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Geology/Lithology of Goru Formation and Wireline response

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Question
Asalaam o Aalikum Sir,
According to literature, Upper Goru Formation is mainly consist of Shale while lower contains Shale and Sand Units.
In Gajwaro-01, Kadanwari-11,Sawan-07 and Sawan-09 the Upper Goru formation is showing less Gamma ray response as compared to Lower Goru formation(top of lower) is showing relatively high GR reading.
Then how we can say lower Goru formation has sand and shale while upper is mainly Shale. If Upper is mainly shale should not it show average GR response high than lower Goru Formation?
Thanks a lot for your time ....

Answer
Dear Atif Ismail,

Thanks for your question. Wireline logs measure rock properties through geophysical signal receiving and processing. These wireline tools capture receiving signals which travelled through rocks, essentially the response of the rock grains and fluids. This is you need to understand that logs do not follow names of the formation rather the given classification of the formations is based on the logs motifs. Gamma Ray (GR)logs are good to differentiate between sandstone/limestone with shales. High GR stands for a rock containing higher radioactive minerals (main source are Potassium, Uranium and Thorium ) which are usually a characteristic of a shale but all sands or limestone are not clean either and thus they may contain radioactive minerals such as Potassium feldspar (KAlSi3O8)or Alkali feldspars, Biotite, Glauconite, Muscovite, Phlogopite, Alunite, Apophyllite, Autunite, Monazite, polyhalite, etc.

Sandstone washed away from granite may contain abundant radioactive minerals so if the rock has either deposited near Granite or sediments have been sourced from granitic wash GR should read high.GR are also recorded high in source rocks as well.  

I can not comment about the naming convention you are referring without looking regional logs pattern and formation properties and their response to GR. The  Upper Goru formation generally  reads high GR as it mainly consists of shale whereas Lower Goru is made up of alternating sandstone and shale so the GR should read according to the lithology. You have to go through other wells in the field and determine how these two formations are marked earlier and based on that criteria you continue marking well tops. However, the presence of sandstone in Upper Goru likely to be due to some geological reasons and you need to see the local geological variation such as some sudden facies change or a minor cut of a channel.

Regards

Tariq  

Geology

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