POTENTIAL IN SUB-DEVONIAN STRATA LISTED FOR ILLINOIS, KENTUCKY, INDIANA

Stephen T. Whitaker
IBEX Geological Consultants
Champaign, Ill.

More than 4.2 billion bbl of oil have been produced in the Illinois basin (Fig. 1) since hydrocarbons were first discovered there more than 100 years ago.

Roughly three-fourths of this production has come from 58 different pay zones in Illinois (Fig. 2), but only four of these zones are in strata older than Devonian. One major reason for this limited sub-Devonian production is that there are relatively few holes that penetrate the deeper strata in the state (Fig. 3).

Seismic data indicate that the sedimentary section may be about 23,000 ft thick in the basin, yet there are no penetrations below 15,000 ft. The limited number of deep tests, coupled with the amount of sedimentary section that has not been contacted, translates to a vast volume of unexplored strata.

This article discusses the potential for stratigraphic plays, structural plays, and source rocks in sub-Devonian strata of the Illinois basin. Although these discussions are based primarily on information from Illinois, they are applicable to the parts of the basin in Kentucky and Indiana as well.

STRATIGRAPHIC PLAYS

Silurian strata contain three active plays and at least one possible future one:

Pinnacle reefs have been found along the flanks of the basin and have as much as 12 million bbl of oil associated with them. There are probably significantly more undiscovered reefs in the basin, but new exploration methods and strategies will be required to locate them.
Numerous stratigraphic accumulations have been found in Silurian rocks along the flanks of an ancient structure known as the Sangamon arch that extended northeastward through central Illinois. These reservoirs formed due to reef development and shoaling in shallow seas.
Erosion at the end of the Lower Devonian period (the sub-Absaroka unconformity) exposed the area, beveled the reef structures, and dolomitized the carbonates. Lack of a proven exploration model has hindered the overall success rate, but there is good potential for additional traps.
In western Illinois (Adams, Brown, Pike, and Schuyler counties) local dolomite reservoirs developed in basal Silurian rocks.
These reservoirs were formed by infilling of shallow paleovalleys cut into the surface of the underlying Maquoketa shale prior to Silurian deposition. The valley-fill carbonates were dolomitized and formed stratigraphic traps in the paleolows.
Buckhorn Consolidated field (Adams and Brown counties, Ill.) is the best example of this type of reservoir ( 1.5 million bbl at 600 ft depth).
A potential future play in the Silurian entails the regional pinchout of porous Niagaran dolomites along the northwestern flank of the basin.

Silurian strata along the flanks of the basin were originally fossiliferous shoals that were locally dolomitized. Dolomitization was further enhanced, along with an ensuing leaching of fossil fragments, when the Silurian strata were exposed during the Devonian period. These porous dolomites pinch out in an arc around the northwestern flank of the basin and may provide numerous hydrocarbon traps.

The impermeable New Albany shale unconformably overlies the Silurian strata along the northwestern flank and forms an excellent vertical seal. Oil shows have been reported in these Niagaran rocks along the perimeter of the basin.

Ordovician rocks have only one documented pay in the basin, but there are at least five potential plays that warrant exploration. Despite good potential for traps in these strata, large areas of the basin have no penetrations into Ordovician rocks.

Current production from Ordovician rocks in Illinois comes from the Trenton (Galena), a fossiliferous limestone. In Illinois, this reservoir produces only where it has been fractured in present-day structures.
A potential new play in the Trenton is indicated by local dolomitization along the flanks of the basin on what are interpreted to be paleohighs.
Although these mapped dolomitic areas are not in the oil-bearing portions of the state, their presence suggests that local dolomitic reservoirs may exist in the Trenton within the oil-productive part of the basin.
These dolomitized areas would occur on features that were high during upper Ordovician time. Such paleofeatures would not necessarily be under present-day structures and would have to be identified through isopach and isochron mapping.
The Joachim formation, which consists of interlayered dolomites, shales, limestones, and local anhydrites, has a high potential for stratigraphic traps. Some oil shows have been reported in this formation on present-day structures, but there has been no production yet.
Localized sandstone and dolomite lenses in the Dutchtown formation may also be good candidates for stratigraphic entrapment. In addition, the organic content of the formation in outcrop is as high as 5%, indicating that it could have potential as a source rock.
The St. Peter and Everton formations are in a facies relationship and contain relatively widespread sandstones. As with most "blanket" formations, the potential for traps is limited. Nevertheless, both formations contain reservoir-quality rocks and have had modest oil shows.
Dolomites of the Ordovician Shakopee and Oneota formations and the Cambrian Eminence, Potosi, Franconia, and upper Eau Claire formations (collectively known as the Knox group) directly underlie the sub-Tippecanoe unconformity.

Some hydrocarbons were recovered from a test of the Shakopee at a depth of about 7,800 ft in the Unocal 1 Cisne well near the center of the basin. Except for this example, no free oil has been recovered from Knox group carbonates in the basin to date. It should be noted, however, that there have been very few tests in these strata.

Cambrian rocks contain at least two potential plays in Illinois. Only very minor shows of gas have been reported, but reservoir-quality rocks have been observed locally.

The lower Eau Claire formation consists of interlayered dolomites, shales, and sandstones. The widely varying lithologies of the Eau Claire suggest that stratigraphic traps may be possible.
Seismic data from northwestern Kentucky and southern Illinois indicate that the untested rift-fill sediments below the Mt. Simon sandstone (Fig. 2) may reach thicknesses of 8,000 ft. Conoco Inc. has drilled an exploration7 well this year in northwestern Kentucky that was scheduled to 14,800 ft. This well presumably penetrated some of this untested section.

STRUCTURAL POTENTIAL

Although structural traps have been almost the exclusive exploration target in the Illinois basin, some types of structural plays have been largely ignored. There has been a tendency to drill on the crests of modern day structures and hope that reservoirs are encountered on the way down. Deeper structures, however, can be offset from their shallower counterparts.

Well and seismic data show that Cambrian strata locally onlap onto the flanks of isolated highs in crystalline basement rocks. Apparently there has been little, if any, effort to search on the flanks of these local anomalies where potentially porous strata are terminated.

Commercially available high-quality seismic data recently acquired in southern Illinois and Kentucky show several deep structures. These horst blocks and ancient anticlines are relatively significant in scale but generally are not expressed in shallower strata.

SOURCE ROCKS, STUDIES

Only a few sub-Devonian shale intervals have been analyzed for their potential as source rocks because suitable samples are lacking.

Recently, samples from three wells drilled in western Illinois by Nyvatex Oil Corp. have revealed that total organic carbon content in the Upper Ordovician Maquoketa shale can be higher than 20%.

Analyses of core samples from the Maquoketa shale, performed by G.L. Salmon at the Illinois State Geological Survey, append previous work and strongly indicate the Maquoketa is a source for oil in the basin.

Numerous oil samples from the Illinois basin (Fig. 4) have been identified as Ordovician-sourced based on analyses done by Salmon. These Ordovician oils have been divided into two types: Type I is mainly derived from cyanobacterial matter; Type 11 is mainly of algal origin. All but one of these samples came from production in the Upper Ordovician Trenton limestone.

The one exception is from the Nyvatex 1-1 Vick well, the northwesternmost producing well in the basin. This well produces from Devonian dolomites which, in that locality, rest unconformably on the Upper Ordovician Maquoketa shale.

The only other documented case of sub-Devonian-sourced oil is a sample recovered from the Ordovician Shakopee dolomite in the Unocal 1 Cisne well in Wayne County (Fig. 5).

Although only 10 ft of oil were recovered during a drillstem test, the sample is nevertheless significant because its analysis points to a Lower Ordovician or possibly Cambrian source (OGJ, Feb. 17, 1992, p. 83).

The lack of rock samples from these strata precludes identifying the actual source bed at this time. Being the first documented case of a sub-Trenton sourced oil in the basin, it raises expectations for finding hydrocarbons in deeper rocks.

CONCLUSIONS

There is significant potential for sub-Devonian hydrocarbon plays in the Illinois basin. Reservoir-quality rocks and traps are known to exist in sub-Devonian strata.

Preliminary results of GC/MS analyses indicate that source beds exist within these strata, but more data are needed to adequately evaluate their significance.

BIBLIOGRAPHY

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