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TitleCrain Book
TagsGeology Nature Porosity
File Size2.2 MB
Total Pages54
Table of Contents
                            Common Logging Scales
	The Gamma Ray Response Equation Solved for Shale Volume
Document Text Contents
Page 1

Crain’s Petrophysical Pocket Pal Copyright 1978 – 2012 E. R. Crain, P.Eng. Page 1 of 54


The complete manual is 193 pages, available at

By E. R. (Ross) Crain, P.Eng. Phone: 1-403-845-2527 ross @

Copyright: E. R. Crain, P.Eng. All Rights Reserved Updated Nov 2011
ISBN 0-9734171-1-0
mailto:[email protected]

Page 2

Crain’s Petrophysical Pocket Pal Copyright 1978 – 2012 E. R. Crain, P.Eng. Page 2 of 54

This document is intended as a sample of the presentation quality and technical content of the Petrophysical
Referemce Manuals offered in “Crain’s Petrophysical Series”. The full version of this Manual is more than 190
pages and a few selected pages are shown here to illustrate the style of writing and the quality of the
illustrations. There are 9 other manuals in the Series. Orders can be placed at
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Compare this sample to course handouts provided elsewhere and decide for yourself which is easier to read
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Page 27

Crain’s Petrophysical Pocket Pal Copyright 1978 – 2012 E. R. Crain, P.Eng. Page 27 of 54


5.00 Pore Volume
The second calculation step in a log analysis is to find shale corrected porosity. Pore volume is the space in
a rock filled with oil, gas, or water. Total porosity includes the bound water in the shale and is called PHIt.
Effective porosity does not include bound water, and is called PHIe. When there is no shale, PHIe equals PHIt.

Logs read total porosity. All our analysis methods correct for shale, so the answers from any method
presented below will give effective porosity. Some analysis methods NEED total porosity as an intermediate
step, so you may also need to calculate it.

Raw log porosity, as presented in the field by the service company, does NOT take into account shale or
lithology effects, so raw log readings should NEVER be used as answers. Log analysis MUST ALWAYS be
done to find the correct porosity. All our analysis methods also account for matrix rock (lithology), but YOU
may be required to define the rock type for some methods. Other methods will define the lithology for you.

YOU MUST choose a method that is appropriate for the available data and for the rock type being analyzed.
The easiest methods are:

** Section 5.01: Porosity From The Sonic Log - use if density neutron combination is not available, or in bad
hole when density log is no good.

** Section 5.02: Porosity From The Density Log - use in preference to sonic if available, lithology is well
known, hole is good, and density neutron combination is not available.

** Section 5.03: Porosity From The Neutron Log - use if both sonic and density are not available.

** Section 5.04: Porosity From The Complex Lithology Density Neutron Crossplot - use in preference to a
single log method except in bad hole where density is no good.

** Section 5.05: Porosity From The Dual Water Density Neutron Crossplot – use in quartz sands with no heavy
minerals, otherwise use Complex Lithology method.

** Section 5.06: Porosity From The Photoelectric Density Neutron Crossplot - use in preference to complex
lithology ONLY if mineral model end points are well known.

In all cases, the results must be trimmed to prevent too high a porosity in shaly zones and in bad hole by
using Section 5.07: Material Balance for Porosity (Maximum Porosity). The META/ESP spreadsheet, available
on the Downloads tab at, handles these models and makes the work relatively painless.

Unfortunately there is no standard logging program, so there is no single foolproof log analysis method.
Each method has its own usage rules. These rules may need to be adjusted to suit local conditions. In the
classroom or when starting work in a new area, you may want to try several methods, and see which matches
core porosity the best.

Calibration of log analysis porosity is usually accomplished by comparing it to conventional core

Page 28

Crain’s Petrophysical Pocket Pal Copyright 1978 – 2012 E. R. Crain, P.Eng. Page 28 of 54

5.01 Porosity From The Sonic Log
The sonic is a simple method and must be employed if more modern density neutron data is not available.
The method shown is called the Wyllie time average equation. Other porosity methods are presented in
following sections.

Other methods for the sonic have been proposed, but they are really specific to certain areas, although this is
not clearly stated in the literature. For example, the Hunt-Raymer transform is appropriate for the US Gulf
Coast, but a poor model for the Lower Cretaceous in Western Canada. The Wyllie approach, when calibrated
to core, is universally applicable.


STEP 1: Calculate shale porosity (PHISSH), a constant for each zone:


DTCSH is a constant for the zone, chosen from the sonic log in a nearby shale.

STEP 2: Calculate porosity from sonic log (PHIsc) for each layer in the zone:

2: PHIs = (DTC – DTCMA) / (DTCW – DTCMA)
3: PHIsc = PHIs – (Vsh * PHISSH)

The sonic porosity (PHIsc), after all corrections are applied, is called the effective porosity, PHIe.


CASE 1: Correct each layer for lack of compaction, ONLY IF DTCSH > 328 (Metric) or DTCSH > 100 (English)

4: PHIe = PHIsc / KCP

CASE 2: Correct each layer for gas effect, ONLY IF PHIsc > PHItrue and gas is known or suspected

5: PHIe = PHIsc * KS


• Use when density log is unavailable, or when density log is affected by bad hole.

• Of the three "one-log" porosity methods, the sonic corrected for shale is the preferred one for wells

that have no density log. However, crossplot methods or the density log corrected for shale are
usually better if the log data is available.

• If lithology is unknown, sonic log corrected for shale is better than density log because the lithology

effect on the sonic is smaller.

• Use the compaction correction KCP only if DTCSH > 100 usec/ft (for English units) or DTCSH > 328
usec/m (for Metric units). In western North America, this is normally required when above 3,000 -
4,000 feet (900 – l,200 meters).

8: KCP = DTCSH / 100 (for English units)

OR 9: KCP = DTCSH / 328 (for Metric units)

• KCP is never less than 1.0.

• Use the gas correction KS only if PHIsc is too high compared to other sources and if gas is known to

be present. The need for this correction is common, but it is unlikely that a gas correction will be
needed in very shaly sands since invasion should be relatively deep.

10: KS = PHItrue / PHIsc

• KS is never greater than 1.0.

Page 53

Crain’s Petrophysical Pocket Pal Copyright 1978 – 2012 E. R. Crain, P.Eng. Page 53 of 54

EXERCISE #1: Shaly Sand – Thick Beds – Log Depths in Meters
Page 3 of 4 Your Name_____________

Page 54

Crain’s Petrophysical Pocket Pal Copyright 1978 – 2012 E. R. Crain, P.Eng. Page 54 of 54

EXERCISE #1: Shaly Sand – Thick Beds – Log Depths in Meters
Page 4 of 4 SANDSTONE SCALE LOG Your Name_____________

42 Pages Omitted 

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