Changes

====Shale====

====Shale====

SP interpretation depends on first recognizing shale, where fairly constant SP readings form a straight “shale baseline” on the log (Figure 1a). Its actual SP value is not significant.

SP 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” (Figure 1c). (For more details on SP shale and sand baselines, see the chapter on “Determination of Water Resistivity” in Part 4.)

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 the chapter on “Determination of Water Resistivity” in Part 4.)

====Tight rocks====

====Tight rocks====

* Fresh borehole fluid in a saline formation. Gives “normal” SP.

* Fresh borehole fluid in a saline formation. Gives “normal” SP.

* Borehole salinity is same as formation. Featureless SP, very low amplitude, may be a straight line, no obvious relationship to beds (Figure 1b).

* Borehole salinity is same as formation. Featureless SP, very low amplitude, may be a straight line, no obvious relationship to beds ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1b]]).

* Saline borehole in a fresher formation. Gives a reversed SP, where sands show positive deflections from the shale baseline.

* Saline borehole in a fresher formation. Gives a reversed SP, where sands show positive deflections from the shale baseline.

====Sandstone====

====Sandstone====

Consolidated sandstone is usually permeable, so expect mudcake to cause a caliper reading that is about [[length::0.5 in.]] smaller than the bit size. Bed boundaries are often accurately delimited (Figure 1).

Consolidated sandstone is usually permeable, so expect mudcake to cause a caliper reading that is about [[length::0.5 in.]] smaller than the bit size. Bed boundaries are often accurately delimited ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1]]).

====Sand====

====Sand====

====Coal====

====Coal====

Medium to high rank coals are often brittle and well-jointed. Such joint blocks cave into the borehole (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 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====

====Limestone====

====Limestone====

Clean limestone reads about 5.0 barns/electron (Figure 2).

Clean limestone reads about 5.0 barns/electron ([[:file:quick-look-lithology-from-logs_fig2.png|Figure 2]]).

====Dolomite====

====Dolomite====

Dolomite should read about 3.0 barns/electron, providing an easy way to distinguish limestone from dolomite (Figure 2) even if gas is present. Note that iron in ferroan dolomite increases readings to resemble limestone.

Dolomite should read about 3.0 barns/electron, providing an easy way to distinguish limestone from dolomite ([[:file:quick-look-lithology-from-logs_fig2.png|Figure 2]]) even if gas is present. Note that iron in ferroan dolomite increases readings to resemble limestone.

====Shale====

====Shale====

====Bound water in shale====

====Bound water in shale====

Some water in shales is chemically bound to clay minerals, whereas some occurs in micropores. Both types raise neutron log readings but represent no effective porosity (Figure 1). Shales consequently have high apparent neutron porosity, but values vary among formations. Often 40% is a good shale cutoff limit, but shale values can be as low as 30%. A local cut-off can often be established by calibration, such as from cores.

Some water in shales is chemically bound to clay minerals, whereas some occurs in micropores. Both types raise neutron log readings but represent no effective porosity ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1]]). Shales consequently have high apparent neutron porosity, but values vary among formations. Often 40% is a good shale cutoff limit, but shale values can be as low as 30%. A local cut-off can often be established by calibration, such as from cores.

==Neutron and density logs combined==

==Neutron and density logs combined==

====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 (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:

* Zircon (with other heavy minerals): thorium or uranium radiation.

* Zircon (with other heavy minerals): thorium or uranium radiation.

* Siderite, pyrite, etc.: no increased radiation.

* Siderite, pyrite, etc.: no increased radiation.

Use the shape of the neutron-density cross-over to provide depositional energy in the same way as an SP or gamma ray log (Figure 1). Thus, a “V” shape is a funnel (coarsening upward) and a “Λ” shape is a bell (fining upward).

Use the shape of the neutron-density cross-over to provide depositional energy in the same way as an SP or gamma ray log ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1]]). Thus, a “V” shape is a funnel (coarsening upward) and a “Λ” shape is a bell (fining upward).

====Sandstone (Gas-Filled)====

====Sandstone (Gas-Filled)====

====Dolomite====

====Dolomite====

Characteristic four to six scale division separation with density to the right of neutron is relatively consistent in clean dolomite (Figure 2). Gas reduces or eliminates the separation; use a Pe value of 3 to confirm dolomite. Locally high natural gamma ray looks like clay, but if neutron-density separation is unchanged, it may be “hot” dolomite (especially in the Permian basin). Check uranium if spectral gamma ray is available.

Characteristic four to six scale division separation with density to the right of neutron is relatively consistent in clean dolomite ([[:file:quick-look-lithology-from-logs_fig2.png|Figure 2]]). Gas reduces or eliminates the separation; use a Pe value of 3 to confirm dolomite. Locally high natural gamma ray looks like clay, but if neutron-density separation is unchanged, it may be “hot” dolomite (especially in the Permian basin). Check uranium if spectral gamma ray is available.

====Shale====

====Shale====

Shale shows a log separation with neutron to the left of density, sometimes displaced by a large amount (Figure 1). At times the separation is only three or four scale divisions, which can resemble dolomite. To distinguish shale, check for the following:

Shale shows a log separation with neutron to the left of density, sometimes displaced by a large amount ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1]]). At times the separation is only three or four scale divisions, which can resemble dolomite. To distinguish shale, check for the following:

* Apparent neutron porosity is too high for the area. Shale neutron readings are often between 30 and 50 porosity units.

* Apparent neutron porosity is too high for the area. Shale neutron readings are often between 30 and 50 porosity units.

====Coal====

====Coal====

Neutron and density logs for coal both read similar very high apparent porosities (Figure 1c). Coals give prominent deflections that do not resemble anything but severe washouts. (Diatomite has a density of about 1.4 g/cm³ and a neutron measurement of about 60 porosity units, so crossover is at least seven scale divisions.)

Neutron and density logs for coal both read similar very high apparent porosities ([[:file:quick-look-lithology-from-logs_fig1.png|Figure 1c]]). Coals give prominent deflections that do not resemble anything but severe washouts. (Diatomite has a density of about 1.4 g/cm³ and a neutron measurement of about 60 porosity units, so crossover is at least seven scale divisions.)

====Complex rock mixtures====

====Complex rock mixtures====