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RESERVOIR CHARACTERISTICS: MAIN CONSOLIDATED FIELD
It is generally accepted that an excess of 250 million barrels of crude oil have been produced from the Main Consolidated Field (MC Field), making it one of the largest oil fields in the Illinois Basin. Essentially all of the associated natural gas has been flared or used for lease operations. The source of the oil in Crawford County is not totally understood. It is thought to be from organic-rock shales and limestones which are inter-bedded with the reservoir rocks. The four factors are Source, Reservoir, Seal and Trap, which control the accumulation of reserves in the MC Field. The Reservoirs, such as the Robinson sands, and the McClosky oolites, have good porosities (storage capacity of rock), and permeabilities (ability for fluids to move through the rock). This allows for the storage and production of significant volumes of oil from these zones. The Seals are tite (low permeability) beds which overlie these reservoirs. All of the production in the MC Field has been from less than 4,000’ with the most important reservoirs being the Robinson sands. The MC Field is a structural Trap with reservoirs, such as the Robinson and McClosky zones folded into elongated, dome-shaped closures, along the regional LaSalle Anticlinal Belt. These folds are the effective trapping mechanism, and are steeper on their west flanks.
Most geologic knowledge of the MC Field is based on subsurface information because the seal, source and reservoir rocks are not exposed at the surface near the field area. Driller’s descriptions from early wells, electric logs from recent wells, core description and analyses from specific intervals along with detailed production and well test data, are the types of information used to characterize these reservoirs.
The environment of deposition, reservoir geology and engineering parameters (i.e. porosity, permeability, etc.), of these reservoirs are extremely diverse. More importantly, these reservoirs are quite varied horizontally, in that each individual producing horizon exhibits significant areal changes in porosity, permeability, as well as rock type between well locations (heterogeneity). This reservoir heterogeneity is both a hindrance, and an asset. If the reservoirs were homogeneous (all the same), one could expect much more efficient drainage of the reserves from fewer wells. However, due to their heterogeneity, the process of fully developing their reserves is much more complicated, and explains why the field is not fully depleted after nearly 90 years of production history. This heterogeneity explains why initial and step-out-drilling, along with workover activity continues to develop new reserves today. Detailed studies of individual reservoirs require an understanding of their original environments or deposition. For example, the Pennsylvanian-age Robinson sands were deposited some 100 million years ago by streams which were flowing towards the west-southwest and were draining stable highlands to the north and east. These sands were deposited as point bars, overbank deposits and channel sands. Knowledge of the type and orientation of these sand bodies aids in predicting their areal extent and in planning drilling, workover and recompletion operations.
Natural gas was known to exist in Northeastern Crawford County, Illinois, near the town of Palestine before 1900. These gas reserves were small, and markets were quite limited. This resulted in a very moderate amount of drilling.
Shortly after the turn of the century, two explorationists operated a partnership called Bendeman and Trees. They discovered oil in Section 15, Oblong Township on the J.W. Shire farm during 1906. This discovery was from the Pennsylvanian-aged Robinson sands and deeper units such as, The Basal Penn, Benoist, Aux Vases, and McClosky zones. By 1920, some 7,000 wells had been drilled for primary production in Crawford County, delineating what came to be known as the Main Consolidated field. Some of these "cable tools" wells were initially completed for greater than 1,000 barrels of oil per day. The Ohio Oil Company (now Marathon Oil), was affiliated with the J.D. Rockefeller Trust which controlled pipelines and purchasing lines in Illinois. By controlling crude oil prices at the purchasing point, they were able to acquire a majority position in the producing properties of the MC Field.
Few wells were drilled between 1930 and 1950 due to the availability of low priced crude oil from fields, such as East Texas. Shortly after World War II, the oil industry began to recognize the potential of large-scale waterflood operations. The Ohio Oil Company initiated an aggressive waterflood development in MC Field during the 1950’s and 1960’s, until once again, low crude prices curtailed continued development. This de-emphasis on capital investment resulted in a sharp rate of production decline in MC Field, until higher prices in the 1970’s boosted the rate of production. This increased infill and step-out drilling, along with an aggressive workover (recompletion) program has resulted in a significant increase in production rate. Some 9,000 rotary wells have been drilled in MC Field from the early 1950’s through present.
The MC Field is an excellent operating environment for waterfloods. Waterflooding is one of the most common and efficient secondary recovery methods. Water is injected into the oil reservoir from injection wells in order to renew a part of the original reservoir energy. In MC Field, the pattern of injection and producing wells is called a 10-acre, 5-spot pattern for the Robinson development. The ten acres refers to the area surrounding a producing well from which, under ideal conditions, the oil will be drained. The 5-spot refers to the fact that in each fully-developed 10-acre area, there are 5 wells. As the water is forced into the reservoir, it spreads out from the injection wells and pushes some of the oil towards the producing wells. Eventually, the water "front" will reach these producers and increasingly large quantities of water will be produced with a decrease in the amount of oil when it is no longer economical to produce the extremely high water-ratio wells, the flood may be discontinued.
The average primary recovery in a typical reservoir may be approximately 15 percent of original oil in place. Properly operated waterfloods should recover an additional 15-20 percent of this original oil in place, thus matching or exceeding the volume of primary production. This leaves a substantial amount of oil in the reservoir, even after very efficient waterflood operations. Other enhanced recovery techniques exist, but for the most part, are more expensive than Waterflooding on a per-barrel basis. Other techniques which have been tested in MC Filed include, Vacuum Recovery, Fire Flooding, Air Injection, and Polymer Flooding. All of these techniques have application, but will show greater economic returns at higher-than-current oil prices.
For more detailed information on waterflooding, please go to our
Oil and Gas section
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