The Mid-Continent Mississippian Limestone is an unconventional carbonate reservoir with a complex depositional and diagenetic history. Oil and gas have been produced from vertical wells for over 50 years, but recent horizontal activity in low porosity, low permeability zones have made it crucial to understand the petrophysical characteristics to better target producing intervals. Because of the wide variability and complexity of pore systems in carbonate reservoirs, simple porosity/permeability transforms developed for siliciclastic reservoirs often provide erroneous results in carbonates. However, laboratory measured sonic velocity response in carbonates, when combined with an analysis of pore system architecture, has been shown to provide a proxy for permeability in carbonates with pore systems at the macro- and micro-scale. This project is focused on testing this relationship for micro-to nanoscale pore systems.
The sonic velocity response (compressional and shear wave) for a sub-set of samples from Mid-Continent Mississippian limestones 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 sonic velocity for low porosity, low permeability carbonates. Porosity values in the horizontal direction ranges from 0.5–7%, although locally porosity values may be as high as 20%. Pore diameter ranges in size from the mesopore (4mm-62.5 μm) to nanopore (1μm-1nm) size, with the majority of the pores in the micro- to nanopore size range. Pores viewed with SEM show the largest pores are mostly oblong to oval shaped, intercrystalline to vuggy mesopores with a diameter of 25–100μm while the smallest are circular shaped intercrystalline to vuggy nanopores with a diameter of 5–10μm with 50–100nm pore throats.
Because pore size and shape have significant influence on the permeability in these reservoirs, sonic velocity data coupled with characterization of macro- to nanoscale pore architecture shows promise of predicting both key reservoir facies and key producing intervals within these unconventional carbonate reservoirs, in particular when tied to primary depositional facies and incorporated within a high-resolution sequence stratigraphic framework.