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 The Mississippian limestone is a petroleum exploration target in northern Oklahoma, and diagenetic events are significant factors in controlling porosity. In this study, paleomagnetic data, supported by petrographic results, were used to determine the origin and timing of diagenetic events in five unoriented cores from northern Oklahoma. Petrographic analysis indicates a complex paragenetic sequence, which includes precipitation of sphalerite and baroque dolomite. Thermal demagnetization removes a low-temperature viscous remanent magnetization (VRM) and a chemical remanent magnetization (CRM) in magnetite. An attempt was made to orient the cores using the VRM but this resulted in a streaked distribution of declinations. The inclinations of the CRM in the specimens in the five cores are similar (mean = −2.6°) and the age of the CRM was determined by comparing the inclinations with the expected inclinations for the study area. This indicates remanence acquisition in the Permian (~310–290 Ma). This is consistent with dates for mineralization in the nearby Tri-State MVT deposit and for a hypothesized Permian hydrothermal alteration event in the study area. The age of the CRM and the presence of sphalerite and baroque dolomite suggest that the CRM was acquired via hydrothermal fluids in the Permian.

ABSTRACT The St. Joe group (Lower Mississippian, Tournaisian) is petrographically and isotopically analyzed using δ 13 C and δ 18 O bulk sample stable isotopes in central, northeastern Oklahoma, and southwestern Missouri. Determined to be conformable in Oklahoma, this group represents deposition in the mid- to outer-ramp setting during one long-term depositional cycle and can be used as a reference section for geochemical chronostratigraphy. Minor diagenetic alteration did not overprint the initial isotope signal, and the resulting curve is similar to those from previous studies and is integrated with published conodont biostratigraphy. The resulting correlation indicates that the St. Joe group was deposited in the upper Tournaisian Stage.

ABSTRACT Mississippian rocks in north-central Oklahoma were deposited on a ramp-shelf system that trended along an approximate northeast–southwest strike and that deepened to the southeast and southwest into the Arkoma and Anadarko basins. The system is bounded on the east by the Ozark uplift. Structure in this area is dominated by extensional and transverse faulting associated with the Transcontinental arch (Nemaha uplift). Shallower water (shelf) depositional settings dominate in the northern part of the study area and deepen toward the south into the Anadarko and Arkoma basins. Sedimentary rocks on the carbonate ramp are dominated by cyclic, partially dolomitized, argillaceous mudstones interbedded with fine-grained wackestones to grainstones. Intergrain pore space is filled by bladed, isopachous, and syntaxial marine calcite cements followed by blocky calcite cements. Limestone is commonly replaced by chert with intergrain open space filled by fine crystalline quartz (chert) cement. Late diagenetic fracture, breccia, and vug (FBV) porosity are filled by calcite and less commonly, by quartz cement that displays a coarse, blocky habit. Carbon and oxygen isotope values for limestones and replacive dolomite are consistent with precipitation from Mississippian seawater and mixed seawater–meteoric water; values for FBV-filling calcite cements indicate precipitation from evolved basinal waters. The 87 Sr/ 86 Sr values of calcite micrite, replacement dolomite, and fracture-filling calcite range from 0.7077 to 0.7112. The lower values are consistent with equilibration with Mississippian seawater through most of the study area. More radiogenic 87 Sr/ 86 Sr values for fracture-filling calcite cements in the northeast part of the study area indicate interaction with continental basement rocks or siliciclastic rocks derived from continental basement. Two-phase (liquid plus vapor) aqueous and petroleum inclusions were observed in FBV-filling calcite and quartz cements. The aqueous inclusions have homogenization temperatures of 48°C to 156°C and salinities ranging from 0 to 25 equivalent weight % NaCl equivalent, and reflect the presence of distinct dilute and saline fluid end-members. Calculated equilibrium δ 18 O water values (VSMOW) for fluids that precipitated fracture-filling calcite cements are variable, ranging from –0.3 to +14.5‰ and do not reflect a single end-member water. Early diagenesis was dominated by seawater-involved cementation, with modification by meteoric water during sea-level low-stands. FBV-filling calcite and quartz represent a later stage of diagenesis associated with petroleum generation and migration. Formation of fractures in the Mississippian section in north-central Oklahoma likely is related to fault movement along the Nemaha ridge instigated by Ouachita tectonism during the Pennsylvanian and extending into the Permian. This timing corresponds with regional flow of saline basinal fluids associated with the orogenic activity. These fluids ascended along faults and contributed to precipitation of FBV-filling cements. Calculated δ 18 O water values for calcite cement in some areas of north-central Oklahoma suggest that cement-depositing fluids approached isotopic equilibrium with the host carbonate rocks. In other areas, however, cement-depositing fluids have oxygen isotope signatures that reflect nonresident fluids whose flow was restricted to fault and fracture pathways, which did not permit isotopic equilibration with the host limestone. In particular, fracture-filling calcite veins from Osage County, with high 87 Sr/ 86 Sr (>0.710) and low δ 13 C values (–2.3‰ to –4.1‰), reflect fluids that retained isotopic characteristics that were derived through interaction with subjacent shale source rocks.

ABSTRACT Fault–fracture related “hydrothermal” carbonate reservoirs such as Albion–Scipio (Michigan) are prolific hydrocarbon reservoirs producing mainly from secondary porosity formed through hydrothermal leaching and brecciation by warm (~60°C–150°C) basinal brines. This paper, for the first time, shows that hydrothermal reservoirs may exist in the Mississippian age limestones (“Miss Lime”) of the southern midcontinent. The chapter is pivoted on the observation of a structural depression in a 3-D seismic volume from Payne County in north-central Oklahoma. The depression involves Pennsylvanian to Ordovician (and possibly deeper) units and is fully enclosed by normal faults. Overall, the fault-bounded depression appears to be a negative flower structure with its main fault extending into the basement providing a path for fluids to flow into the flower structure. Fluid inclusion studies on nearby calcite cements suggest that high-temperature brines, likely sourced from the deeper Ordovician–basement rock, have invaded the larger study area. Multiattribute inversion suggests presence of high porosity (>10%) zones in the Miss Lime depression, but confirmation of brecciation and leaching requires further studies. Nonetheless, the paper posits that several elements of a viable fault–fracture-type play are visible in the Miss Lime depression and recommends that the concept be tested along other regional basement involved faults.

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.