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Geology/formation boundaries

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Question
how to identify boundaries of formation in well log?

Answer
Dear Norliana,

The geological formation are defined on the basis of lithology. The difference from one formation to another is its rock type and its physical properties.

Formation are being identified in outcrop and wells as well on the change of lithology. Each formation has their unique lithological characteristics which are primarily controlled by their mode of deposition.

For example, a formation, A, consist of dominantly (say approximately above 75% of its total thickness) sandstone with shale and other rocks (about 25%).

In the well logs this formation will be picked when thick sandstone body appears by means of very low Gamma Ray (GR) readings, relatively increasing electrical resistivity log and usually with decreasing sonic log  reading. Compare this log event with Mudlog lithological description of wellsite geologist. The top of A formation will be picked on the logs when there is a discrete lithological change from its overlying rocks for example; thick shale overlying the formation A. The shale reads higher GR so you should be able to see sudden decreasing trend of GR when approaching new formation. You can mark the top with lead pencil using a ruler to mark the top where GR reads minimum and note down the depth and write the name of the formation.

Similarly, you have to pick the base of A and top of underlying formation. This  probably is a best way to pick well tops while working on wellsite which also refers as quick and dirty  interpretation. Once you returned back to your office it is strongly advised that you should reexamine your initially picked formation tops with the help of regional wells correlation and if required you may  alter formation tops accordingly.

Be careful that not all the formations are distinctly identifiable as they have  much less difference in their overall lithological behavior between the upper and lower formations.

Not all, but there are many basins of the world where lithology is either clastics rich or limestone rich where differentiation of formation may be a difficult task. To resolve this issue, you get to know the regional lithological pattern and distribution and regional stratigraphic chart before you undertake the well tops picking on logs. Its always a good idea that you should have several well correlation panels of the field you are working on and linking with nearby fields.


Please let me know if you are satisfy and happy with my answer. Send me more again if you have more questions.  

Geology

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