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===Spontaneous potential===
===Spontaneous potential===
[[file:basic-open-hole-tools_fig1.png|thumb|left|{{figure number|1}}A typical log showing SP, gamma ray, dual Induction, and sonic measurements. Copyright: Schlumberger, 1983.]]
[[file:basic-open-hole-tools_fig1.png|thumb|left|{{figure number|1}}A typical log showing SP, gamma ray, dual Induction, and sonic measurements. © Schlumberger, 1983.]]
''Spontaneous potential'' (SP) is a natural voltage or electrical potential that arises due to differences in the ionic activities (relative saltiness) of the drilling mud and the formation waters. This potential can be used to correlate formations between wells, to indicate permeability, and to estimate formation water resistivity. No SP occurs when oil-based mud is used in the borehole. Hydrocarbons and shaliness in the formation suppress the SP. The magnitude of the SP decreases as the resistivity of the mud filtrate and formation waters approach a common resistivity. The direction of SP deflection reverses as the ratio of the resistivity of the mud filtrate (''R''<sub>mf</sub>) to that of the formation water (''R''<sub>w</sub>) reaches 1.0 or more. If there is no contrast in the mud filtrate and formation water salinities, there is no measurable SP. A typical presentation of SP is shown on the left of the log in [[:file:basic-open-hole-tools_fig1.png|Figure 1]].
''Spontaneous potential'' (SP) is a natural voltage or electrical potential that arises due to differences in the ionic activities (relative saltiness) of the drilling mud and the formation waters. This potential can be used to correlate formations between wells, to indicate permeability, and to estimate formation water resistivity. No SP occurs when oil-based mud is used in the borehole. Hydrocarbons and shaliness in the formation suppress the SP. The magnitude of the SP decreases as the resistivity of the mud filtrate and formation waters approach a common resistivity. The direction of SP deflection reverses as the ratio of the resistivity of the mud filtrate (''R''<sub>mf</sub>) to that of the formation water (''R''<sub>w</sub>) reaches 1.0 or more. If there is no contrast in the mud filtrate and formation water salinities, there is no measurable SP. A typical presentation of SP is shown on the left of the log in [[:file:basic-open-hole-tools_fig1.png|Figure 1]].
===Gamma ray===
===Gamma ray===
[[file:basic-open-hole-tools_fig2.png|thumb|{{figure number|2}}A typical log showing density, compensated neutron, Pe, gamma ray, and caliper measurements. Copyright: Schlumberger, 1983.]]
[[file:basic-open-hole-tools_fig2.png|thumb|{{figure number|2}}A typical log showing density, compensated neutron, Pe, gamma ray, and caliper measurements. © Schlumberger, 1983.]]
Gamma rays tools measure the natural radioactivity of the formation. This radioactivity is emitted primarily from potassium in the structure of clay minerals, radioactive salts in the formation waters, radioactive salts bound to the charged surfaces of clay minerals, potassium associated with feldspars, and radioactive minerals associated with igneous rocks and rock fragments. The gamma ray response is used for correlation of formations between wells and for estimating volume shale and/or volume clay minerals.
Gamma rays tools measure the natural radioactivity of the formation. This radioactivity is emitted primarily from potassium in the structure of clay minerals, radioactive salts in the formation waters, radioactive salts bound to the charged surfaces of clay minerals, potassium associated with feldspars, and radioactive minerals associated with igneous rocks and rock fragments. The gamma ray response is used for correlation of formations between wells and for estimating volume shale and/or volume clay minerals.