--> Abstract: Significance of Depositional and Early Diagenetic Controls on Architecture of a Karstic-Overprinted Mississippian (Osagian) Reservoir, Schaben Field, Ness County, Kansas, by E. K. Franseen, T. R. Carr, W. J. Guy, and D. S. Beaty; #90928 (1999).

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FRANSEEN, EVAN K., TIMOTHY R. CARR, WILLARD J. GUY and D. SCOTT BEATY
Kansas Geological Survey, University of Kansas

Abstract: Significance of Depositional and Early Diagenetic Controls on Architecture of a Karstic-Overprinted Mississippian (Osagian) Reservoir, Schaben Field, Ness County, Kansas

Summary

A sedimentologic and diagenetic study of Mississippian (Osagian) cores from the Schaben field, Ness County, Kansas was undertaken to discern the interplay of original depositional facies and early to late diagenetic events in producing the complex architecture of this dominantly cherty dolomite reservoir. The strata represent deposition on a ramp. Basal strata (M0 unit) consist of normal to somewhat restricted marine strata characterized by an abundance of echinoderm-rich facies commonly containing a diverse fauna. Upper strata (M1 unit and above) are dominated by sponge spicule-rich facies and silicified original evaporite minerals indicating restricted evaporitive ramp, lagoon or supratidal environments. An internal unconformity, apparently a subaerial exposure event, separates upper and lower strata (M1 surface). The post-Mississippian unconformity caps the entire sequence.

Macroscopic, microscopic, well-log, petrophysical and minipermeameter data, and oil shows, indicate that depositional facies and early diagenetic events were dominant controls for reservoir characteristics. The best reservoir is the sponge spicule-rich wackestone/packstone facies (SWP) with replaced evaporites. Echinoderm-rich wacke-packstones (EWPG) are also locally important reservoir facies. Burrow mottling was important in creating localized networks for early diagenetic fluids. Early dissolution of grains and dolomitization created the moldic, intercrystalline and vuggy porosity important for favorable reservoir conditions. Abundant early silica cementation and replacement (chert, megaquartz, chalcedony) created complex heterogeneity. Silica replacement and cementation in EWPG facies typically results in impermeable areas whereas silica cementation and replacement in SWP and mudstone-wackestone (MW) facies results in variably tight or porous (tripolitic) areas containing vuggy, moldic, and intercrystalline porosity. Coarse calcite cementation and poikilotopic calcite replacement associated with the M1 surface significantly occluded much of the porosity in underlying M0 strata. Fracturing and brecciation from the post-Mississippian karst, burial and structural events variously enhanced or destroyed reservoir characteristics. Ramp strata were differentially eroded at the post-Mississippian unconformity resulting in complex buried paleotopography.

The most favorable areas for successful production appear to be where SWP facies containing abundant evaporites (M1 unit and above) intersect the post-Mississippian unconformity and form topographic highs. Our data suggest the EWPG facies to be locally favorable reservoirs. However EWPG facies that are dominant in the M0 unit are not likely to be favorable reservoirs because processes associated with the M1 surface significantly occluded much of the porosity. This prediction is confirmed to the northeast of our study area where M0 strata intersect the unconformity and are not productive. Our methods and results provide predictive capability, indicate the potential for deeper stratigraphic traps, and suggest alternate production strategies such as horizontal drilling or targeted infill drilling may be warranted.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas