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Question
Sir i have started to learn and work on Petrophysics. I read the literature and now i have lot of understanding of these logs.Now i am going to interpret a reservoir zone.please guide me how i should read and compare the log reading curves on display and interpret?
I mean i should start to look right from GR the SP and on or ?

Answer
Dear Mr Ahmad,

Thanks for sending me your question.

It's fantastic that you have read a lot about Petrophysics and you are ready start logs interpretation. SP or Spontaneous Potential is the first log run by Schlumberger brothers in a well borehole during 1929 and utility and application of this log has not yet diminished rather great improvement has come recently.

SP log is useful to differentiate between permeable and impermeable beds, define sedimentary facies and also a useful data point to correlate from one well to another specially if wells are in one field. SP is displayed in log track 1 (the extreme left of the log)and reads in millivolt (mv)across the bedding. Sp -ve readings deflect to the left whereas (show permeabiliy) +ve readings are to the right (showing no permeability) and if no defelction means that there is no difference  between the salinity of formation water and drilling mud. SP log is good only in the conductive mud environment which means SP does not work in Oil base mud or air drilling.

Gamma Ray measures the total radioactivity count of the bed. Also, placed in track 1 and measures in Gamma API Units (GAPI)with standard display 0-100 (starting from left). GR reads high in beds having higher contents of radioactive minerals such as shales and feldspar rich sands. Limestone and sandstone usually have very low GR readings depending on their authigenic mineral composition. GR is excellent log for differentiation of sand shale or carbonates shale sequences in the well bore and also an  excellent correlation log.

GR provides great help in logs based sequence stratigraphic analysis to define Maximum flooding surface (highest GR readings), systems tracts (fining upward and coarsening upward trends) and unconformities (sequence boundaries). GR is excellent log to mark the formation tops and to pick the potential source beds (very high GR readings and if exceeds 150 GAPI it is known as hot bed which could possibly be a good source rock).

Resistivity logs are used to define zones of hydrocarbon and water. There are porosity determination logs as well; Neutron logs help to define porous zone and gas bearing zones, Density log also a good tool for porosity and also help to identify lithology whereas Sonic logs measure slowness of the compressinal waves or sound waves which in turn provide porosity estimation.

Keep in mid that due to bed hole conditions or heavy mud log data sometime give unreliable information and one should know those factors which degrade the logs readings and ensure that logs are providing reliable information or not.

Hopefully, you will start your interpretation with great ease and confidence.

Hope, this may help.

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