This article investigates the relationship between rock properties (composition, porosity, and pore architecture) and dry ultrasonic P-wave velocity (VP) 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 VP, 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), VP 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.

No abstract

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 (T2), 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.

Recent work has shown that there is a predictable inverse relationship between laboratory-measured sonic velocity response and porosity in carbonates, which can be reasonably approximated using the empirical Wyllie time-average equation (WTA). The relationship was initially identified in late Cretaceous to Cenozoic age samples collected from the Great Bahama Bank and the Maiella Platform, an exhumed Cretaceous carbonate platform in Italy. We have compared older carbonate samples from different basins and different geologic ages to determine the applicability of this relationship and subsequent correlations to key petrophysical properties to other carbonate basins and other geologic time periods. The data set used for the comparison shows this relationship to be relatively consistent in other depositional basins (Michigan Basin, Paradox Basin) and with samples from older geologic periods (Pennsylvanian, Ordovician, and Mississippian). However, this basic relationship is also observed to vary significantly within a reservoir system and within a depositional basin in samples from different geologic periods (e.g., Silurian- versus Ordovician-age rocks in the Michigan Basin). Although the empirical WTA can generally be applied as a first-order estimate across a wide range of sample ages in carbonates, limited data suggest the relationship between velocity and porosity to be moderately more complex. For instance, in unconventional carbonate reservoirs characterized by predominantly micro- to nanoscale porosity, it is observed that the WTA should be applied as an upper data boundary. In addition, this study has shown that the relationship to the dominant pore type is less direct than in a macropore system in which it can be assumed that the dominant pore type also has the greatest effect on the effective permeability.

Summary Nanopore geometry and mineralogy are key parameters for effective hydrocarbon exploration and production in unconventional reservoirs. This study describes an approach to evaluate relationships between low-frequency complex resistivity spectra (CRS), nanopore geometry, and mineralogy to use CRS to provide estimates of reservoir parameters concerning hydrocarbon saturation, storage, and producibility. For this purpose, the frequency dispersion of CRS was analyzed in 56 mudrock core plugs from the Vaca Muerta Formation (VMF) (Jurassic/Cretaceous) in Argentina, along with cementation factors (m), carbonate content (CO3), and total organic carbon (TOC). To quantify the nanoporosity, a subset of 23 samples was milled with broad ion beam (BIB) and imaged with scanning electron microscopy (SEM); the image grids of these samples were stitched together into high-resolution BIB-SEM mosaics and analyzed with digital image analysis (DIA) techniques. Results show that porosity is the dominant control on electrical properties in the mudrocks analyzed as part of this study. There is no conclusive evidence that pore geometry influences the electrical properties in the analyzed mudrocks. Pore-geometry parameters [dominant pore size (DOMsize) and perimeter over area (PoA)] do not correlate with electrical properties. Instead, mineralogy shows a first-order correlation with electrical properties, where cementation exponents are higher in rocks with high TOC and low CO3 content. CRS can be used to estimate porosity and cementation factors with high correlation coefficients of R2 = 0.71 and R2 = 0.95, respectively. Estimates of the 2D interfacial surface area (ISA2D), which is a function of both pore geometry and porosity, achieve an R2 = 0.59. The results of this study suggest that low-frequency dielectric rock properties, if measured downhole, could be useful to identify primary producing intervals in unconventional reservoirs, and to accurately determine cementation factors independent of formation fluids and porosity.