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. 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.