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==Wireline and LWD logs==
==Wireline and LWD logs==
Wireline logs are run in wells to determine the physical properties of the rock and fluids in the borehole (Table 9). From this, a detailed interpretation can be made of the geology and fluid saturations in the reservoir interval. A brief summary of these logs is provided here. For more details, the textbooks by Serra (1984), Rider (1996), and Luthi (2001) can be consulted.
Wireline logs are run in wells to determine the physical properties of the rock and fluids in the borehole (Table 3). From this, a detailed interpretation can be made of the geology and fluid saturations in the reservoir interval. A brief summary of these logs is provided here. For more details, the textbooks by Serra (1984), Rider (1996), and Luthi (2001) can be consulted.
{{Table 9
{| class = "wikitable"
|-
|+ {{table number|3}}Main open-hole log types
|-
! Log || How it works || What it is used for
|-
| Gamma ray log || Measures the natural gamma-ray response of the rock || Well-log correlation, lithology identification; ideal for recognizing shales
|-
| Spectral gamma-ray log || As above, but with a more sensitive detector to pick out the individual contribution of potassium thorium, and uranium to the gamma-ray response || Knowledge of potassium, thorium, and uranium variation in the rocks can be useful for evaluating mineralogy and depositional environments
|-
| Spontaneous potential log || Measures the potential difference driving the electrical current, that results from salinity differences between the drilling mud and formation water in permeable rocks downhole || Gives a rough indication of lithology and is used for the evaluation of formation water resistivity
|-
| Electrical logs || Measures the electrical properties of the fluid in the rock || Can indicate if hydrocarbons are present or not
|-
| Density and neutron logs || Measures the formation density and volume of fluids in the rock, respectively || An estimate of porosity can be made; also allows the identification of certain lithologies such as limestone, anhydrite, and halite
|-
| Sonic log || Measures how fast an acoustic signal can pass through a rock || An estimate of porosity can be made; also used for seismic calibration
|-
| Nuclear magnetic resonance log || Determines the nuclear magnetic response of the fluids in the rock || provides data that allows porosity and permeability to be estimated
|-
| Dipmeter logs || Measures the electrical or sonic response of the rocks around the borehole || Used to calculate formation dip, pick out faults and other structures, and sometimes determine the sedimentary structure for paleocurrent analysis
|-
| Borehole image logs || Measures a detailed profile of the electrical or sonic response of the rocks in the borehole || Gives an indication of hole conditions that can affect the reliability of the log responses
|-
| Caliper log || Measures the diameter and shape of the borehole || Gives an indication of hole conditions that can affect the reliability of the log responses
|-
| Wireline coring including sidewall coring tool || Takes several short core plugs from the borehole wall || Lithological determination and rock sampling for biostratigraphy
|-
| Checkshot and vertical seismic profile log || Measures velocity data at specific borehole depths || Used to calibrate the seismic response
|-
| Formation tester log || Measures pressures at specific points in the reservoir and can allow small volumes of fluid to be sampled || Establish a pressure profile for the reservoir and define fluid contacts
|}
[[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, 2002). Reprinted with permission from the Geological Society, whose permission is required for further use.]]