Maquoketa paleotopography key to reservoirs in western Illinois

Stephen T. Whitaker
IBEX Geological Consultants Inc.
Champaign, Ill.
John C. Ledbetter
Nyvatex Oil Corp.
Billings, Mont.

Shallow Silurian reservoirs in western Illinois (Fig. 1 [54513 bytes]) have been a primary target for exploration since the late 1950s. It was not until the discovery and development of Buckhorn Consolidated field in the early 1980s, however, that significant drilling efforts for Silurian reservoirs were focused on western Illinois. At Buckhorn, 1.7 million bbl of oil have been produced from a basal Silurian dolomite at about 650 ft.

Drawn by inexpensive drilling and available acreage, hundreds of operators flocked to western Illinois to try their luck. By the late 1980s, however, exploration efforts in western Illinois were curtailed due to the failure to locate additional significant reservoirs. Much of this failure was due to the lack of a suitable geologic model that could be used to explain the reason for reservoir development and thereby guide exploration efforts.

An article¹ by Whitaker and Howard in 1995 presented a geologic model explaining Silurian reservoir development and stratigraphic entrapment of oil at Buckhorn Consolidated field. This model stated that the reservoir and trap at Buckhorn Consolidated field were formed as Silurian dolomite in-filled a shallow paleovalley cut into the underlying Ordovician Maquoketa shale.

Some companies have recently initiated new exploration efforts in the area using this model. These efforts include new geologic mapping coupled with high-resolution seismic, aeromagnetic, and aerogravity surveys. Several relatively large acreage blocks are presently being leased.

Although Buckhorn contains the most significant Silurian reservoir discovered to date in western Illinois, exploration efforts need not concentrate solely on Buckhorn-type traps. Production from other Silurian fields in the area indicates that reservoir development and hydrocarbon entrapment are not limited to Buckhorn look-alikes.

W. Illinois Silurian fields

Silurian production has been established at several locations in western Illinois (Fig. 2 [44541 bytes]). Production varies up to about 70,000 bbl of oil per well, although average reserves per well are about one-half that. These fields are rarely located on any kind of structural closure, which has puzzled previous explorationists.

Oil shows have been reported in numerous wells throughout the region, but commercial production has proven elusive. Discouraged by a lack of success and a failure to understand the trapping mechanisms, many operators saw little reason to secure wire line logs from boreholes. Without adequate data, exploration programs were further hindered.

Based on the information that is available, it is apparent that a key to understanding the entrapment of oil in western Illinois is understanding the topography on the Maquoketa surface immediately prior to deposition of overlying beds. Periods of post-Ordovician, pre-Silurian erosion dissected the Maquoketa shale leaving shallow valleys and isolated ridges and hills that greatly influenced subsequent deposition.

Paleotopography

As described in the July 1995 OGJ article, paleotopography of the Maquoketa shale in western Illinois is estimated by making isopach maps from a reliable datum, the Devonian Laura zone, to the top of the Maquoketa shale (Fig. 7 [16345 bytes]). Thick trends indicate scouring of the Maquoketa surface, whereas thin trends indicate paleotopographic highs.

Isopach maps of Silurian strata are used to verify interpretations made from the Laura-Maquoketa isopach. Thick Silurian strata situated in an area of indicated Maquoketa scouring support a paleovalley interpretation. Thin Silurian strata indicate paleohighs on the Maquoketa surface if supported by a corresponding thin on the Laura-Maquoketa isopach.

Brooklyn field

One example of using Maquoketa paleotopography to explain hydrocarbon entrapment is provided at Brooklyn field in 3n-3w, Schuyler County.

This field, about 15 miles north of Buckhorn field, was discovered in late 1983 with completion of the Fuller & Baxter 1 Blackburn well. The productive zone was from a slightly sandy Silurian dolomitic limestone at a depth about 475 ft. Several offsetting wells were quickly drilled. Ultimately casing was set on 37 tests, although only 12 were commercial.

In its first year, the field produced nearly 133,000 bbl of oil, although production declined dramatically thereafter. Much of this rapid production decline was due to quick profit-taking by multiple operators resulting in overproduction. To date, total production from Brooklyn is about 250,000 bbl of oil.

The Laura-Maquoketa isopach map indicates that at Brooklyn, as at Buckhorn Consolidated field, a shallow valley was incised into the Maquoketa (Fig. 3 [38865 bytes]) prior to Silurian deposition. This valley was infilled by basal Silurian carbonate, resulting in a reservoir and trap. The majority of the production from the field came from the 12 wells in the eastern valley.

The pre-Silurian valley is not as well-formed at Brooklyn as at Buckhorn, which may partly account for the smaller reserves at Brooklyn. Additionally, the Silurian is near its depositional/erosional limit at Brooklyn, and as such is slightly less porous than at Buckhorn.

Plymouth East area

In late 1983 and early 1984, Gedco drilled a series of wells in 4n-42, McDonough County, about 5 miles northwest of Brooklyn. Only three of these wells were completed as producers in what drillers described as Silurian carbonate, with only one, the 2 Woodside, proving commercial.

No field name was given awaiting the completion of a commercial confirmation well. The reservoir in the 2 Woodside is reported as a sandy Silurian limestone at a depth of 463-466 ft that produced 14,000 bbl of oil in its first year. By the time of its plugging in 1995 this well had produced about 50,000 bbl of oil.

Nyvatex Oil Corp. recently drilled a well about 1/3 mile west of the 2 Woodside. This well, the 1 Susan Cheney, has apparently encountered commercial oil from the same sandy horizon as the 2 Woodside and is presently awaiting completion. Additional drillsite locations have been selected based on Nyvatex's proprietary geophysical system.

Regional mapping suggests that the reservoir-bearing strata at Plymouth East may be Middle Devonian in age rather than Silurian. There are no cores or well cuttings of the strata in question, therefore the age of the reservoir cannot be determined with certainty. Although the question of age is not relevant to the trapping mechanism here, it does introduce the fact that Devonian strata, which also produce oil in the area, are also influenced by Maquoketa paleotopography.

A structure map on the Silurian (Devonian?) reveals that there is no structural anomaly associated with the reservoir (Fig. 4 [35547 bytes]). The structurally highest well in the mapped area contained no hydrocarbons, a fact that deterred additional drilling until Nyvatex's recent efforts.

An isopach of the Laura-Maquoketa interval (Fig. 5 [33090 bytes]) indicates that a channel was incised into the Maquoketa surface as at Brooklyn and Buckhorn. Unlike what is observed at these two fields, however, there is no production within the carbonate that infills this valley. Instead, the production is located immediately downdip from the channel along a thin trend that suggests a paleotopographic ridge.

An isopach map of the Silurian (Devonian?) carbonate strata (Fig. 6 [34139 bytes]) reveals a pronounced thick trend that supports the interpretation of an incised valley on the Maquoketa surface infilled with Silurian (Devonian?) sediments. The reservoir facies is found along the thin interval immediately north (down-dip) from the channel. This thin trend duplicates the thin area mapped on the Laura-Maquoketa isopach and supports the ridge interpretation.

The question as to why there is no production within the valley at this location can be answered by examining cross-section A-B (Fig. 7 [16345 bytes]). There are, in fact, zones with observable porosity in the lower portion of the valley-fill that did reveal shows of oil. There is, however, apparently no dense caprock to prevent vertical migration of oil into shallower beds. Instead, the upper portions of the valley-fill, which also contain oil staining, are slightly sandy and chalky with very minor porosity and very little permeability.

The Silurian (Devonian?) to the south (regionally updip) of the channel is dense and impermeable limestone. Along the interpreted ridge immediately north (regionally downdip) of the channel, a thin, sandy, and porous interval was deposited that is capped by an equally thin, but extremely dense and impermeable, limestone. It is in this thin sandy interval within the Silurian limestone along the ridge that the reservoir and trap formed.

Conclusion

Pre-Silurian and pre-Devonian topography of the Ordovician Maquoketa surface had a major influence on the development of reservoir facies within the overlying Silurian and Devonian strata, respectively, carbonates in western Illinois. The most prolific Silurian reservoirs in the area are found where these strata fill valleys incised into the underlying Maquoketa surface. Not all valley-fill sediments are productive, however.

Silurian reservoirs are not limited to valley-fill sediments in western Illinois. As illustrated in the Plymouth East area, some reservoir-quality rock was deposited on Maquoketa topographic highs where slightly higher energy environments existed. Although there are not yet much data on these types of traps, production is capable of being quite commercial as documented at Plymouth East.

A more carefully obtained and thorough collection of data is needed in western Illinois to significantly improve exploration efforts in the area compared to previous efforts. Better logging programs will not only help geologic interpretations but could provide better acoustic information to improve the accuracy of the high-resolution seismic data required to detect the subtle traps.

Additional geologic and geophysical data may help to determine whether some of the Maquoketa paleovalley orientations are influenced by ancient lineaments. Aeromagnetics and aerogravity surveys are being acquired to help determine whether any trends can be defined.

As with all exploration, the more data that are acquired, the more that is learned. With additional knowledge will come the discovery of additional reservoirs in western Illinois.

Reference

1. Whitaker, S.T., and Howard, R.H., Exploration for basal Silurian reservoirs in western Illinois, OGJ, July 31, 1995, pp. 91-96.

The Authors

Stephen Whitaker is president and co-founder of IBEX Geological Consultants Inc., Champaign, Ill. He began his industry career in 1978 with Texaco in Denver and has also worked with Apache Corp. and K&E Petroleum. He joined the Oil and Gas Section at the Illinois State Geological Survey in 1986 and authored numerous articles on new exploration concepts and development methods for the Illinois basin. In 1992 he co-founded IBEX to provide consulting services to the oil industry and promote exploration in Illinois. He received a BS in geology in 1976 from the University of Southern California and an MS in geology from the University of Colorado, Boulder, in 1978.
John C. Ledbetter is president and chief executive of Nyvatex Oil Corp., Billings, Mont. Following military service and 8 years on Wall Street, he founded Nyvatex in 1968 as an investment and development vehicle to apply his theories on chaos mechanics (deep pattern non-linear dynamics) to oil and gas exploration.He helped pioneer exploration of the western Montana overthrust belt in the late 1970s-early 1980s and consulted on exploration of the Wyoming overthrust and the Nevada Basin and Rrange. He is a 1957 graduate of the U.S. Military Academy.