SP-GR-RESISTIVITY RULES

These rules are the basic set for segregating shales from other rock types. Pure shales are seldom zones of interest as oil and gas reservoirs, although many rocks that have been traditionally called "shales" are really silty or sandy shales. These are now zones of interest as "shale gas" reservoirs. Pure shales may be hydrocarbon source rocks and are interesting in different ways than reservoir rocks.  

Crain’s Rule #0:
Gamma ray or SP deflections to the left indicate cleaner sands, deflections to the right are shaly. Draw clean and shale lines, then interpolate linearly between clean and shale lines to visually estimate Shale Volume (Vsh).

Shale beds are not “Zones of Interest”. Everything else, including very shaly sands (Vsh < 0.75) are interesting. Although a zone may be water bearing, it is still a useful source of log analysis information, and is still a zone of interest at this stage. Clean and shaly sands have been marked on the logs shown below (Layers A, B, and C). Everything else are shale beds.

To find clean zones versus shale zones, examine the spontaneous potential (SP) response, gamma ray (GR) response, and density neutron separation. Low values of GR, highly negative values of SP, or density neutron curves falling close to each other usually indicate low shale volume. High GR values, no SP deflection, or large separation on density neutron curves normally indicate high shale volume. Young shales have low resistivity (1 to 4 ohm-m), older shales have medium resistivity (5 to 25 ohm-m). Shale source rocks have higher resistivity (25 to 250 ohm-m) and usually have extra high GR (150 to 300 API units).

Annotated logs showing layers picked on the basis of shale volume. Layer A is a very shaly sand, Layers B and C are clean sands. The layers above A, and between A and B, and below C are shales with medium resistivity (about 20 ohm-m), moderately high GR (100 -120 API units), and SP on the right side of the log track (zero deflection to the left). Clean and shale lines are drawn on the SP and GR logs. Clean lines on the GR can be anywhere between 7 to 45 API units and typically between 15 and 30 API units.
 

NEUTRON DENSITY SEPARATION RULES

These rules are intended to segregate clean rocks into various common minerals, typically quartz, calcite, dolomite, anhydrite, and halite. These are by far the most common minerals in sedimentary rocks. If you prefer rock names, the rules will distinguish sandstone, limestone, dolostone, anhydrite, and rock salt - same stuff, alternate names.

Crain’s Rule #6:
On Limestone Units logs, the density neutron separation for limestone is near zero, dolomite is 8 to 12 porosity units, and anhydrite is 15 or more. Sandstone has up to 7 porosity units crossover.

On Sandstone Units logs, separation for sandstone is near zero, limestone is about 7 porosity units, dolomite is 15 or more, and anhydrite is 22 or more.

Visual determination of lithology (in addition to identifying shale as discussed earlier) is done by noting the quantity of density neutron separation and/or by noting absolute values of the photo electric curve. The rules take a little memory work.

You must know whether the density neutron log is recorded on Sandstone, Limestone, or Dolomite porosity scales, before you apply Crain’s Rule #6. The porosity scale on the log is a function of choices made at the time of logging and have nothing to do with the rocks being logged. Ideally, sand-shale sequences are logged on Sandstone scales and carbonate sequences on Limestone scales. The real world is far from ideal, so you could find any porosity scale in any rock sequence. Take care!

SANDSTONE SCALE LOG 

 Sand – shale identification from gamma ray and density-neutron separation. Small amounts of density neutron separation with a low gamma ray may indicate some heavy minerals in a sandstone. Most minerals are heavier than quartz, so any cementing materials, volcanic rock fragments, or mica will cause some separation.  Both pure quartz (no separation) and quartz with heavy minerals (some separation) are seen here.

LIMESTONE SCALE LOG

 Lithology identification is accomplished by observation of density neutron separation and the gamma ray response, along with a review of core and sample descriptions. Here, calcite, dolomite, and anhydrite layers are easy to see based solely on their neutron density separation values and the corresponding clean GR curve.

PE-GR-DENSITY NEUTRON RULES

The photoelectric effect is often a direct  mineralogy indicator..

Crain’s Rule #7:
PE below 1 is coal, near 2 is sandstone, near 3 is dolomite or shale, and near 5 is limestone or anhydrite. The high density (negative density porosity) of anhydrite will distinguish anhydrite from limestone. High gamma ray will distinguish shale from dolomite.
 

RULE EXCEPTIONS:
High GR log readings coupled with density neutron log readings that are close together, are a sign of radioactive sandstone or limestone. To tell radioactive dolomite zones from shale zones, use a gamma ray spectral log, since the density neutron log will show separation in both cases. The PE value can help differentiate between radioactive dolomite and chlorite shale but not between dolomite and illite rich shale. High thorium values on the gamma ray spectral log indicate the shale.

SONIC DENSITY NEUTRON GR RULES

A combination of neutron density separation rules, plus some "absolute value" rules can be used to identify evaporite minerals. An example is shown below, in which the absolute values for some pure minerals are shown. Some mineral mixtures may be identified by intermediate absolute values plus some local knowledge.

Absolute values of neutron and density porosity for some pure minerals - these are particularly useful for evaporite minerals. Note the backup scales that are needed pr density, neutron, and GR curves that are required to handle some of these minerals

Absolute values of sonic log for some minerals  - same sequence as previous illustration. Numerical algorithms for solving 2 and 3 mineral models are given elsewhere in this Handbook.