Abstract

Recent studies of Holocene ooid shoals have demonstrated that hydrodynamics influence sedimentological differentiation among subenvironments, but the ultimate role that these distinctions have on properties in subsurface analogs is less constrained. This study explores how geological variability is captured petrophysically in a Pennsylvanian oolitic succession (Dewey Limestone, Missourian, Cutter Field, Kansas). In the field, the Dewey Limestone ranges from < 10 ft to more than 32 ft (< 3.0 to >10 m) thick; areas with thick accumulations include porosity of greater than 20% and form NW–SE-trending highs, separated by thins of low porosity (< 15%). An extensive suite of petrophysical logs in one well (Cutter KGS 1) illustrates the character of the strata. Resistivity image logs and dip-meter data motivate a subdivision of the succession into distinct depositional packages. These units include differences in porosity, acoustic (compressional and shear) velocity, velocity anisotropy, and elastic moduli. In addition, apparent porosity exponents (m_a) were computed from log porosity and array induction deep resistivity.

These data are consistent with a conceptual model that the Dewey Limestone in the Cutter Field area represents a tidal ooid shoal. In an elongate tidal sand ridge, above a basal low-energy subtidal tight carbonate, an overlying bioturbated cleaning-upward succession (shoal-flank deposits) grades into a porous oolitic grainstone with reversing dip directions (subtidal shoal), and an upper rippled oolitic grainstone with some bioturbation (shallow subtidal to intertidal shoal. Petrophysically, these units are interpreted to include changes in porosity and pore type, and a general upward increase in anisotropy, pore connectivity, and permeability. Geomorphological, sedimentological, and petrophysical similarities among the Dewey strata, Holocene analogs, and Pleistocene and Pennsylvanian shoals illustrate how petrophysical trends can be influenced strongly by depositional framework and character.