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Logan County Oklahoma
Source-rock reservoirs geochemistry of Devonian–Mississippian mudrocks in central Oklahoma
Oil families and inferred source rocks of the Woodford–Mississippian tight oil play in northcentral Oklahoma
High-Resolution Stable-Isotope Chemostratigraphy in the Mississippian Limestone of North-Central Oklahoma
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 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.