ABSTRACT Petrographic, geochemical, and fluid inclusion analysis of dolomite and calcite cements has been conducted on Mississippian carbonates collected from the surface and subsurface of the southern midcontinent of the United States (Oklahoma, Missouri, Kansas, and Arkansas). Fracture and vug, intergrain, and intragrain porosity are filled with calcite, authigenic quartz, and dolomite cements. Primary limestone porosity is filled partially by early marine and meteoric calcite cements. Equant (blocky) calcite cements were precipitated under seawater or mixed meteoric-seawater conditions in the phreatic zone and in the deep phreatic zone under late (burial) diagenetic conditions. Fracture- and breccia-filling saddle dolomite cements that were observed are late diagenetic and are likely related to the nearby Tri-State Mississippi Valley-type (MVT) mineral district. Carbon and oxygen isotope values of dolomite cements range from δ 18 O (VPDB) = −9.5 to −2.7‰ and from δ 13 C (VPDB) = −4.0 to −0.4‰. Values for calcite cements range from δ 18 O (VPDB) = −11.6 to −1.9‰ and from δ 13 C (VPDB) = −12.2 to +4.6‰. These values are consistent with three types of diagenetic fluids: seawater, seawater modified by meteoric water, and evolved basinal water. Analysis of fluid inclusions in late calcite, dolomite, and quartz cements indicates the presence of both dilute and high salinity end-member fluids. Homogenization temperatures (T h ) of fluid inclusions range from 57°C to 175°C and salinities range from 0 to 25 equivalent weight % NaCl. Fluid inclusion T h values and salinities are consistent with a saline basinal fluid variably diluted by fluids of meteoric or mixed seawater and meteoric origin. Petroleum inclusions were observed in late diagenetic calcite and dolomite cements.The late diagenetic cements filled porosity retained after early diagenetic cementation indicating that some original porosity in the Mississippian carbonate rocks remained open during petroleum migration. Elevated fluid inclusion T h values over a broad region, not just in the Tri-State Mineral District, imply that the regional thermal maturity of rocks may be higher than believed previously. This study indicates that the Mississippian carbonate resource play on the southern midcontinent has a very complex diagenetic history, continuing long after early diagenetic cementation. Possibly the most important diagenetic events affecting these rocks occurred during burial and basinal fluid migration through these strata.

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 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.

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.