The Mississippian-age limestone of the North American midcontinent (NAMC) is a valuable unconventional, very fine-grained, low-porosity and low-permeability mixed carbonate–siliciclastic reservoir in Oklahoma and Kansas. Although over 14,000 vertical wells have been producing oil and gas from these Mississippian-age reservoirs for over 50 years, recent horizontal activity has illustrated how crucial it is to understand the petrophysical and depositional characteristics associated with producing intervals. High-resolution sequence stratigraphic architecture determined for five cores in three areas of the basin have been integrated with key petrophysical data (porosity and permeability), a qualitative and quantitative analysis of the pore architecture, and the acoustic response from representative samples from each core to better understand the distribution of reservoir facies in this unconventional carbonate reservoir. These data can provide insight into how to enhance the predictability of key reservoir intervals within the study area.
The very fine-grained, unconventional reservoir facies within the sample set have a horizontal porosity that ranges from 0.1% to 12.5% (average 2.5%), although porosity values may be as high as 20% locally. Correlative permeability ranges from 0.0001 to 3.4 mD (average 0.05 mD). Horizontal porosity from coarse-grained facies in the “conventional” reservoir facies range from 13% to 45% (average 31%) porosity with correlative permeability ranging from 5.92 to 163 mD (average 43 mD). The variability within the facies provides insight to key characteristics and measurements that allow for enhanced predictability of key petrophysical features (porosity and permeability). The qualitative and quantitative analysis of the pore architecture, completed using an environmental scanning electron microscope (SEM) and digital image analysis, shows the pores are mostly oblong to oval shaped, interparticle, and intercrystalline to vuggy, meso- (4 mm to 62.5 µm) to nanopore (1 µm to 1 nm) size, while pore throat measurements are consistently in the nanopore range. Acoustic response measurements are inversely related to porosity, which is consistent with published case studies using conventional carbonates. A notable difference in the acoustic response from the data set, is a significant shift in the velocity–porosity relationship that is likely a result of the complex micro- to nanopore architecture and postdepositional diagenesis.
Facies preserved in the five cores range from very fine-grained carbonaceous mudstone and wackestones deposited in an outer-ramp environment to moderate to highly bioturbated wackestone and grainstones deposited in middle-ramp environments, and near-shore wackestone to packstones capped by a series of peritidal deposits. All facies exhibit significant overprinting by diagenesis, including weathering and karst development due to subaerial exposure. Each core shows a shallowing, or shoaling, upward succession of facies, which is in agreement with published eustatic sea-level during this period. The sequence stratigraphic architecture determined from detailed facies analysis reveals a similar hierarchy preserved throughout the basin, which is the foundation to predicting key reservoir intervals. The high-resolution sequence stratigraphic architecture is similarly, the foundation to predict intervals with high porosity and high permeability. The highest order sequences (2nd or 3rd order) have a high level of correlation to conventional wire line logs, specifically the gamma-ray log. Augmenting this data with the acoustic response, and qualitative characterization of the macro- to nanoscale pore architecture, provides an example of how integrated studies can enhance predictability of key reservoir facies and producing intervals within unconventional carbonate reservoirs.
