Changes

==Gamma ray logs==

==Gamma ray logs==

The common radioactive elements—potassium, thorium, and uranium—are normally insignificant in [[reservoir fluids]], whereas they are important components of the rock system, especially of clay minerals. [[Basic open hole tools#Gamma ray|Gamma ray]] logs are therefore a good indicator of mineralogy.

The common radioactive elements—potassium, thorium, and uranium—are normally insignificant in [[reservoir fluids]], whereas they are important components of the rock system, especially of clay minerals. [[Basic open hole tools#Gamma ray|Gamma ray]] logs are therefore a good indicator of [[mineralogy]].

===Lithological responses===

===Lithological responses===

! Lithology || Gamma Ray Values (in API units)

! Lithology || Gamma Ray Values (in API units)

|-

|-

| [[Sandstone]] (quartz) || 15–30 (rarely to 200)

| [[Sandstone]] ([[quartz]]) || 15–30 (rarely to 200)

|-

|-

| [[Limestone]] || 10–40

| [[Limestone]] || 10–40

[[file:quick-look-lithology-from-logs_fig1.png|thumb|300px|{{figure number|1}}Characteristic log shapes for different types of sand bodies set in shale, (a) Funnel shape, coarsening upward. Note that this is the shallowest interval, so the shale is least compacted. (b) Cylinder shape, blocky. Note that the SP log is featureless because the borehole salinity is the same as formation salinity. (c) Bell shape, fining upward. Note that coal is present in addition to shale.]]

[[file:quick-look-lithology-from-logs_fig1.png|thumb|300px|{{figure number|1}}Characteristic log shapes for different types of sand bodies set in shale, (a) Funnel shape, coarsening upward. Note that this is the shallowest interval, so the shale is least compacted. (b) Cylinder shape, blocky. Note that the SP log is featureless because the borehole salinity is the same as formation salinity. (c) Bell shape, fining upward. Note that coal is present in addition to shale.]]

The shape of a gamma ray (or SP) log through a sand body is often thought of as a grain size profile. Three basic log shapes are recognized: funnel (coarsening upward), cylinder (blocky), and bell (fining upward) ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1]]). These three shapes can be subdivided into smooth (relatively homogeneous) or serrate (with interbedded thin shales).

The shape of a gamma ray (or SP) log through a sand body is often thought of as a [[grain size]] profile. Three basic log shapes are recognized: funnel (coarsening upward), cylinder (blocky), and bell (fining upward) ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1]]). These three shapes can be subdivided into smooth (relatively homogeneous) or serrate (with interbedded thin shales).

Log shapes typically reflect changing depositional energy from high (clean, coarser sand) to low (shaly, finer sand). An interpretive jump is usually made from depositional energy to depositional process and hence depositional environment. Often this jump is made without seriously considering the intermediate steps. This can be dangerous. Each of the steps is highly ambiguous and must be augmented by other evidence, such as unit thickness, associated rock types, and overall depositional setting. Typically,

Log shapes typically reflect changing depositional energy from high (clean, coarser sand) to low (shaly, finer sand). An interpretive jump is usually made from depositional energy to depositional process and hence depositional environment. Often this jump is made without seriously considering the intermediate steps. This can be dangerous. Each of the steps is highly ambiguous and must be augmented by other evidence, such as unit thickness, associated rock types, and overall depositional setting. Typically,

* ''Radioactive minerals'' in sands, especially K-feldspar, zircon, and mica, can raise sand readings as high as adjacent shales. Gamma ray logs may be useless in immature sands derived from [[basement]] terranes. However, beach placers rich in zircon may be valuable correlative markers if not mistaken for shale.

* ''Radioactive minerals'' in sands, especially K-feldspar, zircon, and mica, can raise sand readings as high as adjacent shales. Gamma ray logs may be useless in immature sands derived from [[basement]] terranes. However, beach placers rich in zircon may be valuable correlative markers if not mistaken for shale.

* ''“Hot” dolomite'', especially common in the Permian basin in the United States, may have gamma ray values up to 200 API units, resembling shale.

* ''“Hot” dolomite'', especially common in the [[Permian basin]] in the United States, may have gamma ray values up to 200 API units, resembling shale.

* ''Radioactive (KCl) muds'' raise the baseline gamma ray zero reading so that apparent values for all rock types are increased, sometimes by about 20 API units.

* ''Radioactive (KCl) muds'' raise the baseline gamma ray zero reading so that apparent values for all rock types are increased, sometimes by about 20 API units.

* ''Evanescent high gamma ray'' readings in sands, present on one logging run but vanished some weeks later, have been observed especially in steamflood conditions. While remaining enigmatic, these may be due to concentrations of radon in the pore space.

* ''Evanescent high gamma ray'' readings in sands, present on one logging run but vanished some weeks later, have been observed especially in steamflood conditions. While remaining enigmatic, these may be due to concentrations of radon in the pore space.

* ''Clay minerals''. Illite clays are rich in potassium, whereas smectite and kaolinite contain thorium. The thorium to potassium ratio can distinguish illitic from smectitic shales and so provide a correlation tool.

* ''Clay minerals''. Illite clays are rich in potassium, whereas smectite and kaolinite contain thorium. The thorium to potassium ratio can distinguish illitic from smectitic shales and so provide a correlation tool.

* ''Organic-rich rocks''. In shales, uranium enrichment is usually associated with organic content and can be a tool for identifying oil source beds. Quantitative relationships between uranium and organic content have been reported, but tend to be inconsistent.

* ''Organic-rich rocks''. In shales, uranium enrichment is usually associated with organic content and can be a tool for identifying oil source beds. Quantitative relationships between uranium and organic content have been reported, but tend to be inconsistent.

* ''Mica sand''. Richly micaceous sands (such as the Rannoch unit of the Brent Sand in the North Sea) appear shaly on gamma ray logs, but can be distinguished because the radiation is all from potassium.

* ''Mica sand''. Richly micaceous sands (such as the Rannoch unit of the Brent Sand in the [[North Sea]]) appear shaly on gamma ray logs, but can be distinguished because the radiation is all from potassium.

* ''“Hot” dolomite''. This type of dolomite can be distinguished from shale because the gamma rays are principally from uranium. The chemical relationship between uranium and the dolomite is unknown.

* ''“Hot” dolomite''. This type of dolomite can be distinguished from shale because the gamma rays are principally from uranium. The chemical relationship between uranium and the dolomite is unknown.

* ''Natural fractures''. Soluble uranium in pore water often precipitates on open fractures, so thin intervals with high uranium count (a “spiky” log) may mark a fractured interval.

* ''Natural fractures''. Soluble uranium in pore water often precipitates on open fractures, so thin intervals with high uranium count (a “spiky” log) may mark a fractured interval.

====Anhydrite and gypsum====

====Anhydrite and gypsum====

Anhydrite and gypsum frequently remain in gauge if pure, but shaly intervals may be washed out.

Anhydrite and [[gypsum]] frequently remain in gauge if pure, but shaly intervals may be washed out.

====Halite and potash salts====

====Halite and potash salts====

====Sandstone====

====Sandstone====

Quartz should read 1.7 to 1.8 barns/electron, but most other minerals can raise the value substantially. Because they are usually present, the log is of limited value.

[[Quartz]] should read 1.7 to 1.8 barns/electron, but most other minerals can raise the value substantially. Because they are usually present, the log is of limited value.

====Limestone====

====Limestone====

===Overlay presentation===

===Overlay presentation===

Manual crossplotting is tedious. A much faster way to visualize rock type is directly from the overlay presentation in which both neutron and density logs are superimposed in the same log track. To do this, a compatible scale must be used so that the porosity components of both logs exactly overlay. Then any offset (or residual) between the two logs is attributable to lithology or to the presence of gas.

Manual crossplotting is tedious. A much faster way to visualize rock type is directly from the overlay presentation in which both neutron and density logs are superimposed in the same log track. To do this, a compatible scale must be used so that the porosity components of both logs exactly overlay. Then any [[offset]] (or residual) between the two logs is attributable to lithology or to the presence of gas.

Both tools are generally calibrated in limestone units, so the compatible scale is defined for freshwater-limestone systems, with theoretical limits as follows:

Both tools are generally calibrated in limestone units, so the compatible scale is defined for freshwater-limestone systems, with theoretical limits as follows:

====Sandstone (Oil or Water Filled)====

====Sandstone (Oil or Water Filled)====

Clean quartz sandstones give the typical two-division neutron-density cross-over with density to the left of neutron ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1]]). The addition of some clay (forming shaly sandstone) increases the neutron reading, reducing log crossover or even reversing it to create separation. Check natural gamma ray for evidence of increasing clay.

Clean [[quartz]] sandstones give the typical two-division neutron-density cross-over with density to the left of neutron ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1]]). The addition of some clay (forming shaly sandstone) increases the neutron reading, reducing log crossover or even reversing it to create separation. Check natural gamma ray for evidence of increasing clay.

Heavier components such as mica increase the density, reducing log cross-over or even reversing it to create separation. Check spectral gamma ray to distinguish the following:

Heavier components such as mica increase the density, reducing log cross-over or even reversing it to create separation. Check spectral gamma ray to distinguish the following:

[[Category:Wireline methods]]

[[Category:Wireline methods]]