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Unconventional Carbonate Reservoir Characterization Using Sonic Velocity and Characterization of Pore Architecture: An Example From the Mid-Continent Mississippian Limestone


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 makes it crucial to understand the petrophysical characteristics to target producing intervals. 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 that quantification of primary reservoir pore types (macro- vs. micro-) may increase the predictability of reservoir permeability within a basin. Facies are characterized by a hierarchy of shoaling upward packages defined by planar bedded mudstone at the base, followed by bioturbated very fine to fine grain sand size crinoid-brachiopod skeletal wackestones, and massively bedded peloidal-skeletal wackestone to grainstones at the top. A sequence stratigraphic hierarchy of shoaling upward cycles are observed in core and wireline logs at third, fourth, and fifth order scale. 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. Porosity in the horizontal direction, in the current data set, 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 porosity in the micro- to nanopore scale. Pores viewed with SEM show the largest pores are mostly oblong to oval shaped intercrystalline to vuggy mesopores with a diameter of 100μm x 25μm, while the smallest are circular shaped intercrystalline to vuggy nanopores with a diameter of 5-10μm with 50-100nm pore throats. Petrophysical analyses have been integrated into high resolution sequence stratigraphic analyses of core and outcrops from Oklahoma, Missouri, and Arkansas. Sonic velocity coupled with characterization of macro- to nanoscale pore architecture, wireline logs and high resolution sequence stratigraphic analyses shows promise of predicting both key reservoir facies and key producing intervals within an unconventional carbonate reservoir.