Borehole Logs
Resistivity - tool measures resistivity (as opposed to resistance) with 2 current electrodes and 2 measurement electrodes. 3 different configurations: short-normal, long-normal, and lateral. Short normal has the smallest distance between 2 adjacent electrodes (16 inches). It is the most sensitive to thin layers but is also influenced by the drilling mud. Lateral log has the longest distance between two adjacent electrodes 18 feet 8 inches). It samples resistivity over a large section of sediment/rock away from the borehole. Lateral log may miss thin beds.
Spontaneous Potential - natural processes produce an electrical current.
Occurrence of Self-Potentials
Liquid-junction - when formaton water and mud filtrate have different salinities, ions migrate from the high salinity fluid to the low salinity fluid. Chloride anions migrate more readily than cations, which produces an electrical current.
Membrane - Ions also migrate through the shale. Cations migrate more readily than chloride anions, which produces an electrical current.
Measurement of self-potentials - Voltage is measured between a ground electrode at the surface and an electrode lowered down the well. When formation water is saltier than the drilling mud, shale has a positive SP response and sand a negative SP response.
Interpretation and Application - SP logs are used to distinguish shale from sand and to estimate percent sand. In addition, it can be used to estimate pore water salinity in clean sands. The SP log is also used in correlation work especially in conjunction with resistivity logs.
Gamma Ray - Natural decay of potassium 40. Feldspars and micas are rich in potassium. High levels of radiation next to clay layers. Log can be run in a cased well.