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Line 124: − ''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 Figure 1.+ − + + + Line 134:
Line 136: − + − − [[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.]] − +
→Correlation and lithology
===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|{{figure number|1}}A typical log showing SP, gamma ray, dual Induction, and sonic measurements. Copyright: 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]].
===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.]]
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.
An advanced version of the gamma ray tool, called the ''spectral gamma ray'', breaks down or segments the detected gamma rays by their different energies using spectral analysis techniques. These segments correspond to the radioactive families of potassium, uranium, and thorium. Uranium frequently occurs as a precipitated salt deposited in a formation from waters having flown through that formation. When this occurs, the uranium counts disguise radioactivity due to mineralogy. The use of the spectral tool allows the removal of gamma ray counts caused by uranium, typically permitting more accurate use of the remaining gamma rays for determining lithology, volume shale, or volume clay. In some local areas, ratios of potassium to thorium have been successfully used to determine some clay types. However, this clay typing has not proven particularly universal and should be attempted with much caution.
An advanced version of the gamma ray tool, called the ''spectral gamma ray'', breaks down or segments the detected gamma rays by their different energies using spectral analysis techniques. These segments correspond to the radioactive families of potassium, uranium, and thorium. Uranium frequently occurs as a precipitated salt deposited in a formation from waters having flown through that formation. When this occurs, the uranium counts disguise radioactivity due to mineralogy. The use of the spectral tool allows the removal of gamma ray counts caused by uranium, typically permitting more accurate use of the remaining gamma rays for determining lithology, volume shale, or volume clay. In some local areas, ratios of potassium to thorium have been successfully used to determine some clay types. However, this clay typing has not proven particularly universal and should be attempted with much caution.
Typical presentations of gamma ray measurements are shown in the logs in both Figures 1 and 2. (For information on the cased hole gamma ray tool, see the chapter on “[[Basic cased hole tools]]” in Part 4.)
Typical presentations of gamma ray measurements are shown in the logs in both [[::file:basic-open-hole-tools_fig1.png|Figures 1]] and [[:file:basic-open-hole-tools_fig2.png|2]]. (For information on the cased hole gamma ray tool, see the chapter on “[[Basic cased hole tools]])
===Photoelectric effect===
===Photoelectric effect===
The photoelectric effect, or Pe, measures a formation's ability to absorb gamma rays. The absorptive abilities of formations vary with lithology. The photoelectric absorption is recorded as a supplementary measurement to the formation density measurement, using common detectors and radioactive sources. Since this measurement is part of the density measurement, the tool is a pad contact tool and is subject to borehole wall rugosity. The measurement is not valid in muds weighted with barite. The recording can be used both for correlation of formations between wells and for determining lithology. A typical presentation of Pe is shown in the log in Figure 2.
The photoelectric effect, or Pe, measures a formation's ability to absorb gamma rays. The absorptive abilities of formations vary with lithology. The photoelectric absorption is recorded as a supplementary measurement to the formation density measurement, using common detectors and radioactive sources. Since this measurement is part of the density measurement, the tool is a pad contact tool and is subject to borehole wall rugosity. The measurement is not valid in muds weighted with barite. The recording can be used both for correlation of formations between wells and for determining lithology. A typical presentation of Pe is shown in the log in [[:file:basic-open-hole-tools_fig2.png|Figure 2]].
==Resistivity==
==Resistivity==