REASONS FOR DEEPER EXPLORATION ON ILLINOIS BASIN'S WESTERN SHELF
Barry W. Koch
Petroleum Geologist
Midland, Tex.
Five recent tests in Western Illinois give strong indications that most of the exploratory tests drilled in this area have not been drilled deep enough.
The five tests, drilled by Nyvatex Oil Corp., Billings, Mont., during 1989-90, encountered shows of hydrocarbons from deeper strata.
Four of these tests were in McDonough County in the south-central portion of the Galesburg plain, and one was in Greene County at the common border of the Galesburg and Springfield plain physiographic divisions.
Most operators have traditionally viewed this area as a one-objective play.
With but few exceptions, when the Siluro-Devonian (Hunton) carbonates have been penetrated and the greenish-gray silty shales of the Brainard shale formation of the Maquoketa group are reached, the tests have been declared to be at total depth.
At least two potential reservoirs exist in the Ordovician rocks lying below the Hunton and above the St. Peter sandstone.
MAQUOKETA GROUP
The Fort Atkinson limestone formation lies between the Brainard and Scales shales.
The Brainard is readily recognized as the greenish-gray silty shale directly below the Siluro-Devonian carbonates. It is familiarly called "Maquoketa" (or locally the "Green Weenie") and is usually 75-100 ft thick.
The Fort Atkinson may be shaly and nearly indistinguishable from the shales above and below but often occurs as a dark brown to black shale in Western Illinois.
However, in at least one test it more closely resembles its classical identity of being a course-grained, crinoidal limestone or fine-grained dolomite.
In this test, the Nyvatex 281A Hood, in 28-4n-2w, McDonough County, Ill., the Fort Atkinson occurs immediately beneath a dense black organic shale as 11 ft of slightly silty, vuggy dolomite with excellent oil shows (Fig. 1). It is obvious that a very different depositional environment existed at Fort Atkinson time in this local area.
Although this test was abandoned because of difficulties encountered in attempting to acidize, this formation should be considered to be a potential oil producing objective.
With enough control obtained by drilling, it should be possible to make predictive models of the depositional environment that could lead to the discovery of the first Fort Atkinson oil field in Western Illinois.
TRENTONIAN STAGE
The Trentonian stage is comprised of the Galena group, which contains the Kimmswick and Decorah subgroups.
"Trenton" production occurs in deeper parts of the Illinois basin, and shows have been recorded from tests in the shallow western portion.
Nyvatex's two tests in McDonough County also reported shows in the upper part of the Trenton. It is generally believed, however, that Trenton production is associated with structural traps with significant vertical closure.
Structural traps of this magnitude at the top of Trenton highs are probably not present in this part of the basin. However, one Nyvatex test ran 75-90 ft high to published maps.
In Nyvatex 14-1 Null Tract 11, in 14-6n-4w, McDonough County, the Hunton section was found to be approximately 90 ft high to the Illinois Geological Survey's structural maps.
The top of the Trenton came in about 75 ft high to predictions. The zone from 700-734 ft is of interest. These rocks "seem to represent deposits put down during the waning phase of the erosional period between Silurian and Devonian time," wrote Fred Caspall, the wellsite geologist.
"The specific rocks ... alternated between beds of green siltstone and light tan limestone. The bottom of the interval grades to a red shale (734-745 ft) often referred to as the 'red beds' by old time drillers and thought to represent a zone of oxidation possibly formed in pre-Silurian time.
"The red shale grades to a grayish green siltstone of classic Maquoketa type in Western Illinois."
This sequence is pointed out to illustrate that the depositional sequence in this portion of the basin may be much more complex than is generally thought.
An understanding of the depositional history is vital to the construction of valid predictive models. The control necessary to decipher this history can only be obtained by geologic information from deeper drilling.
Oil shows were seen in Hunton at 677-695 ft and at the top of Trenton at 824-846 ft. The neutron log also indicated good porosity at the top of Trenton, probably caused by weathering and leaching of an exposed high. This 12% porosity is greater than the 35% normally found at the top of Trenton.
Two recent tests in McDonough County (Figs. 2, 3) and one in Greene County (Fig. 5) penetrated the Guttenberg formation (Upper Decorah) and have recorded intriguing shows of oil.
In Nyvatex 3-1 Provine, in 3-4n-3w, the Trenton show at 820 ft "produced floating oil on the wash bucket... The porosity in this zone is 12%, which coupled with the quality of the oil makes it an interesting show but a very doubtful producer," according to the wellsite geologist's report (Fig. 2).
A second Trenton show occurred at 990-995 ft. Globs of oil floated to the top of the wash bucket, were easily cut with acetone, and had a strong odor. The lower portion was a sand that "literally dripped oil off the strainer when the samples were washed."
The final show of oil in this test occurred from 1,01 5-20 ft in the Guttenberg.
"This zone produced a bright yellow fluorescence ... and a good odor. Oil could be cut from the samples with acetone, and the porosity is a very good 22%," the geologist wrote.
Nyvatex 3-2 Provine, also in 3-4n-3w, had a slight show of oil in the top of Trenton at 835 ft and a more interesting show in Guttenberg from 1,030-38 ft (Fig. 3). Samples from this zone fluoresced and had a good odor, and free oil accumulated on the sample bag.
Caspall said the oil was in very small pore spaces with almost no permeability. Caspall compared it to an oil shale with oil content around 25%. He concluded that the unit is probably a good source rock in the area.
Nyvatex 11-1 Newton, in 11-12n-13w, Greene County, found Guttenberg at 803 ft and had 12 ft of fine to medium crystalline, slightly dolomitic limestone, with 100% bright yellow fluorescence (Fig. 5).
The samples could not be cut, but a slight rainbow of oil occurred on the pit. The rocks apparently contained a heavy proportion of oil locked in rocks with little permeability.
This example is included because it demonstrates that shows in Guttenberg are not limited to a local area. When this formation is found with good porosity and permeability, it will very likely be a discovery.
Each of these tests, one widely separated from the others, found the Guttenberg as a dark brown to black fairly dense limestone lying beneath a black organic shale.
They all exhibited good cut with bright yellow fluorescence, good odor, and some free oil. However, the rocks were too tight to produce.
SERENDIPITY
It is an accepted fact among serious oil hunters that an excellent place to look for new oil is underneath oil fields.
There are geologic reasons for this. This fact is often overlooked, perhaps due to the lure of the romance of remote wildcatting.
However, in recent years more explorationists have gone back to the basics and are drilling deeper tests in old oil fields.
An extravagant example of almost unheard of shallow well flow rates recently occurred in the southern shelf area of the Illinois basin in Clinton County, Ky.
The operator re-entered a well in Concord field that had been completed in 1987 and was producing 3 b/d of oil from about 1,000 ft. The well was deepened 28 ft and reportedly produced 100,000 bbl of oil in 5 weeks in August and September 1990. It is still producing through drill pipe at a rate of 70-80 bbl/hr from fractured carbonates in the Middle Ordovician Stones River.
Within about 1 mile of the well are 25-30 other wells, most of which produce oil from Middle Ordovician Sunnybrook or Stones River. A few produce from Cambro-Ordovician Knox dolomite at 1,700-1,800 ft.
Anecdotal evidence indicates nearby undrilled wildcat acreage jumped in price within days of the discovery from $1-2,500/acre together with rigorous drilling commitments.
CONCLUSIONS
Although the author is aware that some deeper tests had rumored shows in this shallow portion of the Illinois basin, factual reporting by many operators has been incomplete.
For this reason these observations of recent tests are confined to those of which the author has personal knowledge.
It is apparent that if operators would routinely drill wildcats deeper, to total depths of 950-1,150 ft, into the Decorah, one of two things could happen:
- Someone might discover a new oil field by accident from a formation that has previously not produced in this portion of the basin; or
- Enough additional control will accumulate so that more accurate predictive models may be generated that will lead to the discovery of a new oil field.
In order to make any kind of meaningful subsurface geologic interpretation, it is essential that all operators keep accurate lithologic descriptions at the least and, preferably, run wire line logs of the well bore. This information should be submitted to the Illinois Geological Survey.
The costs incurred in drilling an additional 200-300 ft are minimal compared with the expense in time, land, and drilling through the Hunton.
Since the operator probably already has spent several weeks leasing the prospect and days drilling through the Hunton, it seems illogical not to spend 5-6 hr more drilling to test these suggested horizons.
At an increased cost of about 10% the serious operator can triple the number of possible oil productive horizons tested.
A three horizon test is far more attractive than a single horizon test, especially with such a slight increase in costs of time and money.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.