ABSTRACT 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.

This article investigates the relationship between rock properties (composition, porosity, and pore architecture) and dry ultrasonic P-wave velocity ( V P ) of 14 samples representing three facies of the Mid-Continent Mississippian-age Limestone (Miss Lime) units of North–Central Oklahoma. Generally, in carbonate rocks, what drives V P , in addition to bulk porosity (ϕ) and composition, is not straightforward to determine. In this data set, when samples are categorized based on their facies and composition (quartz fraction), V P shows a better trend with dominant pore size rather than ϕ. Results show the dependence of elastic properties on texture and highlight a need for incorporating pore-size distribution in seismic models used for seismic interpretation of low-permeability reservoirs such as the Miss Lime.

Petrophysical characterization and understanding of pore systems and producibility in unconventional reservoirs remains challenging when evaluating reservoir potential. This study’s main objective is to identify and evaluate the controls on petrophysical rock types in unconventional low porosity, low permeability carbonate reservoirs in Mississippian-aged rocks of the southern Midcontinent. Representative samples selected from cores in the study area are calcareous siltstones and grain-rich packstones to grainstones. Rock fabric, pore types, and pore structure of 23 samples were investigated using multiscale image analysis of optical micrographs and scanning electron microscope (SEM) mosaics. Petrographic observations and quantified pore parameters were correlated with nuclear magnetic resonance (NMR) plug measurements of transverse relaxation times ( T 2 ), pore size distribution, and porosity. Results indicate that pore structure, permeability, and NMR response are closely linked to the dominant pore types, pore sizes, and mineralogy, which are distinctive for specific rocks—allowing for petrophysical rock type (PRT) grouping. NMR signature geometry is distinct in each of these rock type groups. Complex mixed mineralogies in these rocks homogenizes porosity and permeability relationships among rocks of different depositional facies, making it difficult to define clear-cut correlative relationships between pore architecture, rock fabric, and petrophysical response. Petrographic assessment indicates that the primary cause of pore-scale heterogeneity and varying petrophysical response is related to postdepositional diagenesis, such as silicification, cementation, dissolution, and mineralization along pores and pore throats, which produce complicated pore systems and affects matrix permeability. These observations confirm that incorporating geologic information such as mineralogy, diagenesis, and pore types/pore architecture into rock typing workflows in carbonate mudrock reservoirs is critical to understanding petrophysical response. Additionally, the distinct geometries in each petrophysical rock type group establishes the viability of using NMR as a rock typing tool based on the correlative relationships between NMR response, pore types, and facies.

Abstract This study shows examples of how fundamental relationships between pore shape, porosity, permeability, and acoustic response differ in carbonate mudrocks with micro- to picoporosity (<62 μm diameter) compared to conventional carbonates with primarily macroporosity (256-4 mm diameter). Quantitative data show that some positive correlations exist between porosity and permeability, similar to those observed in conventional carbonates. However, several expected relationships between properties, such as pore shape and laboratory-measured porosity and permeability, are not readily apparent and appear to be complicated by the internal pore architecture coupled with diagenetic alterations and a multiscale fracture network. Additionally, there is a significant shift in measured sonic velocity relative to values calculated from empirically derived equations that are applicable to conventional carbonates. Deviations from expected quantitative data trends can be partially explained through qualitative observations of the pore types and internal pore geometries. Visual observations show how diagenesis can increase the complexity of the internal pore network by nonsystematically subdividing the pores. When correlated to facies, the internal pore geometry partially clarifies deviations to expected relationships between quantitative pore architecture measurements, porosity, and permeability. Although there is an added level of complexity in the pore architecture of carbonate mudrocks, this study shows there are fundamental relationships that exist between the pore architecture, pore shape, porosity, permeability, acoustic response, facies, and sequence stratigraphic framework with variable levels of predictability that, when used as an integrated data set, can be used to enhance the predictability of key petrophysical properties within these types of reservoir systems.