crain's petrophysical handbook - visual analysis of lithology _ mineralogy

4/20/13 Crain's Petrophysical Handbook - VISUAL ANALYSIS OF LITHOLOGY / MINERALOGY

www.spec2000.net/13-lithvisual.htm 1/11

WELCOME TO

CRAIN'S PETROPHYSICAL HANDBOOK Please be fair to the author. Pay your Shareware fee HERE, and receive the CD-ROM at no extra cost.

VISUAL ANALYSIS OF LITHOLOGY / MINERALOGY

SP GR Resistivity Rules Neutron Density Separation Rules PE GR- Density Neutron Rules Sonic Density Neutron GR Rules Pekiner Separation Applet

SP-GR-RESISTIVITY RULESThese rules are the basic set for segregating shales from other rocktypes. Pure shales are seldom zones of interest as oil and gasreservoirs, although many rocks that have been traditionally called"shales" are really silty or sandy shales. These are now zones ofinterest as "shale gas" reservoirs. Pure shales may be hydrocarbonsource 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 tovisually estimate Shale Volume (Vsh).

Shale beds are not “Zones of Interest”. Everything else, including very shaly sands (Vsh < 0.75) areinteresting. Although a zone may be water bearing, it is still a useful source of log analysis information, andis 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 neutroncurves falling close to each other usually indicate low shale volume. High GR values, no SP deflection, or largeseparation on density neutron curves normally indicate high shale volume. Young shales have low resistivity (1to 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).

http://www.spec2000.net/00-orders.htm

http://www.spec2000.net/13-lithvisual.htm#b1




http://www.spec2000.net/text109fp/PekinerLith.htm



Annotated logs showing layers picked on the basis of shale volume. Layer A is a very shaly sand, Layers B andC 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 (zerodeflection 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 RULESThese 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 youprefer rock names, the rules will distinguish sandstone, limestone, dolostone, anhydrite, and rock salt - samestuff, 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 unitscrossover.

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 thequantity of density neutron separation and/or by noting absolute values of the photo electric curve. The rulestake a little memory work. You must know whether the density neutron log is recorded on Sandstone, Limestone, or Dolomite porosityscales, before you apply Crain’s Rule #6. The porosity scale on the log is a function of choices made at thetime of logging and have nothing to do with the rocks being logged. Ideally, sand-shale sequences arelogged 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 neutronseparation with a low gamma ray may indicate some heavy minerals in a sandstone. Most minerals are heavierthan 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 rayresponse, 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 islimestone or anhydrite. The high density (negative density porosity) of anhydrite will distinguish anhydritefrom limestone. High gamma ray will distinguish shale from dolomite.

SUMMARY OF LITHOLOGYRULES

ROCK N–D N–D PE GR (SS) (LS)SAND 0 - 7 2 LOLIME 7 0 5 LODOLO 15+ 8+ 3 LOANHY 22+ 15+ 5 LOSALT - 37 - 45 4.5 LOSHLE 20+ 13+ 3.5 HI

Memorize this table, or keep a copyin your wallet. Practice the skill anduse it in your daily work.

THINK LIKE A DETECTIVE: 1. Find the evidence 2. Assess the evidence 3. Postulate all possibilities 4. Eliminate the impossible 5: Select the answer that fits bestwith the evidence

Remember: logs are not perfect andthese rules are not perfect. Adjustthe rules to suit your experience.Mineral mixtures are common, sothink in terms of what is possible ineach case.

On the log at the right, the evidenceand conclusion is shown for 6 layerswith different lithology.

This is a LIMESTONE scale log ==>

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 shalezones, use a gamma ray spectral log, since the density neutron log will show separation in both cases. ThePE value can help differentiate between radioactive dolomite and chlorite shale but not between dolomiteand illite rich shale. High thorium values on the gamma ray spectral log indicate the shale.



SONIC DENSITY NEUTRON GR RULESA combination of neutron density separation rules, plus some "absolute value" rules can be used to identifyevaporite minerals. An example is shown below, in which the absolute values for some pure minerals areshown. Some mineral mixtures may be identified by intermediate absolute values plus some localknowledge.



Absolute values of neutron and density porosity for some pure minerals - these are particularly useful forevaporite 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 algorithmsfor solving 2 and 3 mineral models are given elsewhere in this Handbook.



ABSOLUTE VALUES OF LOG readings (and some derived terms) FOR SOME PURE MINERALS* PHINMA DENSMA DELTMA MLITH NLITH PE UMA

Clean Quartz – 0.028 2650 182 0.802 0.623 1.82 4.8 Calcite 0.000 2710 155 0.822 0.585 5.09 13.8 Dolomite 0.005 2870 144 0.769 0.532 3.13 9.0 Anhydrite 0.002 2950 164 0.707 0.512 5.08 15.0 Gypsum 0.507 2350 172 1.002 0.365 4.04 9.5 Mica Muscovite 0.165 2830 155 0.768 0.456 2.40 6.8 Biotite 0.225 3200 182 0.601 0.352 8.59 27.5 Clay Kaolinite 0.491 2640 211 0.753 0.310 1.47 3.9 Glauconite 0.175 2830 182 0.723 0.451 4.77 13.5 Illite 0.158 2770 212 0.696 0.476 3.03 8.4 Chlorite 0.428 2870 212 0.658 0.306 4.77 13.7 Montmorillonite 0.115 2620 212 0.760 0.546 1.64 4.3 Barite 0.002 4080 229 0.383 0.324 261 1065 NaFeld Albite – 0.013 2580 155 0.889 0.641 1.70 4.4 Anorthite – 0.018 2740 148 0.820 0.585 3.14 8.6 K-Feld Orthoclase – 0.011 2540 226 0.772 0.656 2.87 7.3 Iron Siderite 0.129 3910 144 0.494 0.299 14.3 56.2 Ankerite 0.057 3080 150 0.683 0.453 8.37 25.8 Pyrite – 0.019 5000 130 0.370 0.255 16.4 82.2 Evaps Fluorite – 0.006 3120 150 0.670 0.475 6.66 20.8 Halite – 0.018 2030 220 1.172 0.988 4.72 9.6 Sylvite – 0.041 1860 242 0.295 0.270 8.76 16.3 Carnalite 0.584 1560 256 1.959 0.743 4.29 6.7 Coal Anthracite 0.414 1470 345 1.757 1.247 0.20 0.3 Lignite 0.542 1190 525 1.460 2.411 0.25 0.3

Copyright © E. R. (Ross) Crain, P.Eng. emailRead the Fine Print

mailto:[email protected]?subject=Inquiry%20About:

http://www.spec2000.net/00-fineprint.htm

crain's petrophysical handbook - visual analysis of lithology _ mineralogy

Documents