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Geology/petrophysics

Question
QUESTION: electrical and resistivity logs are electrical logs and GR,Neutron and Density are nuclear logs.why we did not put electrical logs in same track and nuclear also in same track ?

WE can display any logs with any combination but the problem is the units of each log.
The idea of multiple logs display on one track is very much depend on logs units compatibility.

Resistivity (Induction and Laterolog)are measured from decimal to several hundred ohms )0.1 to 1000) whereas the display range of density, Sonic, GR and neutron are 40 to 140 microsecond per ft (USFT), 1.0 to 3.5 gram/cc, 0 to 200 GAPI and 0.0 to .10 (in decimal) respectively. So its very logical to select a track that best show the logs readings without confusion or conflicting logs. Therefore, all electrical logs are displayed in log scale and  others in linear scales.

If we combine resistivity logs with a log scale with GR the reading of GR of linear scale would be very difficult to read. Also some logs have good relationship with other logs behavior and they are best if plot on the same track either they will follow or will deflect from each other which it self bears some geological meanings. For example, If density increases neutron porosity decreases or vice versa, sonic increases when density increases or porosity decreases; the relationship of the logs either following each other or defecting.

Logs track are usually not more than 2 inch wide and if we plot several logs it would be difficult to read and almost impossible if we have combine logs scale and linear scale. therefore as a standard practice, 3 tracks are used. First track is linear scale (left hand side)reads, Bit size (in inch units) caliper (in inch units) GR (0 to 150 GAPI units)and SP (0-10 +/- mV).

Log scale unit reads in multiple starting from 0.1 and then first ten smaller units read from 0.1 to 1, next ten smaller units read from 10 to 100, followed by next 10 units read from 100 to 1000. (lowest number to the left).

In linear scale each unit reads their added respective number. Density starts from 2 and if there are 10 division in the log then after 2 next unit will read 2.1, 2.2, 2.3......3.0 g/cc. Sonic if starts from 40, the next number will be 50, 60, 70, 80........140.

Second track is Resistivity with log scale reads from 0.1 to 1000 Mohms, and you can combine Density, sonic and neutron or make a separate track for sonic logs display as you like.

Regards

---------- FOLLOW-UP ----------

QUESTION: Sir if i had three track.I mean GR,SP AND CALI---RESISTIVITY--NEUTRON,DENSITY,SONIC......
Which would be the best order to read and compare all these tracks.?

Track One(extreme left); 4 curves, Bit size, SP, Caliper, Gamma Ray and SP
Track two (middle); Resistivity (induction or laterolog with MSFL, shallow and deep resistivity curves)

Track three (extreme right)Neutron, density and Sonic.

This is based on industry standard practice of log display method but you may do as you feel comfortable for your interpretation but keep in mind that this display has a logic. You may add fourth track as well with sonic (delta T) if you like.

Every log curve should be distinct to each other and must be properly titled with scale.

Hope, this may help,

Regards

Syed Tariq Hasany

Experience

Petroleum Exploration

Organizations
Petronas Carigali Sdn Bhd

Publications
In Pakistani and Asian Petroleum Confertence special Publications

Education/Credentials
BSc Honours in Geology and Master of Science degree in Petroleum Geology

Awards and Honors
Active member AAPG, SPE and PAPG Treasurer PAPG and AAPG Visiting Geoscientist