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Lower
Paleozoic Reservoirs in the Illinois Basin
by Beverly Seyler, Illinois State
Geological Survey
In a PTTC workshop preceding the
annual meeting of the Illinois Oil & Gas Association, staff
from the Illinois Geological Survey presented highlights
from a three-year DOE-supported study of Lower Paleozoic
reservoirs in the Illinois Basin. Some research highlights
are listed below. Information is certainly food for thought
with today’s oil prices.
Secondary Porosity in the Trenton (Galena) Dolomite of
Northern Illinois
Karstification in the Trenton
occurred in the following sequence: Galena deposition (mid-Ord),
uplift and meteoric karstification (mid-Ord), Maquoketa
through Devonian deposition (mid-Ord thru Dev), hydrothermal
karstification (late Paleozoic), uplift, erosion, and
further meteoric karstification (late Paleozoic thru
present). Secondary porosity occurs as conduits, fractures,
and matrix vugs. Fractures and conduits are arranged in an
orthogonal pattern with conduits forming at the fracture
junctions. Primary fluid flow occurs through conduits,
followed by flow from fractures, with fluid storage
occurring in the matrix. Source beds for the Trenton are the
overlying Maquoketa shale and the underlying Guttenberg
(Decorah) shale. Hydrocarbon can be trapped in faulted karst
domes.
Trenton Oil Fields and Potential Production in Illinois
Trenton production in Illinois is
confined to areas with pronounced structural closure at the
base of the Ordovician Maquoketa Shale. Several fields
produce from permeable grainstone intervals in which oil is
trapped by overlying dense limestone strata. Production from
dolomitized fractured intervals has been reported in other
fields. There are 35 separate Trenton pools, most having
been discovered from the early 1940s to the late 1960s. Size
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ranges from 10 - 2,000 acres at depths up to 4,000 feet. Up
to five million barrels per field has been produced. All
fields produce on structures and are most likely fractured.
Most reservoirs have low porosity. Some reservoirs are
dolomitic. There is potential for fault-based Trenton
reservoirs in Illinois similar to the Albion Scipio Field
200 miles to the north in the Michigan Basin.
Trenton Production at Centralia and Westfield
There is a shift in structural
closure with depth. The Mississippian Barlow Limestone
contoured structure map shows an elongate anticline with
north and south regions of closure separated by a small
saddle. The Trenton Limestone contoured structure map has
closure on the north end of the anticline and the crest as
mapped on Barlow has shifted to the south. Trenton
production is confined to the anticlinal closure mapped on
the Trenton. The reservoir interval is composed of
grainstone containing dissolution-enhanced porosity.
There is potential for deeper
Trenton grainstone reservoirs throughout the Illinois Basin.
Trenton structural closure may not be reflected by shallower
horizons. Westfield in eastern Illinois produces on a dome
with 100 feet of closure on the Trenton. The reservoir is
fractured limestone with dolomite reported in some
intervals. There are three porous intervals in Westfield.
Major fracture trends based on alignment of high initial
production show fractures oriented northeast-southwest and
another set oriented perpendicular to the primary set. Many
wells exhibit flush production associated with fractures.
Horizontal Trenton wells at Westfield generally have much
higher production rates than surrounding vertical wells.
East-west horizontal legs have higher production rates than
north-south legs. Major fracture trends may be caused by
basement faults. There is the potential for significant
fault related oil discoveries on the flanks of structures.
Siluro-Devonian Carbonates in Illinois: Play Analysis
This presentation highlighted a
strategy for determining which plays |
have the best potential for commercial success. The strategy
was illustrated by comparing Devonian and Silurian plays in
the Illinois Basin. The strategy looked at discoveries in
the last 10 years, comparing field size, initial production
and estimating the low, medium and high range of production.
For both Silurian and Devonian wells, log normal
distributions of initial production were compiled. Median
initial production from Silurian wells was 35 Bopd, compared
to 128 Bopd for Devonian wells. Additionally, several
examples of trend surface analysis and other mapping
techniques were illustrated as tools to identify higher
potential prospect areas.
Silurian Carbonate Reservoirs, Mount Auburn Trend Along the
Sangamon Arch
The Silurian deposits of the
Sangamon Arch are bounded by the pre-Middle Devonian
(sub-Kaskaskia) and post Ordovician interregional
unconformities. As a result of pre-Upper Devonian erosion,
no Lower and Middle Devonian rocks are present on the
Sangamon Arch. The Silurian rocks of the Sangamon Arch have
produced for over 60 years with cumulative production being
more than 20 million barrels of oil from dolomitized
carbonates in the upper part of the Silurian succession. In
the Mount Auburn trend, production thus far has been chiefly
from the uppermost part of the Niagaran Series; most wells
did not test the deeper reservoirs. This study identified
permeability pinch outs at several horizons along the Mount
Auburn trend in Macon, Christian and Sangamon Counties. The
reservoirs display shallowing-upward cycles and include
dolomitized bioclastic grainstone facies in the upper part
and dolomitized coral reef/rudstone facies in the lower part
of the Niagaran succession. The reservoirs are of limited
lateral extent. The highest production is associated with
these lower reservoirs that most wells have not tested. |
Eastern
Region