Dictionary:SP

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1. Source Point or ShotPoint. 2. Spontaneous potential or self-potential, a well log of the difference between the potential of a movable electrode in the borehole and a fixed reference electrode at the surface. The SP results from electrochemical SP and electrokinetic potentials (q.v.) which are present at the interfacebetween permeable beds adjacent to shale. In impermeable shales, the SP is fairly constant at the shale-baseline value (see Figure S-17). In permeable formations the deflection depends on the contrast between the ion content of the formation water and the drilling fluid, the clay content, the bed thickness, invasion, bed-boundary effects, etc. In thick, permeable, clean, nonshale formations, the SP has the fairly constant sand-line value, which changes if the salinity of the formation water changes. In sands containing disseminated clay (shale), the SP will not reach the sand line and a pseudostatic SP value will be recorded. The SP is positive with respect to the shale baseline in sands filled with fluids fresher than the borehole fluid. See also SSP. An SP log (like the gamma-ray log, with which it generally correlates) is used to distinguish sands from shales and to determine qualitatively the abundance of clay-particles in a sand body, also spoken of as ‘‘clean’’ or ‘‘dirty’’ and as ‘‘coarsening’’ or ‘‘fining’’ (see Figure C-6). 3. The natural ground voltage observed between nearby nonpolarizing electrodes in field surveying. In many mineralized areas this is caused by electrochemical reaction at an electrically conducting sulfide body. In geothermal areas, SP can be caused by the motion of ions (streaming potential) or from contrasts in temperature. Compare induced polarization.