3D MODEL IDENTIFIES UNSWEPT OIL IN ILLINOIS'S KING FIELD
Hannes E. Leetaru
Illinois State Geological Survey
Champaign, Ill.
Three-dimensional modeling can be used to enhance the understanding of depositional history and lateral and vertical heterogeneity of reservoirs.
As part of its reservoir characterization program, the Illinois State Geological Survey generated a 3D model of the facies distribution at King field.
King field, 75 miles east-southeast of St. Louis in southeastern Jefferson County, Ill., extends over 1,700 acres and has 108 wells producing mainly from the Mississippian Aux Vases formation at a depth of about 2,750 ft.
The field has produced more than 4.1 million bbl of oil since its discovery in 1942, and the stock tank original oil in place is estimated to have been 16 million bbl of oil.
STRATIGRAPHY, STRUCTURE
A King field type log shows important stratigraphic markers. The Aux Vases formation is overlain by and is transitional with the carbonate-dominated Renault formation, a 10 ft thick brown limestone with negligible porosity.
Although the Renault formation is laterally persistent over most of King field, the limestones within the formation may be locally absent. The discontinuous nature of these limestones, together with the similarity of the electric log responses of the Renault and Aux Vases limestones, can lead to correlation difficulties.
The Aux Vases is underlain by oolitic limestones of the Karnak member of the Ste. Genevieve formation. The structure map (Fig. 1) of the top of the Renault formation shows 40 ft of closure along a north trending anticline that is about 3.5 miles long and 1.5 miles wide.
PALEOGEOGRAPHY
Regional studies indicate that the Aux Vases was deposited in relatively shallow water environments throughout the Illinois basin.1
The Aux Vases formation at King field is interpreted to have been deposited in a nearshore, mixed carbonate-siliciclastic environment.
Generally, the conditions that permit marine carbonate deposition within a siliciclastic sequence are (1) low input of siliciclastic sediment and (2) an extremely broad tidal zone and corresponding exceptionally wide facies belts that parallel the shoreline, such as occur in epeiric seas.2
Laporte3 described the deposition of sediment near mean sea level in an epeiric sea as producing a complex facies mosaic. Minor fluctuations in sea level in such a setting can cause rapid changes in depositional environment.
AUX VASES RESERVOIRS
The Aux Vases reservoirs at King field are porous and permeable sandstones that are rarely thicker than 20 ft.
Reservoir-quality sandstones grade laterally into siltstones, nonporous calcareous sandstones, sandy limestones or shales; such transitions commonly occur within distances as short as the minimum well spacing (660 ft).
The rapid facies transitions are illustrated by cross section A-A' (Fig. 2). The calcareous facies (blue color) and the siltstone-shale facies (brown and light brown) separate the reservoir sandstone (yellow) into individual compartments.
Reservoir compartmentalization is readily apparent in Fig. 3, which is identical to cross section A-A', except that facies with low permeability have been subtracted and only the reservoir sandstone bodies are displayed.
3D MODELING
On a fieldwide basis, the vertical and lateral variations in lithofacies can best be visualized using three-dimensional modeling. A 3D model of the distribution of the various lithofacies at King field (Fig. 4) was generated using SGMTM software by StratamodelTM on a Silicon GraphicsTM workstation.
The sandstone lithofacies (yellow) is a fine-grained quartz arenite, characterized by high spontaneous potential and low resistivity on electric logs.
The best reservoir sandstone contains more than 80% quartz and less than 10% feldspar. Various proportions of calcite, quartz, and clay mineral cements are present in most of the sandstone.
The detrital quartz and feldspar grains are very fine grained to medium grained and moderately well sorted. In hand specimen, the best reservoir rock is usually a light brown to light green, fine-grained, friable sandstone. Rocks from the poorer producing wells are much less friable and have pore spaces partially occluded with calcite and silica cements.
Core analyses from the better reservoir facies had an average porosity of 27% and an average permeability of more than 700 md. The available evidence indicates that the reservoir sandstones were deposited as tidal channels and as offshore bars.
Clay minerals identified by X-ray diffraction constitute less than 10% of the bulk volume of the reservoir rocks. These clays consist of various proportions of illite, mixed-layered illite-smectite, and chlorite. A large percentage of the diagenetic clay in the more porous rocks is a type of iron-rich chlorite.
The best quality reservoir rocks have a thin relatively continuous dusting of clay mineral around each framework quartz grain.
In the less porous parts of the Aux Vases, these clay mineral coatings are more discontinuous, and calcite and quartz cement are more abundant. In general, continuous clay mineral coatings seem to inhibit the formation of quartz cement.4
Most of the effective porosity in the reservoir facies at King field is primary, but some secondary porosity was formed by dissolution of feldspar grains. Microporosity is associated with partially dissolved feldspars and diagenetic clay minerals.
The immobile water contained in the microporosity can result in anomalously high values for water saturation in calculations from wire line log resistivity data.
The calcareous lithofacies (blue, Fig. 4) composed of both sandy limestone and a highly calcareous sandstone, apparently were deposited in an offshore high-energy environment. The sandy limestone lithofacies is composed of fossil fragments, ooids, quartz grains, and minor feldspar grains.
Many of the carbonate grains are heavily abraded. The calcareous sandstone lithofacies is composed predominantly of quartz and feldspar grains and carbonate fossil fragments. Both lithologies contain abundant whole and broken echino-derm plates that have acted as nuclei for precipitation of calcite cement.
Although sediments deposited in offshore environments commonly are widespread,5 at King field the offshore, high-energy, calcareous facies apparently is restricted. This limited distribution suggests that the calcareous facies was truncated by channels interpreted to be of tidal origin and composed of the Aux Vases reservoir sandstones.
There is some minor production from the calcareous facies; however, in most areas carbonate cement occludes all of the pore space in these rocks. Consequently, the calcareous lithofacies generally forms as permeability barriers that increase reservoir compartmentalization.
Laminated green shale and light green siltstone comprise the siltstone-shale lithofacies (light brown-brown, Fig. 4). This lithofacies contains little or no effective porosity or permeability and is interpreted to have been deposited in low-energy offshore and tidal-flat environments.
3D DEPOSITIONAL HISTORY
The geometry of the Aux Vases reservoir facies and their relationship to other lithofacies can be better understood by using the model to traverse up through the formation. In this model the Aux Vases was divided into 21 layers.
Almost all of the oldest layers (layer 3, Fig. 5) at King field consist of shales and siltstones deposited in the distal portion of the Aux Vases progradational system.
Sandstone of the reservoir facies (layer 7, Fig. 6) is first deposited in the northeast corner of the field. Although relatively clean and apparently of reservoir quality, this sandstone is stratigraphically low in the section and therefore water-bearing. By the time of layer 13 (Fig. 7) siliciclastic deposition has inundated the King field area.
The Aux Vases depocenter had begun to migrate away from the King field area by layer 19 time (Fig. 8), and shales and calcareous lithofacies are onlapping the reservoir. Finally, in layer 21 (Fig. 9), the siliciclastics were no longer being deposited and the limestones of the Aux Vases and Renault formations created an effective seal for the entrapment of hydrocarbons.
LITHOFACIES RELATIONSHIPS
The mixture of carbonate and siliciclastic lithofacies at King field was probably caused by temporal fluctuations in the supply of siliciclastic sediment coupled with the presence of an extremely broad tidal zone.
A decrease in the input of siliciclastic sediment could give the appearance of a relative rise in sea level, when base level is raised and marine sedimentation prevails. Also, lateral shifting of the depocenter can cause significant changes in sediment influx resulting in many lateral and vertical transitions between siliciclastic and carbonate lithofacies.
RESERVOIR MANAGEMENT
The mixed carbonate-siliciclastic, nearshore, shallow marine depositional environment of the Aux Vases formed in a heterogeneous reservoir at King field composed of intercalated sandstones, siltstones, shales, and carbonates.
Lateral and vertical compartmentalization of the reservoir by facies that change over distances shorter than the interwell spacing make delineation of the true limits of the field difficult.
Dry and abandoned wells do not necessarily signify the limits of the Aux Vases reservoir at King field. For example, oil production from a separate compartment has been found down-dip of dry holes in which the Aux Vases apparently contained water.
Reservoir heterogeneity must be considered in determining the ideal well spacing for primary and especially secondary recovery. The present 10 acre well spacing at King field may not be adequate for effective oil recovery.
Previously unrecognized compartmentalization of the Aux Vases reservoir at King field has allowed large portions of the reservoir to remain unswept by waterflood projects.
The areal distribution of the best reservoir facies and the potential for compartmentalization are shown (Fig. 10).
Geologically targeted infill drilling, combined with a well-designed waterflooding program based on three-dimensional reservoir modeling could recover an additional 1-2 million bbl of oil from this reservoir.
ACKNOWLEDGMENTS
This research was conducted by the Illinois State Geological Survey with support provided by U.S. Department of Energy grant DE-FG22-89BC1450 and the State of Illinois through Department of Energy and Natural Resources Grant AE-45. Portions of this article were previously reported in an ISGS publication, Illinois Petroleum 135, "Reservoir heterogeneity and improved oil recovery of the Aux Vases (Mississippian) formation at King field, Jefferson County, Ill.," published in 1991.
REFERENCES
- Swann, D.H., and Bell, A.H., Habitat of oil in the Illinois basin, in L.G. Weeks, editor, Habitat of Oil, AAPG, Tulsa, Okla., 1958, pp. 447-472.
- Selley, R.C., Ancient sedimentary environments, Cornell University Press, Ithaca, N.Y., 1978, 287 p
- Laporte, L., Carbonate deposition near mean sea-level and resultant facies mosaic, Manlius formation (Lower Devonian) of New York State, AAPG Bull., Vol. 51, 1967, pp. 74-101.
- Pittman, E.D., and Lumsden, D.N., Relationship between chlorite coatings on quartz grains and porosity, Spiro sand, Oklahoma, Journal of Sedimentary Petrology, Vol. 38, No. 2, 1968, pp. 668-670.
- Weber, K.J., Influence of common sedimentary structures on fluid flow in reservoir models, Journal of Petroleum Technology, March 1982, pp. 665-672.
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