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Abstract: Porosity Patterns in Upper Pennsylvanian Limestones of Kansas: Prediction of Oil and Gas—Leg Thickness in Subsurface Reservoirs on the Basis of Outcrop Samples

H. N. Fisk Laboratory of Sedimentology, Department of Geology, University of Cincinnati, Cincinnati, OH 45221—0013 ([email protected])

Knowledge of oil- and gas-leg thickness within a stratified hydrocarbon reservoir is important for reservoir volume calculations. In areas of active drilling, these parameters are constrained through petrophysical and core analyses. The present study demonstrates constraint of these parameters through petrographic and geochemical analysis of outcrop samples representing lateral equivalents of subsurface reservoir units. The procedure used may facilitate reservoir volume estimates in areas with little or no subsurface control.

Samples of the Missourian Stage (Upper Pennsylvanian) Swope, Dennis, Westerville, Iola, Wyandotte, Plattsburg, and Stanton limestones were collected from outcrop and shallow drill-core in eastern Kansas.

Within the regressive limestone member of each unit (10 - 60 ft thick), porosity types and abundance were observed to vary systematically in the stratigraphic dimension: (1) the upper portion is a 1 - 5 ft thick zone of moderate porosity (5 - 15%) of mainly micromoldic character; (2) the middle portion is a 5 - 25 ft thick zone of strong porosity (10 - 30%) of mainly oomoldic character, containing abundant residual (dead) oil; and (3) the lower portion a 5 - 25 ft thick zone of limited porosity (<5%) in which secondary (oomoldic) porosity was largely filled with coarselycrystalline calcite spar or baroque dolomite.

The paragenesis of the study units reflects two main diagenetic stages: (1) an early (meteoric) event marked by dissolution or recrystallization of mineralogically-labile grains, precipitation of 13C-depleted interparticle cements, and deposition of geopetal silt, and (2) a late (burial) event marked by enhancement of 2Þ porosity, migration of hydrocarbon-bearing brines, and precipitation of mold-filling, 18O-depleted cements. The latter stage was associated with a stratified (gas-oil- water) fluid column, producing the vertical sequence of porosity types and secondary cements described above. Recognition of differential diagenetic patterns associated with stratification of pore fluids can facilitate determination of oil- and gasleg thickness.

AAPG Search and Discovery Article #90930©1998 AAPG Eastern Section, Columbus, Ohio