Interpreting sediment structure from fine-scale resistivity imaging

Figure 1. Structural detail of a dune-bedded sandstone: (a) rock surface; (b) electrical image: length 4mm (current 12 mA); (c) electrical image: length 1 mm (current 30 mA)

BGS has been developing laboratory techniques to produce fine-scale resistivity images of sedimentary core material as a means to investigate fine sediment structure. The techniques complement other core-log procedures for the detailed characterisation of hydrocarbon reservoir rocks and have wider applications in sedimentological investigation.

Resistivity imaging

The methodology developed uses the well-established relationship between electrical resistivity of porous rock and its porosity and fluid saturation. Both contact and non-contact techniques are being developed, with the aim of producing quantitative resistivity data at a resolution similar to that achieved by modern downhole imaging devices.

The system is composed of a timer, constant-current source, signal conditioning modules and hardware and software for data acquisition and robot control. A current is driven into the saline saturated rock core sample and generates potentials that are measured at points along the core surface. The system uses a robotically controlled voltage electrode capable of micron-scale movement for the capture of high-resolution resistivity data. A two-dimensional resistivity map of the core sample is produced, revealing the fine structure of the rock core and providing detail on porosity and grainsize-distribution. Porous rocks can be studied using the system in constant current, variable voltage mode. Low-porosity rocks can be investigated in constant voltage, variable current mode.

Further information

Haslam, E., Gunn, D. A., Jackson, P.D. and Lovell, M.A. 2011. Fine scale resistivity structure from contact measurement of potential field. Petrophysics.

Contact

Contact Ed Haslam for further information