PREDICTABILITY OF CARBONATE PETROPHYSICAL PROPERTIES USING SONIC VELOCITY AND CHARACTERIZATION OF PORE ARCHITECTURE IN FINE GRAINED CARBONATES: AN EXAMPLE FROM THE MISSISSIPPIAN LIMESTONE OF THE MID-CONTINENT, UNITED STATES

The Mississippian Limestone of the Mid-Continent United States, is present in the subsurface of northern Oklahoma and western Kansas and outcrops in Missouri and Arkansas. Although there is some siliciclastic input, the majority of the sediment was carbonate deposition in an epeiric seaway within a generally regressive environment. Typical facies include a shallowing upward sequence of planar bedded mudstone, bioturbated very fine to fine grain sand size crinoid-brachiopod skeletal wackestone/packstone, traction deposits with large scale cross-bedding, and massively bedded peloidal-skeletal wackestone to grainstones. The depositional sequence stratigraphic hierarchy is observed in core and correlates to wireline logs at the third, fourth and fifth order scale.

Most of the Mississippian Limestone is a very low porosity and low permeability carbonate. Porosity in the current data set ranges from 0.5-10%, although locally porosity values may be as high as 20%, and permeability ranges from 0.001mD to 10mD but most is less than 1mD. Pore diameter ranges in size from the mesopore (4mm-62.5 µm) to nanopore (1µm-1nm) size, with the majority of the porosity in the micro- to nanopore range. Pores viewed with SEM show the largest pores are oblong to oval, intercrystalline to vuggy mesopores, and the smallest are circular intercrystalline to vuggy nanopores.

Sonic velocity, or acoustic response, in carbonate rocks have predictable trends based on porosity, pore architecture and location within a sequence stratigraphic framework. Previous work has shown quantification of primary pore types may increase the predictability of petrophysical properties that can in turn be associated with the original depositional environment.

Acoustic response (compressional and shear wave) for a sub-set of samples from the Mississippian Limestone varies from 6500 to 5000m/sec (Vp) and 4500-2500m/sec (Vs). Overall trends of the data confirm observations from previous studies regarding the expected range of acoustic response for low porosity, low permeability carbonates. Therefore, integrating petrophysical characteristics with the acoustic response shows promise of using acoustic response to predict facies and key petrophysical parameters within these Mississippian Limestones.