Resistivity/porosity/velocity relationships from downhole logs: an aid for evaluating pore morphology

Relationships between downhole resistivity/velocity logs and porosity styles, controlled by cementation/dissolution/recrystallization, are investigated using core and downhole logging data from the carbonate-rich sediments encountered during Leg 133 of the Ocean Drilling Program (ODP), northeastern Australia off the Great Barrier Reef. It is shown that although resistivity and velocity are controlled by porosity, the "connectiveness" of the solid phase (velocity) and fluid phase (resistivity) may be more important controls than porosity in these environments. Velocity logs in this environment are shown to be controlled primarily by the bulk properties of the solid fraction and fluid phases, with interparticle cementation causing significant increases in velocity during lithification. Downhole electrical resistivity logs, in contrast to velocity logs, are shown to respond to the fluid phase in these marine carbonate sediments, where the terrigenous clay fraction is small. Resistivity is shown to be dependent on the connectivity of the pore space in addition to its bulk porosity. The Archie m exponent (derived from resistivity and porosity) is shown (1) to increase to values above 3 when poorly connected vuggy or moldic styles dominate porosity and (2) to be independent of the strength of interparticle cements. Velocity and resistivity log responses are compared in different diagenetic carbonate environments. The effects of diagenetic processes in shallow-water reefal carbonates are contrasted with the effects of the normal compaction/lithification depth profile in carbonate-rich hemipelagic sediments. Signatures are identified in velocity/resistivity responses that are diagnostic of different diagenetically controlled porosity styles. For example, log responses from well-connected porosity developed in a dolomite, are distinguished from that in sediments having similar porosity, but extensive recrystallization, which may block the pore throats. A dimensionless velocity/resistivity ratio (VR) is proposed to quantify and to enhance the identification of cementation/dissolution/connectivity styles. A theoretical model is proposed that will predict the velocity vs. resistivity (and hence VR) relationships attributed to these different porosity styles. Within a lithified rock, a high VR ratio is shown to represent well-connected porosity, whereas a low VR ratio is shown to represent poorly connected porosity where the pore throats are blocked. This VR ratio is used to identify two substantially different trends within two Miocene reef units identified at Site 816 (Davies, McKenzie, Palmer-Julson, et al., 1991). Davies, McKenzie, Palmer-Julson, et al. (1991) and Pigram et al. (this volume) showed that there is evidence of subaerial exposure and fresh water diagenesis within both these reef units. We observed a trend of increasing fabric destruction with depth within the deeper Miocene reef unit at Site 816. Although dolomitization is complete within this unit, matrix recrystalization is seen to increase with depth. We postulate that recrystallization (where the matrix is now composed of dolomite rhombs having an open intercrystalline porosity) has lead to the generation of connected porosity at Site 816 that is seen as high values of the VR ratio. Thin layers having very high VR ratios were identified in the lower unit. One such layer was studied in more detail using formation microscanner (FMS) images. This 0.6-m-thick layer of distinctly lower resistivity has an internal structure that suggests that this may be a solution feature and might constitute a significant pathway for fluid flow in the formation.