has drilled and logged numerous tests in the area which have provided much-needed data. With these new sets of data it is evident that there are local areas in western Illinois that contain significantly thicker sections of Maquoketa than would be expected based on regional mapping. These isolated thicks form mounds, or domes, that cause draping of overlying Devonian and Silurian strata. Some hydrocarbons were recovered from the Silurian and Devonian strata on these domes and were apparently detected by the proprietary geophysical tools being used by Nyvatex.

The origin of these domes is open to conjecture at this time. Analyses of the logs, cores, and samples, retrieved from the tests drilled by Nyvatex into these domes suggest that the Maquoketa "mounds" were more resistant to compaction in these anomalously thick locations. It is not yet possible to determine whether this greater resistance is due to biothermal activity or some other form of localized cementation. Since the Devonian and Silurian

strata overlying these mounds do not exhibit significant thinning as would be expected over erosional highs, differential compaction is postulated as a primary cause for the creation of the mounds. Pre-Silurian erosional patterns on the Maquoketa surface would probably have been influenced by the more resistant nature of the shales in these isolated areas as well as by the presence of any slight topographic expression of these features.

SUMMARY

Basal Silurian reservoirs in western Illinois developed due to dolomitization of carbonate that filled shallow valleys incised in the underlying Maquoketa shale. Exploration for these reservoirs should utilize all of the clues that have been presented here. It will be critical to continue gathering data from the area via quality wireline logs, cores, samples, and geophysical studies. It is unlikely that the Buckhorn-Siloam-Kellerville complex is unique in western Illinois.

ACKNOWLEDGMENTS

The authors thank Ned Price with Midland Minerals Corp. and Red Smith with Texas Geophysical for supplying data to us for this study. The authors are also extremely grateful to John Ledbetter with Nyvatex for providing cores, logs, analyses, and his observations from recent drilling in the study area.

REFERENCES

1. Howard, R. H., Oil and gas in the Adams-Brown-Schuyler County area, Illinois, Illinois State Geological Survey Circular 325, 1961, 23 p.

2. Harper, J. D., Pilkington, K., and Beach, B., and B. Beach, Illinois Brown County play: Technical geology a must for exploration, OGJ, Vol. 85, No. 9, 1987, pp. 47-48.

3. Harper, J. D., Pilkington, K., and Beach, B., Technical approach, creative geologic exploration models keys to success in Illinois, OGJ, Vol. 85, No. 10, 1987, pp. 46-47.

4. Crockett, J. E., Seyler, B. J., and Whitaker, S. T., Buckhorn Consolidated Field, in Geology and Petroleum Production of the Illinois Basin Vol. 2, Illinois and Indiana-Kentucky Geological Societies joint Publication, 1988, pp. 51-51.

5. Montgomery, S. L., Western Illinois: current debate on a shallow play, Petroleum Information Petroleum Frontiers, Vol. 4, No. 4, 1988, 65 p.

6. Buschbach, T. C., Cambrian and Ordovician strata of northeastern Illinois, Illinois State Geological Survey, Report of Investigations 218, 1964, 90 p. 10.