Natural fractures are common in the unconventional “Mississippian Limestone” play of the US Southern Mid-Continent region. Owing to their narrow width, vertical cores provide limited data on the distribution of fracture attributes (e.g., kinematic aperture, height, and spacing) in relation to fracture abundance. For the purpose of searching for an outcrop analog that provides an extensive view of lateral fracture distribution, this study uses an outcrop with Mississippian-aged strata in northwestern Arkansas. Targeting the Reeds Spring Formation, this study aims to characterize the type, attributes, and distribution of natural fractures and to test the outcrop’s suitability as a fracture analog for the subsurface. In the outcrop, planar and nodular beds of lime mudstone and chert contain near-vertical cemented fractures. Fracture types mainly include ptygmatic and opening-mode fractures. Ptygmatic fractures are the most common fracture type in both lime mudstone and chert, whereas the opening-mode fractures are present mostly in chert. Bedding structures, which are defined by lime mudstone–chert variations, affect fracture growth, as indicated by the observation that perfect bed-bounded, top- or base-bounded, and confined fractures collectively account for the majority of the fracture population. In terms of fracture intensity, chert shows a higher average value as compared with lime mudstone. Negative exponential and power law are present as the statistical patterns between cumulative frequency and fracture height, kinematic aperture, aspect ratio, and spacing. The best-fitting distribution pattern and the coefficient of determination vary with lithology, fracture type, and fracture height. These patterns likely point to a cooperative role of lithology, fracture type, and fracture-bedding relationships, as well as the dynamics of rock mechanical properties, in affecting these fracture attributes. In comparison with the cores, this outcrop may serve as a fracture analog for the Mississippian Limestone play in northernmost Oklahoma–southernmost Kansas where cherty facies are widespread, but not for the play areas in north-central Oklahoma, which are dominated by mixed carbonate–siliciclastic facies.

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.

Marine low-magnesium calcite concretions are widespread in many siliciclastic and mixed carbonate–siliciclastic shelf and basinal settings. The process of concretion formation is generally well established and involves microbial influence (mostly sulfate reduction to oxidize organic material at or just below the seafloor). The microbes produce interstitial fluids that are conducive to abundant, and apparently rapid, precipitation of calcite cements. Pervasive cementation generates well-indurated beds or isolated flattened “pods” that are commonly confined to specific stratigraphic horizons. Stratabound concretions can be important as fluid-flow barriers during subsequent burial and compaction. Thin-section and scanning electron microscopy of Cenozoic and Mesozoic concretions has revealed a dense occurrence of small (mostly 2–10 μm), equant, mostly subhedral calcite crystals. The best resolution of both techniques is, however, unable to adequately characterize crystal boundaries, the distribution of clays or organic matter, or the nature of the pores within the calcite matrix. Here, we used scanning electron microscopy to examine ion-micromilled surfaces of concretions from Upper Miocene and Upper Jurassic strata. Results indicate that the dominant crystal size is 1 to 3 μm (mean 2.08 μm; standard deviation = 1.42 μm). Pores were formed at the intersections of calcite crystals by the constriction of the fluid-filled interstitial space, likely prior to dewatering and initial compaction. These (micro) pores are of the “type III, fitted fused” variety. Two-dimensional pore shapes analyzed on micromilled surfaces are near-equidimensional (length/width = ~1–1.5), oval (length/width = 1.5–5), and elongate (length/width = >5) forms. Equidimensional and oval pores occur at the intersections of calcite crystals (along with clay minerals and organic material). Elongate pores of uncertain origin are found at the boundaries between adjacent calcite crystals. The helium pycnometer porosity of the plugs associated with the Upper Jurassic micromilled sample is consistent with a relatively low total porosity, with values of 0.38, 0.58, and 0.82%. Micromilled surfaces improve our understanding of two-dimensional crystal structure and porosity within the matrix of marine concretions. The size and shape of cement crystals and pores suggest that relatively early, rapid, and pervasive precipitation produced a homogeneous mass of calcite and small isolated pores. The resultant low porosity and permeability formed a rock that was diagenetically stable and resistant to chemical and physical modification later during burial.

ABSTRACT Multiple orders of depositional cyclicity in the Mayes Group of northeastern Oklahoma are delineated by refined depositional facies associations and stratigraphic surfaces. Facies associations include deep subtidal facies, shallow subtidal facies (including distal and proximal subfacies), carbonate shoal facies, and shoal crest facies. The Mayes Group records a primary transgressive–regressive depositional cycle bounded below by a major unconformity (sub-Mayes unconformity) and above by an important provincial conodont biostratigraphic boundary and widespread flooding surface at the base of the Fayetteville Shale. Within the Mayes Group, two secondary transgressive–regressive depositional cycles are separated by an interpreted unconformity. The lower Mayes cycle comprises the Bayou Manard and Lindsey Bridge members of the Pryor Creek Formation, whereas the Ordnance Plant Member is grouped with the Hindsville Formation in the upper Mayes cycle. Present in both the lower and upper Mayes cycles are high-frequency shallowing-upward cycles bounded by flooding surfaces. Evaluating the distribution of facies and stratigraphic surfaces within a framework of multiple orders of depositional cyclicity is essential to interpreting the geologic evolution of the southern mid-continent during the Meramecian and Chesterian, and impacts oil and gas production by improving our understanding of reservoir compartmentalization.

ABSTRACT Exploration for hydrocarbons in Mississippian strata in Kansas and Oklahoma began in the 1900s. Early production came from open-hole completions in vertical wellbores at the apex of structural and stratigraphic traps. In the mid-20th century, cased-hole completions and hydraulic fracture stimulation allowed development of lower permeability zones. Recently operators began to explore and develop transition zones and low-permeability facies with horizontal drilling. The petroleum system that includes these accumulations consists of two hydrocarbon kitchens in the Arkoma and Anadarko basins, which have been generating oil and gas from the Woodford Shale since the beginning of the Pennsylvanian. Hydrocarbons charged out of the basins and along the fractured terrain of the Cherokee platform into reservoirs from Kinderhookian to Chesterian age across the carbonate facies belt. The distribution of these reservoirs, including limestones, dolomites, and cherts, along with structural configuration, governs the relative abundance and location of oil, gas, and water in each trap. The past decade saw over four thousand laterals targeting Mississippian reservoirs, including shales in unconventional traps, and the greatest rise in oil production in the region since the 1920s. High associated water volumes have created escalating operational costs and are correlative with earthquake activity.

ABSTRACT Mississippian (Tournaisian–Viséan) carbonate mounds in the Compton and Pierson limestones, Ozark region, North America, have been called Waulsortian. However, European Waulsortian mounds contain features such as geopetals with multigenerations of carbonate mud (polymuds) and stromatactis cavity systems that are rare to absent in Ozark mounds. To determine similarities and differences, examine their origins, and clarify nomenclature, mounds in the Compton and Pierson limestones are compared with Waulsortian mounds in the Feltrim Limestone, Ireland. Features considered included mound size, geometry, style of aggradation, composition, depositional setting, and diagenetic history. Mounds in the Compton and Pierson limestones are <10 m (33 ft) thick and form singular knoll-form or aggregates with a strong lateral growth component. In contrast, individual Waulsortian mounds in the Feltrim Limestone range from 5 to > 30 m ( 16 – 100 ft ) thick, but coalesce and vertically aggrade to form complexes that exceed 500 m ( 1600 ft ) . Pierson mounds are crinoidal and grain-rich, whereas Compton and Feltrim mounds are bryozoan-rich and mud-dominated. All mounds have similar cement stratigraphy and diagenetic histories. Mud-rich Compton mounds and Feltrim mounds are interpreted as deeper water than skeletal-rich Pierson mounds. Limited accommodation constrained Compton and Pierson mound size and forced lateral aggradation. Subsidence-driven accommodation in the Dublin Basin allowed Feltrim mounds to grow larger, coalesce, and aggrade vertically. Three types of mounds are recognized: true Waulsortian in the Feltrim Limestone, mud-cored Waulsortian-type Compton and Pierson mounds, and Pierson transported bioaccumulation mounds. Small dimensions of Waulsortian-type Pierson and Compton mounds limit their potential as oil and gas reservoirs, whereas Pierson crinoidal sediment piles are known to form reservoir-size accumulations.

ABSTRACT Production-scale variability in Mississippian reservoirs of the U.S. midcontinent is poorly understood, largely due to distant spacing of subsurface wells and the lack of outcrops that provide a 3-D distribution of potential reservoir facies. This study utilizes exposures of Upper Mississippian (Meramecian to Chesterian in age) lithofacies in a quarry to develop a 1st-order 3-D facies model at a production or enhanced-production scale (<40 ac [<16.2 ha]). By utilizing photogrammetry to stitch and georeference high-resolution aerial photos, 3-D representations of outcropping walls and pavement were created at a submeter resolution and serve as valuable tools for the visualization of bed and facies relationships in 3-D space. Drone-based aerial and orthogonal photography was used to capture images and create 3-D models of inaccessible outcrop areas. These models were then imported as base surfaces to geostatistical reservoir modeling software (Petrel), in which they were integrated with petrographic and sequence-stratigraphic data to model facies and porosity relationships. Various algorithms and variogram lengths were tested and compared to high-resolution outcrop data to determine the most appropriate workflow for future subsurface modeling. The Petrel-based facies and porosity models illustrate the lateral and vertical variability that exists in outcrop while providing detailed approximations of subsurface reservoir heterogeneity.

ABSTRACT Few detailed diagenetic studies have been carried out on the Mississippian limestone of north-central Oklahoma, U.S.A. Facies analysis, petrographic observations, and stable isotope data are integrated to investigate diagenetic history. This progradational succession of heterozoan–biosiliceous carbonates accumulated on the southern margin of the Burlington–Anadarko shelf. Diagenetic products related to mesogenesis are pervasive, whereas those related to eogenesis and hydrothermal alteration are localized. A pervasive burial diagenetic overprint is consistent with patterns in stable isotopic data, the bulk of which define a trend of large decreases in δ 18 O ( − 1.5 ‰ to − 7.5 ‰ ) accompanied by relatively small decreases in δ 13 C ( + 3.5 ‰ to + 1.5 ‰ ) values. Microbioclastic skeletal wackestone–packstones are prominent. Due to low diagenetic potential, these facies entered the burial realm essentially unlithified. They are characterized by features that point to chemical compaction as the primary driver for lithification. Spicule-rich facies experienced a different diagenetic pathway, with silicification leading to lithification prior to physical compaction. Late-stage fracturing and hydrothermal alteration, significant elsewhere in the basin, is only locally developed. Porosity is associated largely with late-stage fractures and solution seams resulting from chemical compaction.

ABSTRACT Natural fractures are common in several unconventional reservoirs in the U.S. and around the world and, even when sealed with cements, can facilitate the propagation of induced fractures during hydraulic fracturing. This study is focused on correlating fracture types and intensity to distinct petrophysically significant facies and to an established sequence stratigraphic framework in the unconventional carbonate reservoirs of the “Mississippian limestone” of the U.S. midcontinent region. Four fracture types are observed: ptygmatic, vertical extension, shear, and mixed types of fractures. Most of the fractures have been completely sealed with predominantly calcite cement. Fractured zones are vertically heterogeneous at various scales, indicating the variability in rock mechanical properties. At the millimeter scale, fractures are commonly discontinuous and exhibit variable kinematic aperture. At the centimeter scale, ptygmatic fractures exhibit variable termination modes in relation to bedding planes, suggesting a mineralogical control on rock mechanical properties. At the meter scale, the highest fracture abundance corresponds to facies with the highest calcite content. The mineralogical control of fracture distribution is also represented by the higher fracture intensity within the regressive phases of “third-order” sequences, indicating the value of sequence stratigraphic approach in characterizing and predicting fracture distribution in these unconventional reservoirs.

ABSTRACT Four conodont biozones, including three subzones, are interpreted within a revised lithostratigraphic framework for the upper Boone Group and Mayes Group in northeastern Oklahoma and adjacent parts of Missouri, Kansas, and Arkansas. Although revised lithostratigraphy is principally based on observed lithologic characteristics and stratigraphic relationships, conodont biostratigraphic data played an important role in correlation and final organization of units. Within the upper Boone Group, Biozone 1 (lower Meramecian) includes the Ritchey Formation and the Tahlequah limestone and Biozone 2 (middle Meramecian) includes the Moccasin Bend Formation and Quapaw Limestone. The Mayes Group spans Biozone 3 and Biozone 4. Biozone 3 (upper Meramecian) is represented by the Bayou Manard Member of the Pryor Creek Formation (new name). Biozone 4 marks the appearance of definitive Chesterian conodont fauna. The lower two subzones within Biozone 4 correspond to the Lindsey Bridge (Biozone 4L) and Ordnance Plant (Biozone 4M) members of the Pryor Creek Formation, whereas the upper subzone consists of the Hindsville Formation (Biozone 4U). Documentation of conodont taxa and recognition of the proposed biozones provides relative time constraints for genetically meaningful interpretations of regional geology and subsequent evaluation of the Mayes Group and upper Boone Group within a broader interregional context.

ABSTRACT Lithologies, depositional environments, stratigraphic architecture, and conodont biostratigraphy of Lower to Middle Mississippian rocks in the western Ozarks comprise five depositional sequences in ramps on the southern Burlington shelf. Aggradational ramps in the Kinderhookian to early Osagean St. Joe group were relatively strongly overprinted by Ouachita-related tectonism involving inferred recurrent passage of fore-bulge highs and associated basins across central and southern parts of the outcrop area. Significant effects of tectonism are southward facies shallowing onto the broad Kanoka ridge paleotopographic high associated with locally extensive marine and lesser subaerial erosion, sediment thickening and deposition of generally northward down-lapping, resedimented wedges with dislodged reef blocks and conglomerates into relatively rapidly subsiding basins, and formation of a regionally extensive paleosol at the top of the group. Back-stepping subsidence due to middle Osagean foundering of the Kanoka ridge was followed by rapid, long-distance progradation of middle- and outer-ramp facies in the Bentonville and Reeds Spring limestones. Tectonism at this time resulted variously in local folding, uplift, marine and subaerial erosion, and reversal of shelf bathymetry. Southward erosion of the Reeds Spring and Bentonville occurred at least in Oklahoma on rejuvenated segments of the Kanoka ridge. Overlying lower Meramecian limestones are shallow-water deposits truncated by a major unconformity.

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 Mississippian depositional systems in the subsurface of Oklahoma consist of a mix of carbonates and siliciclastic rocks that were variously interpreted as deposited on a regional shelf, ramp, or distally steepened ramp. These varied interpretations resulted in significantly different models for associated facies types and distribution, including potential reservoir types and the distribution of these units that may occur in the subsurface. Fundamental differences in the facies types and distribution of a shelf and shelf margin system versus a ramp or distally steepened ramp include the varying regional distribution for high- and low-energy facies, reef facies, and downslope mass transport deposits. Recent work in both the subsurface of Oklahoma, as well as local outcrops in Arkansas and Missouri, indicates that the facies were deposited on a distally steepened ramp due to the lateral facies distribution and the vertical facies successions identified throughout the system. The presence and characteristics associated with debris flows as described in this study, especially when defined within the context of a sequence stratigraphic hierarchy, supports the interpretation of a distally steepened ramp conceptual model and provides insight into similar mass transport deposits that may occur in the subsurface.

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 Mississippian limestone of the midcontinent United States is a complex and highly heterogeneous hydrocarbon play. Its heterogeneity is largely due to the mixed siliciclastic and carbonate nature of the midcontinent Mississippian system, which yields complex reservoir lithologies and distributions that are laterally discontinuous and difficult to predict. The purpose of this study is to apply stable-isotope chemostratigraphy, a relatively recent method for addressing industry-related correlation problems, as an additional reservoir characterization tool that provides insight into chemical attributes of Mississippian-aged sedimentation and how these chemical signatures can be used for potential chronostratigraphic applications. High-resolution sampling (every 0.3 m [1 ft]) of one subsurface core for carbon and oxygen stable isotopes has revealed predictable patterns related to facies and vertical stacking patterns as well as to globally recognized secular changes in ocean chemistry. The chemostratigraphic approach applied herein suggests more frequent third-order cyclicity than recently defined in other subsurface data sets within the basin, which is more consistent with global ties to the individual North American stages and within the Mississippian overall. In addition, δ 18 O values suggest a level of predictability at the fourth-order scale related to shallowing-upward packages and mixed meteoric input at cycle tops. Overall, stable isotope curves closely match those of well-established Mississippian global carbon cycling and have been used to suggest time boundaries in this area of the depositional system.

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 Organic-rich Mississippian carbonates and oil samples from wells completed in the Mississippian and Woodford zones in northern Oklahoma were sampled and geochemically assessed to evaluate charge history. Rock and oil samples were collected from the Cherokee platform and the Anadarko shelf. Samples were analyzed using gas chromatography and gas chromatography-mass spectrometry (GC-MS) techniques for quantitative analysis of diamondoids and saturate and aromatic biomarkers. Results indicate Mississippian hydrocarbon source rocks have generation potential and reached the early oil window thermal maturity. Extracted bitumen from Mississippian rocks and related oils show unique biomarker signatures such as the presence of extended tricyclic terpanes and high input of C 27 relative to C 28 and C 29 in regular and rearranged steranes. The extent of cracking, as measured by diamondoids, reveals a dramatic change in diamondoids concentration between areas east and west, respectively, of the Nemaha uplift. The higher concentration of diamondoids and biomarkers observed west of the Nemaha uplift indicates mixing of uncracked oil with cracked oil migrating out of the Anadarko Basin. This mix of uncracked and cracked oils west of the uplift suggests episodic hydrocarbon charge and a long-distance component to the migration model. In contrast, the Mississippian samples from east of the Nemaha uplift are depleted in diamondoids, suggesting limited migration distance and localized hydrocarbon generation under lower thermal stress.