ALBERT

Description: The electrical resistivity log has proven to be a powerful tool for lithology discrimination, correlation, porosity evaluation, hydrocarbon indication, and calculation of water saturation. Carbonate rocks develop a variety of pore types that can span several orders of magnitude in size and complexity. A link between the electrical resistivity and the carbonate pore structure has been inferred, although no detailed understanding of this relationship exists. Seventy-one plugs from outcrops and boreholes of carbonates from five different areas and ages were measured for electrical resistivity properties and quantitatively analyzed for pore structure using digital image analysis from thin sections. The analysis shows that in addition to porosity, the combined effect of microporosity, pore network complexity, pore size of the macropores, and absolute number of pores are all influential for the flow of electric charge. Samples with small pores and an intricate pore network have a low cementation factor, whereas samples with large pores and a simple pore network have high values for cementation factor. Samples with separate-vug porosity have the highest cementation factor. The results reveal that (1) in carbonate rocks, both pore structure and the absolute number of pores (and pore connections) seem more important in controlling the electrical resistivity, instead of the size of the pore throats, as suggested by previous modeling studies; (2) samples with high resistivity can have high permeability; large simple pores facilitate flow of fluid, but fewer numbers of pores limit the flow of electric charge; and (3) pore-structure characteristics can be estimated from electrical resistivity data and used to improve permeability estimates and refine calculations of water saturation.

Description: 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.

Description: The term “beef” describes bedding-parallel calcite veins found commonly in the organic-rich matrix of unconventional resource plays. Although some authors have interpreted beef to be an early diagenetic feature, these calcite veins are commonly attributed to precipitation at high temperatures and localized overpressure during hydrocarbon generation. The temperature at which the beef formed is thus crucial to ascertain the process of beef genesis. We use the novel methodology of clumped isotope analysis to constrain both the temperature at which beef forms and the isotopic composition of fluids present during formation.For this study, we use beef from basinal sections of the Vaca Muerta Formation in the Neuquén Basin, where veins are commonly up to approximately 10 cm (∼4 in.) thick and are laterally continuous over 1 km (0.6 mi). The calcite veins occur in isolation or in association with concretions and ash layers. Sequence stratigraphic boundaries have little influence on distribution, and only a low correlation between beef and total organic content or beef and ash layers exists. The internal crystal structure of beef varies largely, suggesting both syntaxial and antitaxial growth forms. The δ18O values of beef range from approximately −12‰ to −9‰, and the δ13C values vary between approximately −1‰ and 1‰. The surrounding mudstone and concretion fracture fills (calcite) show little difference isotopically when compared to the beef itself. The δ18O values of nearby concretions range from approximately −3.5‰ to 1‰, and the δ13C values vary between approximately 6‰ and 11‰.Clumped isotope analysis of beef in the Vaca Muerta Formation indicates temperatures between approximately 140°C and 195°C, whereas the surrounding mudstones vary from approximately 120°C to 150°C. The corresponding formation fluid δ18Ow values range from 8.5 to 14.5‰. These temperature data are higher than the maximum temperatures suggested by published studies modeling the basin’s thermal and burial histories. If these models are correct, the clumped isotope data indicate that the growth of beef in the Vaca Muerta Formation required the input of hydrothermal fluids from greater depths. Alternatively, the geothermal gradient or burial depth was underestimated in these models.

Description: Carbonate rocks commonly contain a variety of pore types that can vary in size over several orders of magnitude. Traditional pore-type classifications describe these pore structures but are inadequate for correlations to the rock's physical properties. We introduce a digital image analysis (DIA) method that produces quantitative pore-space parameters, which can be linked to physical properties in carbonates, in particular sonic velocity and permeability. The DIA parameters, derived from thin sections, capture two-dimensional pore size (DomSize), roundness (γ), aspect ratio (AR), and pore network complexity (PoA). Comparing these DIA parameters to porosity, permeability, and P-wave velocity shows that, in addition to porosity, the combined effect of microporosity, the pore network complexity, and pore size of the macropores is most influential for the acoustic behavior. Combining these parameters with porosity improves the coefficient of determination ( R 2) velocity estimates from 0.542 to 0.840. The analysis shows that samples with large simple pores and a small amount of microporosity display higher acoustic velocity at a given porosity than samples with small, complicated pores. Estimates of permeability from porosity alone are very ineffective ( R 2 = 0.143) but can be improved when pore geometry information PoA ( R 2 = 0.415) and DomSize ( R 2 = 0.383) are incorporated. Furthermore, results from the correlation of DIA parameters to acoustic data reveal that (1) intergrain and/or intercrystalline and separate-vug porosity cannot always be separated using sonic logs, (2) P-wave velocity is not solely controlled by the percentage of spherical porosity, and (3) quantitative pore geometry characteristics can be estimated from acoustic data and used to improve permeability estimates. Ralf J. Weger was a postdoctoral researcher with the Comparative Sedimentology Laboratory at the University of Miami when the article was written. He received his B.S. degree in systems analysis (2000) and his Ph.D. in marine geology and geophysics (2006) from the University of Miami. His dissertation focuses on quantitative pore- and rock-type parameters in carbonates and their relationship to velocity deviations. His main interests range from processing and visualization of geophysical data to petrophysical characterization of carbonate rocks. Gregor P. Eberli is a professor in the Division of Marine Geology and Geophysics at the University of Miami and the Director of the Comparative Sedimentology Laboratory. He received his Ph.D. from the Swiss Institute of Technology (ETH) in Zürich, Switzerland. His research integrates the sedimentology, stratigraphy, and petrophysics of carbonates. With laboratory experiments and seismic modeling, his group tries to understand the physical expression of carbonates on log and in seismic data. He was a distinguished lecturer for AAPG (1996/97), Joint Oceanographic Institutions (1997/1998), and the European Association of Geoscientists and Engineers (2005/2006). Gregor T. Bächle graduated from the University of Tübingen in 1999 with a Diploma (equivalent to M.Sc. degree) in geology. In 2001, he joined the Comparative Sedimentology Laboratory (CSL) with a Scholarship of the German Academic Exchange Service to obtain a Ph.D. from the University of Tübingen. From 2004 to 2008, he was a research associate in the CSL, managing the rock physics laboratory. He is currently working for ExxonMobil Upstream Research Company, Quantitative Interpretation, Houston, Texas. Jose Luis Massaferro is a geology manager in Repsol YPF's exploration office in Argentina. He received his Ph.D. from the University of Miami in 1997. He was a Fulbright Fellow while pursuing his studies in Miami. Prior to his Ph.D. studies, he worked for Texaco as a geologist. In 1998, he joined Shell E&P and was involved in different projects, including 3-D seismic volume interpretation, high-resolution sequence stratigraphy, and kinematic modeling of compressional structures. In 2005, he joined Repsol in Madrid. Yue-Feng Sun is an associate professor at Texas A&M University. He received his Ph.D. (1994) from Columbia University. He has 25 years of experience as a geoscientist in the industry and academia. His professional interests include carbonate rock physics, poroelasticity, poroelectrodynamics, reservoir geophysics, and petroleum geology. He is a member of AAPG, the American Geophysical Union, American Physical Society, and the Society of Exploration Geophysicists.

Description: Electrical and fluid flow properties of porous media are directly related to the morphology of pores and the connectivity of the pore network. Both are closely linked to the amount and type of intrinsic microporosity in carbonate rocks, which is not resolved by conventional techniques. Broad-ion-beam (BIB) milling produces high-quality true-two-dimensional cross sections for scanning electron microscopy (SEM) and enables accurate quantification of carbonate microporosity for the first time. The combination of BIB-SEM mosaics with optical micrographs yields a multiscale digital image analysis (MsDIA) spanning six orders of magnitude. In this paper, the pore structures of 12 different carbonate rock samples from various rock types are quantified using MsDIA. Mercury injection capillary pressure measurements are used to assess pore-throat properties. The quantified pore-structure parameters are correlated with plug measurements of electrical resistivity and permeability. Results indicate that petrophysical properties are closely linked to the type of microporosity, which is distinctive for a certain rock type. Rock types with crystalline microporosity, such as mudstone and dolomite, generally show good connectivity, in which the size of the pore-network determines if the rock favors either hydraulic or electric flow. Rock types with intercement or micromoldic microporosity, such as bindstone and travertine, show variations in connectivity due to layering and moldic micropores of biological origin. Furthermore, pore-size distributions (PSD) follow a power law in all samples, despite their depositional and diagenetic differences. The slope of the PSD correlates with the electric properties, in which samples with a steeper slope show lower cementation factors. The linearity of the power law distribution enables predictions of pore populations outside the investigated length scales.

Description: The electrical resistivity log has proven to be a powerful tool for lithology discrimination, correlation, porosity evaluation, hydrocarbon indication, and calculation of water saturation. Carbonate rocks develop a variety of pore types that can span several orders of magnitude in size and complexity. A link between the electrical resistivity and the carbonate pore structure has been inferred, although no detailed understanding of this relationship exists. Seventy-one plugs from outcrops and boreholes of carbonates from five different areas and ages were measured for electrical resistivity properties and quantitatively analyzed for pore structure using digital image analysis from thin sections. The analysis shows that in addition to porosity, the combined effect of microporosity, pore network complexity, pore size of the macropores, and absolute number of pores are all influential for the flow of electric charge. Samples with small pores and an intricate pore network have a low cementation factor, whereas samples with large pores and a simple pore network have high values for cementation factor. Samples with separate-vug porosity have the highest cementation factor. The results reveal that (1) in carbonate rocks, both pore structure and the absolute number of pores (and pore connections) seem more important in controlling the electrical resistivity, instead of the size of the pore throats, as suggested by previous modeling studies; (2) samples with high resistivity can have high permeability; large simple pores facilitate flow of fluid, but fewer numbers of pores limit the flow of electric charge; and (3) pore-structure characteristics can be estimated from electrical resistivity data and used to improve permeability estimates and refine calculations of water saturation. Klaas Verwer is a research geologist at Statoil in Bergen, Norway. He received his Ph.D. from the Vrije Universiteit in Amsterdam, Netherlands. His thesis and postdoctoral research revolves around carbonate sedimentology, outcrop analog work using digital field technologies, and petrophysics of carbonates. Currently, he is working as a research geologist in Statoil, where he is involved in outcrop analog studies and pore-structure characterization and its related physical properties in carbonates. Gregor P. Eberli is a professor in the Division of Marine Geology and Geophysics at the University of Miami and the director of the Comparative Sedimentology Laboratory. He received his Ph.D. from the Swiss Institute of Technology (ETH) in Zürich, Switzerland. His research integrates the sedimentology, stratigraphy, and petrophysics of carbonates. With laboratory experiments and seismic modeling, his group tries to understand the physical expression of carbonates on log and in seismic data. He was a distinguished lecturer for AAPG (1996–1997), Joint Oceanographic Institutions (1997–1998), and the European Association of Geoscientists and Engineers (2005–2006). Ralf J. Weger is an assistant scientist with the Comparative Sedimentology Laboratory at the University of Miami. He received his B.S. degree in systems analysis (2000) and his Ph.D. in marine geology and geophysics (2006) from the University of Miami. His work focuses on quantitative pore- and rock-type parameters in carbonates and their relationship to velocity deviations. His main interests range from processing and visualization of geophysical data to petrophysical characterization of carbonate rocks.