CYPRESS SANDSTONE IN THE ILLINOIS BASIN

Based on a workshop co-sponsored by PTTC's Midwest Region and the Illinois State Geological Survey on April 5, 2000 in Mt. Vernon, Illinois.

BOTTOM LINE

Recognition of reservoir compartments in the Cypress sandstone is essential to properly evaluate the recovery potential and to design a reservoir development and management program.

PROBLEM ADDRESSED

Although production from the Cypress Formation in the Illinois Basin has exceeded 1 billion bbl of oil, significant reserves remain to be exploited. Production of remaining Cypress reserves will require a management plan that can deal with highly compartmentalized reservoirs. Recent research by the Illinois State Geological Survey (ISGS) helps define the stratigraphic, structural, diagenetic, and depositional features of the Cypress that will be of practical use to producers. Regional mapping by ISGS highlights likely areas for exploration.

KEY WORDS:

Basement faults, Completion strategies, Dual porosity, Fracture porosity, Tight gas, Trenton-Black River Group

SPEAKERS

Petrography of the Cypress
Beverly Seyler, Illinois State Geological Survey

Stratigraphy and Structural Overview, Reservoir Architecture
John Grube, Illinois State Geological Survey

Facies Analysis, Core Workshop David Morse, Illinois State Geological Survey
Production Case Study-Waterflooding Jim Blumenthal, Consultant

Cypress Exploration in Western Illinois
Tom Partin , Consultant

TECHNOLOGY OVERVIEW

Sandstones in the Upper Mississippian age Cypress Formation comprise some of the most prolific reservoirs in the Illinois Basin. More than 200 Cypress oil fields have produced in excess of 1 billion barrels of petroleum. Cypress sandstone reservoirs are commonly compartmentalized, resulting in reservoir management problems that are typical of Chesterian sandstones throughout the Illinois Basin. Cypress reservoirs remain a key exploration target and are also prime candidates for enhanced and improved petroleum recovery techniques due to large reserves and the presence of economic quantities of bypassed oil. New information presented in this workshop includes a discussion of compartmentalization of reservoirs, identification of facies, and regional interpretation of structural influence on deposition of Cypress sandstone provinces.

Stratigraphic Setting
The Illinois Basin lies within the interior of the North American craton. Interpretation of Upper Mississippian sediments, including the Cypress Sandstone, indicates that the Basin had a low gradient, southward dipping sea floor and a low subsidence rate relative to the sedimentation rate. Little accommodation space, combined with shallow water depths, a high tidal range and actively prograding deltas, caused sediment reworking that resulted in thin, discontinuous but widespread stacked deposits. These sands prograded from the northeast toward the south and southwest.

Shallow tidal to marine depositional settings (including tidal flat and channels, as well as estuarine and paleosol deposits) are the most common. Parallel, linear, thin, stacked sandstones of tidal current ridges associated with deltas also formed in the Cypress. Today such ridges form in coastal areas with more than 10 ft of tidal range in upper mesotidal to macrotidal environments. Numerous upper Cypress sandstone bodies, including those at Tamaroa and Richview fields, and individual compartments of stacked sandstones in the lower to middle Cypress at Lawrence Field, show linear ridge geometries. Stratigraphic relationships of lower and middle Cypress were complicated by a major relative sea level drop that led to development of soil horizons, red beds, and subaerial exposure of most or all of the basin. During subsequent sea level rise, thick sands filled portions of eroded valleys.

Structural Setting
Many of the present day structural features of the Illinois Basin originated prior to the Mississippian. The old structures were reactivated late in the Pennsylvanian or in post-Pennsylvanian time. Contemporaneous structural activity during Chesterian time (Late Mississippian) is indicated by syndepositional thinning over prominent basinal elevated structures. Most production from the Cypress is from these anticlinal features, including the LaSalle Anticlinal Belt, the Du Quoin Monocline (and related Salem, Louden and Mattoon anticlines), the Sparta Shelf, and the Clay City Anticline.

Structural-stratigraphic combinations are the dominant trapping mechanism for Cypress sandstones. Production is commonly established where stratigraphically-controlled, reservoir quality sandstone drapes across or occurs near fold axes. Structural closure is not always required for hydrocarbon production. Relative compaction of shale over sandstone or limestone reservoir rock can also produce traps. Examples include Tamaroa and Bartelso fields.

Diagenesis Influences Reservoir Quality
Diagenetic clays (kaolinite, chlorite, and mixed-layered illite/smectite) must be accounted for to minimize formation damage during drilling, completion, or well enhancement. Kaolinite is most common and is weakly bonded to pore walls. It can be "shocked" loose by fresh water, and then migrates to pore throats, causing migration of fines that will fill pore throats. Kaolinite residue that is lining pores formed after feldspar leaching is not as likely to be dislodged as pore-filling kaolinite. Therefore, the danger of fresh-water shocking cannot be gauged solely by the amount of kaolinite.

Iron-rich chlorite and ferroan calcite are soluble when exposed to HCl during completion and may result in precipitation of insoluble iron hydroxides. Mixed-layered illite/smectite may swell when exposed to introduced water. The amount of swelling is usually minimal when brine is the contacting fluid, but contact with fresh water may produce a great amount of swelling, thereby reducing permeability.

Regional Mapping
To define the gross sandstone trends within the Cypress Formation, regional mapping was undertaken. Six depositional provinces were identified based on thickness of the Cypress sandstone, its position within the stratigraphic section, and electric log (SP) character. Province 1, the easternmost, is composed of thin, stacked sandstones that are associated with the LaSalle anticline and contain most of the prolific Cypress reservoirs. Provinces 2, 3, and 4, located in the heart of the Illinois Basin, commonly contain thick sandstones with blocky SP curves. Province 5 is associated with anticlinal features where sandstone thickness decreases and blocky SP curves are not common. Provinces 2-5 are less productive than Province 1. Province 6 is associated with the Sparta Shelf where Cypress sandstones are thinner than in provinces 2, 3, and 4. In Province 6, which has moderate Cypress production, stacked sandstone benches are common.

Net sandstone maps show major fairways of thick sandstones in the central portion of the Illinois Basin, bordered to the east by the LaSalle Anticline-Wabash Valley Fault System, to the south by the Cottage Grove Fault System, to the west by the DuQuoin Monocline and Louden Anticline, and to the north by Cypress subcrop.

Province 1 Cypress sandstones have produced a significant amount of reserves due to their structural setting, although they may not display the best overall reservoir characteristics. Massive Cypress sandstones are rarely productive. Lack of a seal, insufficient closure, or active hydrodynamic flushing are responsible for the lack of reservoirs in the thickest Cypress sandstones. In the Illinois Basin, multiple, thin, stacked, partially-charged reservoirs are the norm.

Compartmentalized Marine Bar Reservoirs
Vertically stacked, lenticular, ridge-forming marine bar sandstone reservoirs are common in the middle and upper part of the Cypress. The ridges are typically subparallel and oriented in a northeast to north direction. Reservoirs vary from 1-2 miles in length, are up to 1/2 mile wide, and may be greater than 40 ft thick. Deposits this thick are generally accumulations of vertically stacked lenses that are individually less than 10 ft thick and are separated by 1-10 ft of shale or less permeable sandstone. These sandstones were deposited in tidallyinfluenced, shallow marine environments.

Optimal waterflood recovery from marine ridge reservoirs requires that production and injection wells penetrate laterally continuous compartments. Mapping of individual compartments, which defines fluid communication, is critical and requires careful, detailed correlation.

Marine Bar Reservoirs-Field Examples
Results of field studies show that understanding the distribution of reservoir compartments and using effective reservoir management strategies can significantly improve recovery. Compartments can be effectively drained where they are geologically well defined. Compartments can be detected by various techniques, including comparison of recovery factors of genetically similar sandstones within a field, using packers to separate commingled intervals, analyzing fluid recoveries and pressure, making detailed core-to-log calibrations, and analyzing pressure data from waterflood programs.

Tamaroa Field
Cypress production from Tamaroa Field is typical of that from vertically stacked, subparallel marine bars separated by thin shales. The trap is provided by draping of lenticular sandstones over structural crests. Producing sand bodies occur at a depth of 1,150 ft, are less than 10 ft thick, 1/4 to 1/2 mile wide, and less than 2 miles long. Reservoir volumetric calculations indicate that recovery efficiencies among the Tamaroa pools range from 5% to 43%. This range is related to the degree to which the geology of the reservoir was understood and employed in the drilling and development plan. Remaining recoverable reserves are estimated at more than 600,000 bbls.

Richview Field
Oil is produced from northeast trending, verticallystacked, lenticular Cypress sandstones at 1,500 ft. They are up to 1/2 mile wide, range from 1-2 miles long, and average 10 ft thick. Discrete reservoir compartments are created within the field by shale beds that separate the vertically stacked sandstone lenses. Trapping occurs where the compartmentalized sandstones lap onto or drape over a structural saddle. Reservoir pressure maintenance has strongly influenced ultimate cumulative recovery at Richview Field. Since waterflooding was initiated within a year of primary production, recovery efficiency is high-about 45%. It is recommended that reservoir continuity and flow unit correlation be evaluated through use of field pressure analyses, including pulse, interference, build-up, draw-down and tracer tests.

Lawrence Field
The Cypress Formation has produced 200 million bbls of oil from 4,000 wells in Lawrence field since 1907. Its sandstone geometries were formed in a near-shore, shallow marine setting. In the central portion of the field, five distinct sandstone intervals are separated by thin impermeable shales. Although the total sand package is up to 60 ft thick, individual sandstones average only 10 ft thick. Oil production is primarily from the middle three sandstones. Tightly-cemented calcareous intervals less than one foot thick occur within the sandstones and act as barriers to fluid flow. Compartmentalization also occurs at a smaller scale where discontinuous shale layers that are a fraction of an inch thick form oil trapping compartments. About 39.5% of the original oil in place (OOIP) has been produced at Lawrence Field. The immobile oil volume is a significant 46.5% OOIP. Bypassed mobile oil is approximately 14% OOIP.

Waterflooding Case Study
Upper Cypress sandstone waterflood success at Sailor Springs Consolidated field is highly dependent on flow unit identification. Examples show that sandstone bodies mapped and defined by gross sandstone isopachs do not necessarily flood successfully. Offsetting wells drilled on ten-acre spacing may also appear to produce from grossly correlative sandstones although no communication is established during water injection. Thus, more detailed sandstone mapping is required to define the nature of the compartments or flow units.

Cypress Exploration in Western Illinois
Upper Cypress sandstones in the southwestern portion of the Illinois Basin in St. Clair and Randolph Counties have tested significant amounts of natural gas from depths of less than 600 ft. These sandstones have geometries and proportions similar to those found in the more established portion of the Basin to the east. Structural closure is also established in several locations, although more control is necessary to define reservoir capacity. Due to low pressure and lack of infrastructure, economics have not warranted the development of these potential reservoirs to date. Local industry may find a feasible application.

CONNECTIONS:

Jim Blumenthal
Consultant
P. O. Box 419, Olney, IL 62450
Phone 618-395-7023

John Grube
Illinois State Geological Survey
615 East Peabody Dr.,
Champaign, Illinois 61820
Phone 217-244-2389, Fax: 217-244-2785, E-mail grube@geoserv.isgs.uiuc.edu

David Morse
Illinois State Geological Survey
615 East Peabody Dr.,
Champaign, Illinois 61820
Phone 217-244-5527, Fax 217-244-2785, E-mail morse@geoserv.isgs.uiuc.edu

Tom Partin
Consultant
6823 Lincoln Ave.,
Evansville, IN 47715
Phone 812-476-1060

Beverly Seyler
Illinois State Geological Survey
615 East Peabody Dr.,
Champaign, Illinois 61820
Phone 217-244-1716, Fax: 217-244-2785, E-mail seyler@geoserv.isgs.uiuc.edu

For information on PTTC’s Midwest region and its activities contact:
David G. Morse, Petroleum Geologist, Oil and Gas Section, Illinois State Geological Survey,
Natural Resources, Bldg., 615 E. Peabody Dr., Champaign, IL 61820
Phone 217-244-5527, Fax 217-333-2830, E-mail morse@geoserv.isgs.uiuc.edu

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