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Quick-look lithology from logs (view source)
Revision as of 19:39, 18 January 2022
, 19:39, 18 January 2022added Category:Methods in Exploration 10 using HotCat
(For more details on these logs, see [[Basic open hole tools]]. Also, [[Difficult lithologies]] covers logging tool response in sedimentary minerals.)
(For more details on these logs, see [[Basic open hole tools]]. Also, [[Difficult lithologies]] covers logging tool response in sedimentary minerals.)
Borehole imaging tools such as the Formation MicroScanner are invaluable for detailed purposes, including bedding character and sedimentary structures, but are much less commonly available. (For more details, see [[Borehole imaging devices]].)
[[Borehole imaging devices|Borehole imaging tools]] such as the Formation MicroScanner are invaluable for detailed purposes, including bedding character and sedimentary structures, but are much less commonly available.
==[[Basic open hole tools#Gamma ray|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. 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===
{| class = "wikitable"
{| class = "wikitable"
|-
|-
! Lithology
! Lithology || Gamma Ray Values (in API units)
|-
|-
| Sandstone (quartz)
| [[Sandstone]] ([[quartz]]) || 15–30 (rarely to 200)
| 15–30 (rarely to 200)
|-
|-
| Limestone
| [[Limestone]] || 10–40
| 10–40
|-
|-
| Dolomite
| [[Dolomite]] || 15–40 (rarely to 200)
| 15–40 (rarely to 200)
|-
|-
| Shale
| [[Shale]] || 60–150
| 60–150
|-
|-
| Organic-rich shale
| [[Oil shale|Organic-rich shale]] || 100–250
| 100–250
|-
|-
| Anhydrite, halite
| [[Anhydrite]], [[halite]] || 8–15
| 8–15
|-
|-
| Sylvite (KCI)
| [[Sylvite]] (KCI) || 350–500
| 350–500
|-
|-
| Coal
| [[Coal]] || 15–150 (any value possible)
| 15–150 (any value possible)
|}
|}
[[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,
* Funnel shapes imply upward-increasing energy, which may be found in distributary mouth bars, delta lobe fringes, deep sea fans, and other environments.
* Funnel shapes imply upward-increasing energy, which may be found in distributary mouth bars, delta lobe fringes, deep sea fans, and other environments.
* Cylinder shapes reflect relatively constant energy levels and can include eolian dunes, low sinuosity distributary channels, and beaches.
* Cylinder shapes reflect relatively constant energy levels and can include eolian dunes, low sinuosity distributary channels, and beaches.
* Bell shapes represent waning-current sequences, which can include alluvial point bars, deltaic distributaries, and deep sea fan channels.
* Bell shapes represent waning-current sequences, which can include [[alluvial]] point bars, deltaic distributaries, and deep sea fan channels.
In fact, grain size has no effect on gamma ray logs. The log shapes reflect shaliness, that is, clay and mica content of the sand. Because most sands reflect a hydrodynamic equilibrium, clay content does usually correlate (inversely) with grain size. However, in the following examples, clay content and grain size do not correlate, resulting in misleading log shapes:
In fact, grain size has no effect on gamma ray logs. The log shapes reflect shaliness, that is, clay and mica content of the sand. Because most sands reflect a hydrodynamic equilibrium, clay content does usually correlate (inversely) with grain size. However, in the following examples, clay content and grain size do not correlate, resulting in misleading log shapes:
* Very fine, clean sand above coarser sand may show a cylinder shape.
* Very fine, clean sand above coarser sand may show a cylinder shape.
* Clay clasts concentrated near the base of a channel may give a funnel shape.
* Clay clasts concentrated near the base of a channel may give a funnel shape.
* Clay added later due to bioturbation or mechanical infiltration at the top of a gravel may create a bell shape.
* Clay added later due to [[bioturbation]] or mechanical infiltration at the top of a gravel may create a bell shape.
(For more details on using log shape to interpret depositional environment, see [[Lithofacies and environmental analysis of clastic depositional systems]].)
(For more details on using log shape to interpret depositional environment, see [[Lithofacies and environmental analysis of clastic depositional systems]].)
===Problems and exceptions===
===Problems and exceptions===
* ''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.
* ''Uranium prospecting''. Most of the “uranium” signal actually comes from the tenth decay process in the uranium series, the decay of bismuth-214. This is separated in time from the original uranium by half-lives in excess of 10<sup>9</sup> years, so the relatively soluble uranium may have moved away during the interim even though the log still records its presence.
* ''Uranium prospecting''. Most of the “uranium” signal actually comes from the tenth decay process in the uranium series, the decay of bismuth-214. This is separated in time from the original uranium by half-lives in excess of 10<sup>9</sup> years, so the relatively soluble uranium may have moved away during the interim even though the log still records its presence.
==[[Basic open hole tools#Spontaneous potential|Spontaneous potential]] (SP) logs==
==Spontaneous potential (SP) logs==
===Lithological responses===
===Lithological responses===
====Shale====
====Shale====
[[Basic open hole tools#Spontaneous potential|Spontaneous potential]] interpretation depends on first recognizing shale, where fairly constant SP readings form a straight “shale baseline” on the log ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1a]]). Its actual SP value is not significant.
====Sandstone====
====Sandstone====
The potential differences around a sand/shale contact deflect the SP from the shale baseline. The deflection is negative for a normal salinity contrast (borehole fresher than formation). Little change occurs within a sand interval, so a clean sand shows a straight-line “sand line” ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1c]]). (For more details on SP shale and sand baselines, see [[Determination of water resistivity]].)
The potential differences around a sand-shale contact deflect the SP from the shale baseline. The deflection is negative for a normal salinity contrast (borehole fresher than formation). Little change occurs within a sand interval, so a clean sand shows a straight-line “sand line” ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1c]]). (For more details on SP shale and sand baselines, see [[Determination of water resistivity]].)
====Tight rocks====
====Tight rocks====
An SP log is of little use in the absence of boundaries between shale beds and permeable beds. In relatively tight rocks (carbonates, evaporites, etc.), the SP wanders aimlessly, with no sharp usable deflections.
An SP log is of little use in the absence of boundaries between shale beds and permeable beds. In relatively tight rocks (carbonates, [[evaporite]]s, etc.), the SP wanders aimlessly, with no sharp usable deflections.
====Log shapes====
====Log shapes====
* ''Hydrocarbons''. The SP is generated in water. High hydrocarbon saturation reduces the SP, making sands appear more shaly.
* ''Hydrocarbons''. The SP is generated in water. High hydrocarbon saturation reduces the SP, making sands appear more shaly.
==[[Basic open hole tools#Calipers|Caliper logs]]==
==Caliper logs==
===Property measured===
===Property measured===
For lithological purposes, the critical data are caliper readings ''relative'' to bit size. There are three scenarios:
For lithological purposes, the critical data are [[Basic open hole tools#Calipers|caliper]] readings ''relative'' to bit size. There are three scenarios:
{| class = "wikitable"
{| class = "wikitable"
|-
|-
| Hard, inert rock || Hole in gauge || Caliper = bit size
|-
|-
| Hard, inert rock
| Soft or [[Brittleness|brittle]] rock || Hole washes out || Caliper > bit size
| Hole in gauge
| Caliper = bit size
|-
|-
| Soft or brittle rock
| Permeable rock || Mudcake builds up || Caliper
|-
| Mudcake builds up
| Caliper
|}
|}
====Shale====
====Shale====
Shale frequently spalls into the borehole, especially in the minimum principal stress direction. This leads to elliptical boreholes identifiable with multiple arm calipers, as on a dipmeter.
Shale frequently spalls into the borehole, especially in the minimum principal stress direction. This leads to elliptical boreholes identifiable with multiple arm calipers, as on a [[dipmeter]].
====Coal====
====Coal====
Medium to high rank coals are often brittle and well-jointed. Such joint blocks cave into the borehole ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1c]]) leaving deep washouts as thick as the coal seam (frequently only [[length::1 ft]] or so). Not all coals behave this way.
Medium to high rank coals are often [[Brittleness|brittle]] and well-jointed. Such joint blocks cave into the borehole ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1c]]) leaving deep washouts as thick as the coal seam (frequently only [[length::1 ft]] or so). Not all coals behave this way.
====Carbonates====
====Carbonates====
====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====
====Evaporites====
====Evaporites====
Individual evaporitic minerals (such as anhydrite, halite, sylvite, and carnallite) have well-defined densities and generate straight-line density logs with little variation ([[:file:quick-look-lithology-from-logs_fig2.png|Figure 2]]).
Individual evaporitic minerals (such as anhydrite, halite, sylvite, and carnallite) have well-defined densities and generate straight-line [[density log]]s with little variation ([[:file:quick-look-lithology-from-logs_fig2.png|Figure 2]]).
====Coal====
====Coal====
====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====
* ''Gypsum and anhydrite''. The typical neutron porosity value in anhydrite (CaSO<sub>4</sub>) is close to zero, but that in gypsum (CaSO<sub>4</sub> • 2H<sub>2</sub>O) is much higher—up to 60%.
* ''Gypsum and anhydrite''. The typical neutron porosity value in anhydrite (CaSO<sub>4</sub>) is close to zero, but that in gypsum (CaSO<sub>4</sub> • 2H<sub>2</sub>O) is much higher—up to 60%.
* ''Potash Evaporites''. Sylvite is anhydrous with a near-zero neutron porosity, but carnallite (KMgCl<sub>3</sub> • 6H<sub>2</sub>O) gives neutron values of 30% to 60%.
* ''Potash [[Evaporite]]s''. Sylvite is anhydrous with a near-zero neutron porosity, but carnallite (KMgCl<sub>3</sub> • 6H<sub>2</sub>O) gives neutron values of 30% to 60%.
====Bound water in shale====
====Bound water in shale====
==Neutron and density logs combined==
==Neutron and density logs combined==
Neutron and density logs each react to both lithology and porosity, so by analyzing the two logs together, one can begin to distinguish lithology from porosity. Neutron and density logs, together with a caliper measurement recorded by the density tool and a natural gamma ray log, are commonly run as a combination. This is the most powerful of the commonly available log suites for general purpose determination of lithology.
Neutron and [[density logs]] each react to both lithology and porosity, so by analyzing the two logs together, one can begin to distinguish lithology from porosity. Neutron and density logs, together with a caliper measurement recorded by the density tool and a natural gamma ray log, are commonly run as a combination. This is the most powerful of the commonly available log suites for general purpose determination of lithology.
===Crossplotting===
===Crossplotting===
===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:
{| class = "wikitable"
{| class = "wikitable"
|-
|-
!
! || All [[Porosity]] (H2O) || No Porosity (CaCO3)
|-
|-
| Neutron (p.u.)
| Neutron (p.u.) || 100 || 0
| 100
| 0
|-
|-
| Density (g/cm<sup>3</sup> )
| Density (g/cm<sup>3</sup> ) || 1.0 || 2.71
| 1.0
| 2.71
|}
|}
{| class = "wikitable"
{| class = "wikitable"
|-
|-
| Neutron (p.u.) || 45 || 30 || 15 || 0 || –15
|-
|-
| Density (g/cm<sup>3</sup> ) || 1.95 || 2.20 || 2.45 || 2.70 || 2.95
| Density (g/cm<sup>3</sup> )
| 1.95
| 2.20
| 2.45
| 2.70
| 2.95
|}
|}
|}
|}
On these scales, any offset of neutron and density logs is maintained regardless of porosity. Offsets are due to rock differences in density and neutron-absorbing properties (capture cross section). Ideal relationships for the three main liquid-filled porous rocks are as follows:
On these scales, any offset of neutron and [[density logs]] is maintained regardless of porosity. Offsets are due to rock differences in density and neutron-absorbing properties (capture [[cross section]]). Ideal relationships for the three main liquid-filled porous rocks are as follows:
====Sandstone====
====Sandstone====
====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]]
[[Category:Methods in Exploration 10]]