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

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

Question 1 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 2

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	Mudstone
B	Caprock
C	Shale
D	Seal
E	Sandtone

Question 2 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 3

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

Question 3 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 4

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	Inaccuracies in measurements due to mud infiltration
D	Tight oil
E	Signal interference or poor tool calibration

Question 4 Explanation: 

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

Question 5

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

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

Question 5 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 6

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	Dolomite
C	Argillaceous limestone
D	Shale
E	Limestone
F	Calcite

Question 6 Explanation: 

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

Question 7

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

Question 8

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

Question 9

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

Question 9 Explanation: 

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

Question 10

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

Question 11

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

Question 11 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 12

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

Question 12 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 13

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 4
C	Fracture 4 and 5
D	Fracture 7
E	Fracture 1 and 2

Question 14

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

Question 15

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	2047 m (+/- 5m)
D	2077 m (+/- 5m)
E	2090 m (+/- 5m)

Question 15 Explanation: 

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

Question 16

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 1047 m and 1060 m
D	Between 1066 m and 1071 m
E	Between 1066 m and 1078 m

Question 16 Explanation: 

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

Question 17

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

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

Question 17 Explanation: 

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

Question 18

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

Question 19

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	Limestone
B	Dolomite
C	Glauconitic Sandstone
D	Sandstone
E	Anhydrite

Question 19 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 20

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

Question 20 Explanation: 

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

Question 21

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	50 degrees Southeast
C	40 degrees Southeast
D	150 degrees Northwest
E	60 degrees Southeast

Question 22

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

Question 22 Explanation: 

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

Question 23

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

Question 23 Explanation: 

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

Question 24

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

Question 24 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 25

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

Question 26

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

Question 27

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 1079 m
C	At 1067 m
D	At 1046.5 m
E	At 1027 m

Question 27 Explanation: 

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

Question 28

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	From KB down to 9600 ft.
D	Between 9500 ft and 9600 ft.

Question 28 Explanation: 

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

Question 29

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

Question 30

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

Question 30 Explanation: 

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

Question 31

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

Question 31 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 32

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

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

Question 34

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

Question 34 Explanation: 

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

Question 35

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	Limestone
D	Shale
E	Sandstone

Question 36

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

Question 37

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

Question 38

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

Question 38 Explanation: 

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

Question 39

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

Question 39 Explanation: 

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

Question 40

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

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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.