Mississippian "Chat" intervals in south-central Kansas have produced more than 20 million bbl of oil as of December 1983, but only 8-12% of estimated oil in place was produced. Generally, poor "Chat" production has been attributed to low API gravity and low-viscosity oils, producing from high-porosity, low-permeability in-situ chert intervals. Production is from Mississippian "Chat" residual, tripolitic, in-situ chert intervals and overlying Pennsylvanian Cherokee sands.

One reservoir initially classified as having low API gravity and low-viscosity oil is the Hardtner field in southern Barber County, Kansas, near the axis of Pratt anticline. Previous studies using structure, isopach, net-pay, and combinations of porosity and water saturation maps have had limited success in defining hydrocarbon migration paths or increasing reserves through drilling. Recent studies indicate that, by mapping the reservoir parameters of relative oil-water permeability, water- and oil-wet rock conditions, and phase-drive mechanism, producibility may be better estimated.

Reservoir core analysis and geophysical well logs indicate both water-wet and oil-wet rock conditions are present within 80-ac spacings. Further core analysis indicates pore-throat geometries are related to the dissolution of sponge spicules present in the cherts.

The present study uses variables of relative permeability, paleotopography, pore-throat tortuosity, and coordination numbers as input for multivariant vector-analytical algorithms (CABFAC-QMODE family of algorithms). Results of this and other "Chat" reservoir studies suggest a practical evaluation criteria for more effective exploitation drilling and increased efficiency in enhanced recovery programs.

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