Petrophysical (Well) Log Interpretation or Petrophysics Log Interpretation is a specialized area of science that deals with borehole data. In Canada, it is typically taught with physical core log interpretation and other geological and engineering fundamental concepts in most universities. Technical educational institutions and professional organizations offer courses in well log interpretation as a stand along subject. You can learn in depth at CRAIN’S PETROPHYSICAL HANDBOOK (www.spec2000.net). This page is best viewed on desktop computers due to small font size in well logs.

Disclaimer: While every reasonable effort is made to ensure that the information provided is accurate, no guarantees for the currency or accuracy of information are made. It takes several proof readings and rewrites to bring the quiz to an exceptional level. If you find an error, please contact me as soon as possible. Please indicate the question ID-Number or description because server may randomize the questions and answers.

Petrophysical Log Interpretation

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Question 1

The following temperature log is obtained from a productoon well. You were told to provide an assement for geothermal gradient for the general area. Which part of the following log is best stuited for geothermal anaysis? (ID-PPL-40)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/30-tmprlog.htm

A	Between 9550 ft to 9650 ft.
B	From KB down to 9500 ft.
C	Between 9500 ft and 9600 ft.
D	From KB down to 9600 ft.

Question 1 Explanation: 

Note, when analyzing the general geothermal gradient, it is best to use temperature logs with minimum interference due to petroleum production.

Question 2

In which of the following interval would you expect to have hydrocarbons? Click on the image if it is hard to read. (ID-PPL-26)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/01-crainsrules.htm

A	Between 1071 m and 1078 m
B	Between 1049 m and 1055 m
C	Between 1066 m and 1078 m
D	Between 1066 m and 1071 m
E	Between 1047 m and 1060 m

Question 2 Explanation: 

Based on the gamma ray (GR), D-phi and N-phi log responses on the right hand side log.

Question 3

Other than the obvious large pay zone, this well log also shows areas of oil in some shale dominated regions. This phenomena is caused by which of the following? (ID-PPL-13)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net

A	Borehole collapse
B	Poor tool pads in the well
C	Tight oil
D	Inaccuracies in measurements due to mud infiltration
E	Signal interference or poor tool calibration

Question 3 Explanation: 

While other listed answers are also possible, typically tight oil can provide such well log responses in shale dominated regions.

Question 4

What is the likely lithology at 4225 m? (ID-PPL-33)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/13-lithvisual.htm

A	Shaly sandstone
B	Gas-bearing calcite
C	Shale interbeded sandstone
D	Feldspatic (K-spar rich) arenite
E	Gas-bearing dolomite
F	Gas-bearing sandstone

Question 4 Explanation: 

The crossover effect between D-Phi and N-Phi density curves is indicative of gas-bearing formation. The characteristics of the PE and the GR curves are indicative of calcite (most likely).

Question 5

What is the most likely lithology at 2050 m on the following log? (ID-PPL-27)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net

A	Anhydrite
B	Dolomite
C	Shale
D	Limestone
E	Sandstone

Question 6

Calculate the neutron density between 3025 m and 3065 m? The log is calibrated with sandstone matrix. (ID-PPL-22)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/13-lithvisual.htm

A	~ 0.36
B	~ 0.24
C	~ 0.48
D	~ 0.18
E	~ 0.27

Question 6 Explanation: 

You take the average of peaks within the interval for density reading. To do this, you determine the peak values (+/-) and draw a line in the center. The scale is between 0 and 0.6 with 20 increments. Hence 0.6/20 = 0.03. Each line is 0.03 and to the average line in this particular interval, there are 8 lines from 0. That will give you a value of about 0.24. No other answer come close it so it should be easy to pick. Check the image below for more info.

Question 7

What is the depth interval of pay zone? (ID-PPL-11)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net

A	Between ~1070 m and ~1078 m
B	Between ~1042 m and ~1043 m
C	Between ~1066 m and ~1071 m
D	Between ~1066 m and ~1078 m
E	Between ~1025 m and ~1028 m

Question 7 Explanation: 

The core oil saturation (So) data is important in determining the pay zone in this particular log.

Question 8

Which of the interval is shale on the following well log? (ID-PPL-04)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/18-shalegas.htm

A	Between 1203 m to 1239.5 m
B	Between 1211.5 m to 1239.5 m
C	Between 1212.5 m to 1223.5 m
D	Between 1209.5 m to 1211.5 m

Question 8 Explanation: 

Shale result in separation between Phi-D and Phi-D with gamma ray to the right.

Question 9

What is the net:gross value for the interval between 3000 m and 3075 m? Assume your company uses the sand cut-off at 90 API. The log is calibrated with sandstone matrix. (ID-PPL-08)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/13-lithvisual.htm

A	~1.0
B	~0.38
C	0.0
D	~75
E	~0.013

Question 9 Explanation: 

Gross thickness = 3075 m - 3000 m = 75 m
Net sand interval = 3075 m - 3000 m = 75 m (read off the GR curve)
Hence, net:gross = 75 m/75 m = 1.0

Question 10

Where you would you place the conservative estimate of top boundary for the pay zone? Click on the image if it is hard to read. (ID-PPL-31)

A	At around 2158 m of depth
B	At around 2105 m of depth
C	At around 2177 m of depth
D	At around 2102 m of depth
E	At around 2110 m of depth

Question 10 Explanation: 

The deflection of the ILD and SFLU resistivity curves start around 2110 m of depth.

Question 11

At what depth would you expect to have gas production? (ID-PPL-39)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/30-tmprlog.htm

A	9540 ft
B	9500 ft
C	9640 ft
D	9600 ft
E	958 0ft

Question 12

If you must choose one location that is most likely to have 100% clean sand, where would that be? The log is calibrated with sandstone matrix. (ID-PPL-21)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/13-lithvisual.htm

A	At 3095 m
B	At 2995 m
C	At 2893 m
D	At 3011 m
E	At 2955 m

Question 12 Explanation: 

Clean sand has the smallest separation between D-phi and N-phi on sandstone calibrated logs.

Question 13

What is the dominant lithology on the following log? (ID-PPL-14)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net

A	Glauconitic Sandstone
B	Sandstone
C	Limestone
D	Anhydrite
E	Dolomite

Question 13 Explanation: 

Dolomite has low Gamma Ray (GR) response with about 3.14 PE response. It is not sandstone because the PE response is too high to be sandstone.

Question 14

Where you would you place the bottom boundary (conservative estimate) of the dominant pay zone? Click on the image if it is hard to read. (ID-PPL-32)

A	At around 2145m of depth
B	At around 2102 m of depth
C	At around 2110 m of depth
D	At around 2180 m of depth
E	At around 2158 m of depth

Question 14 Explanation: 

The deflection of the ILD and SFLU resistivity curves start around 2180 m of depth.

Question 15

What is the lithology between 281 and 282 m of depth? Click on the image if it is hard to read. (ID-PPL-36)

A	Sandstone
B	Shale
C	Clean limestone
D	Interbedded clean sandstone and shale
E	Coal

Question 16

Which of these fracture(s) identified in the image log is most likely at around 90-degrees to the borehole vertical axis? Assume a perfectly vertical borehole. (ID-PPL-35)

Original at: image log; Borehole image log analysis for sedimentary environment and clay volume interpretation by A. Shahinpour (NTUN) 2013

A	It is impossible to determine.
B	Fracture 4 and 5
C	Fracture 4, 6 and 7
D	Fracture 3, 8 and 9
E	Fracture 5 and 6

Question 17

What is the best geological explanation for the Gamma Ray (GR) response between 2970 m and 2998 m? The log is calibrated with sandstone matrix. (ID-PPL-18)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/13-lithvisual.htm

A	Prograding marine shelf
B	Slope channel deposit
C	Channel point bar deposit
D	Transgressive marine shelf
E	Interbeds of coal and sand

Question 18

On the following typical well log response chart, where would you most likely to find illite? (ID-PPL-17)

A	Just above position I
B	Just above position II
C	At position III
D	Just below position II
E	At position I

Question 18 Explanation: 

The clay mineral illite is associated with high K content. The SGR response for K is very high Just above position II. Additionally pure clay minerals have lower GR response than organic shale.

Question 19

In which interval would you expect to find coal according to the following well log responses? The log is calibrated with sandstone matrix. (ID-PPL-05)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/13-lithvisual.htm

A	Between 2945 m and 2970 m
B	Between 2970 m and 3075 m
C	Between 3075 m and 3085 m
D	At 3075 m
E	Between 2890 m and 2895 m

Question 19 Explanation: 

Coal has a very strong Phi-D and Phi-N response.

Question 20

What depositional environment would best describes the characteristics shown between 280 and 320 m? Click on the image if it is hard to read. (ID-PPL-38)

A	Braided channel
B	Trangessive marine shelf
C	Super-fan depositional lobes
D	Channel-point bar
E	Slope channel
F	Prograding delta

Question 21

What is the likely lithology at 3075 m? The log is calibrated with sandstone matrix. The log is calibrated with sandstone matrix. (ID-PPL-20)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/13-lithvisual.htm

A	Quartz arenite
B	Dolomitic pelmicrite
C	Shaly sandstone
D	Shale
E	Limy dolomite

Question 21 Explanation: 

Notice the separation of D-phi and N-phi curves with the response of the PEF curve.

Question 22

What is highlighted in red on the following well log? (ID-PPL-03)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/18-shalegas.htm

A	Water invaded zone
B	Coal seam regions
C	Water saturated zone
D	Oil saturated zone
E	Gas saturated zone

Question 23

On the following typical well log response chart, what is the best description for the lithology shown at position I? (ID-PPL-15)

A	Heavy metals
B	Interbeds of clean sand with shale
C	Glauconitic sandstone
D	Organic rich black shale
E	Gas bearing sandstone

Question 24

What is labelled as 1 to 4 on the following image log? (ID-PPL-29)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/22-fracloc5.htm

A	Coal seams
B	Effective porosity
C	Fractures
D	Bedding contacts
E	Hydrocarbon saturated zones
F	Lithological contacts

Question 25

An ideal fracture plain is shown in the schematic diagram (first figure; cylinder and plain). The next figure is an image log with several fractures labeled 1 to 9. Which of these fracture(s) identified in the image log is most likely represented by the schematic diagram? (ID-PPL-34)


Original at: schematic diagram; http://www.google.com/patents/US20110064277
image log; Borehole image log analysis for sedimentary environment and clay volume interpretation by A. Shahinpour (NTUN) 2013

A	Fracture 8 and 9
B	Fracture 1 and 2
C	Fracture 4
D	Fracture 4 and 5
E	Fracture 7

Question 26

Where would you expect a region saturated with hydrocarbon? (ID-PPL-28)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net

A	2047 m (+/- 5m)
B	2057 m (+/- 5m)
C	2077 m (+/- 5m)
D	2090 m (+/- 5m)
E	2047 m (+/- 5m)

Question 26 Explanation: 

The gamma ray (GR) indicate sandstone (sand) hence high porosity. The ILD and SFL readings are almost exactly the same.

Question 27

On the following typical well log response chart, what lithology is shown at position II? (ID-PPL-16)

A	Heavy metals
B	Organic rich black shale
C	Coal
D	Low density shale
E	Shaly sandstone

Question 27 Explanation: 

Organic rich black shale has a significantly higher Gamma Ray (GR) reading than the shale base line labeled on the diagram.

Question 28

What type of fluid is most likely to be present between 300 and 315 m of depth? Click on the image if it is hard to read. (ID-PPL-37)

A	Natural gas
B	Low density bitumen
C	Fresh water
D	Oil (petroleum)
E	Brine water

Question 29

There are five coal seams on the following log. Where would you place them? (ID-PPL-25)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/07-eslog.htm

A	At around 414 m , 451 m , 457 m , 464 m and 475 m
B	At around 412 m , 414 m , 422 m , 470 m and 475 m
C	At around 416 m , 451 m , 457 m , 470 m and 475 m
D	At around 414 m , 422 m , 464 m , 470 m and 477 m
E	At around 416 m , 451 m , 464 m , 470 m and 475 m

Question 30

At which depth interval would you expect a very high water saturation in the formation? (ID-PPL-12)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net

A	Between ~1025 m and ~1028 m
B	Between ~1042 m and ~1043 m
C	Between ~1070 m and ~1078 m
D	Between ~1066 m and ~1078 m
E	Between ~1066 m and ~1071 m

Question 30 Explanation: 

Sandy formations are important for high porosity needed for fluid saturation. The is determined using the Gamma Ray (GR) response to the left. Additionally the core oil saturation log (So) is important in determining the depths at which the water is most likely to be found.

Question 31

At which depth would you suspect borehole collapse? Click on the image if it is hard to read. (ID-PPL-07)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/01-crainsrules.htm

A	At 1023 m
B	At 1046.5 m
C	At 1079 m
D	At 1027 m
E	At 1067 m

Question 31 Explanation: 

Borehole collapse is inferred from the behavior of the caliper log.

Question 32

How would you characterize the lithology above 1025 m of depth? (ID-PPL-19)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net

A	Sandtone
B	Seal
C	Mudstone
D	Caprock
E	Shale

Question 32 Explanation: 

Using just the log itself, we can say that it is shale. However, without additional data from cores and other avenues, it is difficult to determine if it actually act as a cap or a seal. Also, in this particular log, you can see the geologist has decided where the shale-sand cut off should be and depending on that, you may even classify area above 1025 m as tight shale and not a cap rock.

Question 33

Why did the bit size change at 1500 ft of depth? (ID-PPL-10)

Image credit: United States Geological Survey, online at USGS database

A	Intentionally changed the drill bit
B	Drilling mud infiltration
C	Borehole collapse
D	Soft lithological facies
E	High pressure due to increased in depth

Question 34

Which of the following Resistivity curve most likely to measure true formation characteristics? (ID-PPL-02)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/07-eslog.htm

A	AT90 (dashed red line)
B	AT30 (dashed blue line)
C	AT10 (black line)
D	It is difficult to determine with the given log.

Question 34 Explanation: 

The resistivity curve AT90 measures the most deepest lithological characteristics. The AT10 is most likely to be effected by the drilling mud and near borehole fractures induced by drilling.

Question 35

What is the lithology at 1073 m? Click on the image if it is hard to read. (ID-PPL-06)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/01-crainsrules.htm

A	Dolomite
B	Dolomite with clay interbeds
C	Carbonaceous sandstone
D	Clean sand
E	Limestone with sand interbeds

Question 35 Explanation: 

Note the very strong logs deflect to the left on both gamma ray and spontaneous potential response curves. Additionally, in the field you may not get very clear logs.

Question 36

According to the following Spontaneous Potential (SP) and Resistivity log, where would you place the top (upper) boundary for a possible gas saturated zone? (ID-PPL-01)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/07-eslog.htm

A	Around 470 m of depth
B	Around 465 m of depth
C	Around 451 m of depth
D	Around 422 m of depth
E	Around 460 m of depth

Question 37

What is the lithology at 4192 m? The log is calibrated with limestone matrix. (ID-PPL-24)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/13-lithvisual.htm

A	Argillaceous dolomite
B	Shale
C	Dolomite
D	Calcite
E	Anhydrite
F	Argillaceous limestone

Question 37 Explanation: 

Neutron density is at zero for calcite on this log because the scale is set to limestone calibrated logs.

Question 38

What is the lithology at 4097 m? The log is calibrated with limestone matrix. (ID-PPL-23)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/13-lithvisual.htm

A	Anhydrite
B	Limestone
C	Shale
D	Dolomite
E	Calcite
F	Argillaceous limestone

Question 38 Explanation: 

Derived from the separation between D-phi and N-phi curves and the gamma ray response.

Question 39

What is the most likely cause for sharp leftward response of the Spontaneous Potential (SP) on the following log? (ID-PPL-09)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/01-whatisalog.htm

A	Sandstone formation
B	High concentration of gas ("gas effect")
C	Coal bearing formation
D	Presence of hydrocarbons
E	High porosity formation

Question 40

What is the dip of the geologic feature labeled with 2? (ID-PPL-30)

Image credit: Crain's Petrophysical Handbook, online at https://www.spec2000.net
Original at: https://www.spec2000.net/22-fracloc5.htm

A	30 degrees Northwest
B	60 degrees Southeast
C	50 degrees Southeast
D	150 degrees Northwest
E	40 degrees Southeast

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Credits: Based on the excellent class notes provided by, Dr. Rudi Meyer during Fall 2014 and the generous support from CRAIN’S PETROPHYSICAL HANDBOOK (spec2000.net) by supplying the petrophysical well log images. Some of the welllogs are produced by E.R. Crain, P.Eng. Those logs obtianed from E.R. Crain is the copyrighted intellectual property of the author.
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Supplementary Materials

Petrophysical Techniques; Geology/Geophysics (449)
Introduction to Petroleum Geology (577)
CRAIN’S PETROPHYSICAL HANDBOOK (www.spec2000.net)

If you are a company or an organization willing to donate copies of well logs, please contact me. I really appreciate your contributions and proper credits will be provided.