PREDICTABILITY OF CARBONATE PETROPHYSICAL PROPERTIES USING SONIC VELOCITY AND CHARACTERIZATION OF PORE ARCHITECTURE IN CARBONATE MUDROCKS: AN EXAMPLE FROM THE MISSISSIPPIAN LIMESTONE OF THE MID-CONTINENT

The Mississippian Limestone of the Mid-Continent, present in the subsurface of northern Oklahoma and western Kansas with outcrops in Arkansas and Missouri, has a complex depositional, diagenetic and structural history. Most of the Mississippian Limestones are low porosity, low-permeability carbonates that were deposited in an epeiric seaway in response to eustatic and relative sea level changes. Typical facies include a planar bedded mudstone, bioturbated very fine to fine grain sand size crinoid-brachiopod skeletal wackestone/packstone, and massively bedded peloidal-skeletal wackestone to grainstones. The depositional sequence stratigraphic hierarchy is observed in core and correlated to wireline logs at the third, fourth, and fifth order scale.

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 or oval intercrystalline and 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 including associated depositional environments and corresponding facies dependent diagenetic changes. Overall trends of acoustic response data from Mississippi Limestone samples [6500-5000m/sec compressional wave (Vp), 4500-2500m/sec shear wave (Vs)] confirm observations from previous work regarding expected results for low porosity, low permeability carbonates. Integrating petrophysical analyses with acoustic response, high resolution sequence stratigraphic analyses from core, and wireline log response, shows promise for predicting facies and major diagenetic characteristics within these Mississippian Limestones.