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[[File:M91Ch6FG43.JPG|thumb|300px|{{figure number|4}}Gamma-ray, density, neutron, and sonic log response of a sandstone and shale sequence. This example is from well 16/29a-9 in the Fleming field, UK North Sea (from Stuart, 2002). Reprinted with permission from the Geological Society, whose permission is required for further use.]]

[[File:M91Ch6FG43.JPG|thumb|300px|{{figure number|4}}Gamma-ray, density, neutron, and sonic log response of a sandstone and shale sequence. This example is from well 16/29a-9 in the Fleming field, UK North Sea (from Stuart<ref name=Stuart_2002>Stuart, I. A., 2002, The Armada development, UK central North Sea: The Fleming, Drake and Hawkins gas-condensate fields, in J. G. Gluyas and H. M. Hichens, eds., United Kingdom oil and gas fields, commemorative millennium volume: Geological Society (London) Memoir 20, p. 139–151.</ref>). Reprinted with permission from the Geological Society of London, whose permission is required for further use.]]

==Gamma-ray logs==

==Gamma-ray logs==

==Spectral gamma-ray logs==

==Spectral gamma-ray logs==

Spectral gamma-ray logs measure the relative contribution of potassium, thorium, and uranium to the overall gamma-ray response. A high potassium content generally indicates the presence of minerals such as potassium feldspar and mica. Thorium is associated with the mineral monazite, a common heavy mineral in sandstones sourced from acid igneous rocks (Hurst and Milodowski, 1996). Uranium is commonly found absorbed onto organic material and clay in marine shales (Serra, 1984).

Spectral gamma-ray logs measure the relative contribution of potassium, thorium, and uranium to the overall gamma-ray response. A high potassium content generally indicates the presence of minerals such as potassium feldspar and mica. Thorium is associated with the mineral monazite, a common heavy mineral in sandstones sourced from acid igneous rocks.<ref name=Hurstandmilodowski_1996>Hurst, A., and A. Milodowski, 1996, Thorium distribution in some North Sea sandstones: Implications for petrophysical evaluation: Petroleum Geoscience, v. 2, no. 1, p. 69–68.</ref> Uranium is commonly found absorbed onto organic material and clay in marine shales.<ref name=Serra_1984 />

Spectral gamma-ray logs are used less frequently than the other types of log, although in certain situations they can pick out features that the other logs will not (Hancock, 1992). For example, the spectral gamma-ray log response can be used to identify a zone of potassium feldspar dissolution in leached sandstone below an unconformity.

Spectral gamma-ray logs are used less frequently than the other types of log, although in certain situations they can pick out features that the other logs will not.<ref name=Hancock_1992>Hancock, N. J., 1992, [[Quick-look lithology from logs]], in D. Morton-Thompson and A. M. Woods, eds., [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 174–179.</ref> For example, the spectral gamma-ray log response can be used to identify a zone of potassium feldspar dissolution in leached sandstone below an unconformity.

==Density and neutron logs==

==Density and neutron logs==

Density and neutron logs are primarily used for estimating the porosity. Density logs measure the bulk density of a formation, a function of the rock matrix density emitted from the log and the density of the fluids in the pore space, according to the degree by which the energy of gamma rays is progressively absorbed and scattered by electrons in the rock. The principle behind the density log is that, for a rock with a given grain and fluid density, the higher the porosity, the less dense the formation will be. A neutron log bombards the formation with neutrons to detect energy changes as a result of collisions with hydrogen atoms. Hydrogen is found in the water (and oil) molecules filling the pore space. Thus the neutron log gives an indication of the formation porosity (Rider, 1996).

Density and neutron logs are primarily used for estimating the porosity. Density logs measure the bulk density of a formation, a function of the rock matrix density emitted from the log and the density of the fluids in the pore space, according to the degree by which the energy of gamma rays is progressively absorbed and scattered by electrons in the rock. The principle behind the density log is that, for a rock with a given grain and fluid density, the higher the porosity, the less dense the formation will be. A neutron log bombards the formation with neutrons to detect energy changes as a result of collisions with hydrogen atoms. Hydrogen is found in the water (and oil) molecules filling the pore space. Thus the neutron log gives an indication of the formation porosity.<ref name=Rider_1996 />

The logs also have specific geological uses. They can be used to pick out cemented intervals in sandstones. Carbonate-cemented intervals will show a distinctive response on these logs.

The logs also have specific geological uses. They can be used to pick out cemented intervals in sandstones. Carbonate-cemented intervals will show a distinctive response on these logs.

==Sonic logs==

==Sonic logs==

A sonic log measures the time it takes for a sound pulse to travel from a transmitter to a receiver via the formation (Rider, 1996). Sonic logs can be used for measuring porosity but are more commonly used by the geophysicist as they give velocity information for calibrating seismic data. Velocity data allow the geophysicist to convert the time taken for a seismic wave to travel down and back from a specific seismic reflector into an equivalent subsurface depth. The geologist can use sonic logs to pick out coals and poorly consolidated sandstones.

A sonic log measures the time it takes for a sound pulse to travel from a transmitter to a receiver via the formation.<ref name=Rider_1996 /> Sonic logs can be used for measuring porosity but are more commonly used by the geophysicist as they give velocity information for calibrating seismic data. Velocity data allow the geophysicist to convert the time taken for a seismic wave to travel down and back from a specific seismic reflector into an equivalent subsurface depth. The geologist can use sonic logs to pick out coals and poorly consolidated sandstones.

==Electrical logs==

==Electrical logs==

Electrical logs measure the resistivity of the rock and its contained fluids to the passage of an electrical current (Rider, 1996). A high-resistivity response within a porous rock is an indication of hydrocarbons. The logs can also help to recognize certain lithologies. Tight cemented intervals will have a high-resistivity response and these can be picked out in combination with the density and neutron log response.

Electrical logs measure the resistivity of the rock and its contained fluids to the passage of an electrical current.<ref name=Rider_1996 /> A high-resistivity response within a porous rock is an indication of hydrocarbons. The logs can also help to recognize certain lithologies. Tight cemented intervals will have a high-resistivity response and these can be picked out in combination with the density and neutron log response.

==Nuclear magnetic resonance logs==

==Nuclear magnetic resonance logs==

==Dipmeter logs==

==Dipmeter logs==

Dipmeter logs measure the variation in electrical or sonic response around the circumference of the borehole. From this, formation dip and sometimes the orientation of sedimentary structures can be determined (Bourke, 1992; Cameron, 1992).

Dipmeter logs measure the variation in electrical or sonic response around the circumference of the borehole. From this, formation dip and sometimes the orientation of sedimentary structures can be determined.<ref name=Bourke_1992>Bourke, L. T., 1992, Sedimentological borehole image analysis in clastic rocks: A systematic approach to interpretation, in A. Hurst, C. M. Griffiths, and P. F. Worthington, eds., Geological application of wire-line logs II: Geological Society Special Publication 65, p. 31–42.</ref> <ref name=Cameron_1992>Cameron, G. I. F., 1992, Analysis of dipmeter data for sedimentary orientation, in A. Hurst, C. M. Griffiths, and P. F. Worthington, eds., Geological application of wire-line logs II: Geological Society Special Publication 65, p. 141–154.</ref>

==Borehole image logs==

==Borehole image logs==

Borehole image logs give a detailed electrical or sonic map of the borehole wall (Luthi, 1992). This enables geological information such as formation dip, sedimentary structures, faulting, and fracturing to be imaged. The dip and azimuths of these features are measured from the image logs. The logs are especially useful for the structural characterization of heavily faulted and fractured reservoirs. They also show thin beds in reservoir intervals where most conventional logs do not have the resolution to detect them.

Borehole image logs give a detailed electrical or sonic map of the borehole wall.<ref name=Luthi_1992>Luthi, S. M., 1992, [[Borehole imaging devices]] in D. Morton-Thompson and A. M. Woods, eds., [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 163–166.</ref> This enables geological information such as formation dip, sedimentary structures, faulting, and fracturing to be imaged. The dip and azimuths of these features are measured from the image logs. The logs are especially useful for the structural characterization of heavily faulted and fractured reservoirs. They also show thin beds in reservoir intervals where most conventional logs do not have the resolution to detect them.

==Formation tester logs==

==Formation tester logs==

When these tests are conducted in a virgin reservoir preproduction, it may be possible to define the depth of the free-water level. This will correspond to the intersection of the water and oil (gas) gradients. Postproduction, formation tester data can give information on where the reservoir is separating into zones of different pressures as a result of depletion ([[:file:M91Ch06FG44.JPG|Figure 5]]).

When these tests are conducted in a virgin reservoir preproduction, it may be possible to define the depth of the free-water level. This will correspond to the intersection of the water and oil (gas) gradients. Postproduction, formation tester data can give information on where the reservoir is separating into zones of different pressures as a result of depletion ([[:file:M91Ch06FG44.JPG|Figure 5]]).

The raw log data will show the rate at which the pressure built up for each test, and a crude assessment of the formation permeability can be made from this (Smolen, 1992a).

The raw log data will show the rate at which the pressure built up for each test, and a crude assessment of the formation permeability can be made from this.<ref name=Smolen_1992a>Smolen, J. J., 1992, [[Wire-line formation testers]], in D. Morton-Thompson and A. M. Woods, eds., [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 154–157.</ref>

==Wireline coring==

==Wireline coring==

Wireline methods such as sidewall coring allow the retrieval of several short plug-type cores from the borehole wall. A series of wire-attached, hollow steel bullets are fired horizontally into the borehole wall from the wireline tool (Rider, 1996). Sidewall cores are mainly used for lithology determination and biostratigraphic analysis.

Wireline methods such as sidewall coring allow the retrieval of several short plug-type cores from the borehole wall. A series of wire-attached, hollow steel bullets are fired horizontally into the borehole wall from the wireline too.<ref name=Rider_1996 /> Sidewall cores are mainly used for lithology determination and biostratigraphic analysis.

==Checkshot and vertical seismic profiles==

STARTHERE==Checkshot and vertical seismic profiles==

Checkshots and vertical seismic profiles (VSPs) are used by the geophysicist to record velocity information in a well. A checkshot survey is taken at different depths down the borehole (Hardage, 1992). A log with a geophone for detecting seismic signals is run in the hole at the same time as a seismic source is activated at the surface. The distance between the source and the log is established, and the time taken for the signal to travel to the log is measured. From this, an accurate velocity can be calculated.

Checkshots and vertical seismic profiles (VSPs) are used by the geophysicist to record velocity information in a well. A checkshot survey is taken at different depths down the borehole (Hardage, 1992). A log with a geophone for detecting seismic signals is run in the hole at the same time as a seismic source is activated at the surface. The distance between the source and the log is established, and the time taken for the signal to travel to the log is measured. From this, an accurate velocity can be calculated.