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The spontaneous potential (SP) log was one of the earliest measurements used in the petroleum industry, and it has continued to play a significant role in well log interpretation. Most wells today have this type of log included in their log suites. Primarily, the SP log is used for determining gross lithology (i.e., reservoir vs. nonreservoir) through its ability to distinguish permeable zones (such as sandstones) from impermeable zones (such as shales). It is also used to correlate zones between wells. However, as will be discussed later in this chapter, the SP log has several other uses that are perhaps equally important.

The SP log is a record of direct current (DC) voltage (or potential) that develops naturally (or spontaneously) between a moveable electrode in the well bore and a fixed electrode located at the surface (Doll, 1948). It is measured in millivolts (mV). Electric voltages arising primarily from electrochemical factors within the borehole and the adjacent rock create the SP log response. These electrochemical factors are brought about by differences in salinities between mud filtrate and formation water within permeable beds. Salinity of a fluid is inversely proportional to its resistivity, and in practice salinity is indicated by mud filtrate resistivity (Rmf) and formation water resistivity (Rw). Because a conductive fluid is needed in the borehole for the generation of these voltages, the SP cannot be used in nonconductive (e.g., oil-base) drilling muds or in air-filled holes.

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