You are here:

Geology/formation boundaries


how to identify boundaries of formation in well log?

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.  


All Answers

Answers by Expert:

Ask Experts


Syed Tariq Hasany


MY HUMBLE REQUEST. BEFORE SENDING A QUESTION PLEASE READ THE FOLLOWING CAREFULY. ANSWERING ONLY PUBLIC QUESTIONS. Ask questions of exploration of Petroleum, and specific regions for specific answers about Pakistan, Central Asia, Middle East and Far East. Ask questions of Business side of the petroleum Geology. Love to guide students and also Career related issues PRIVATE QUESTIONS on I am not expert of Hard rocks (Igneous and metamorphic rocks)Volcanic activity and related petrology, mineralogy and its exploration. BE SPECIFOC WHAT YOU WANT TO KNOW AND ASK WITH DIGNITY AND RESPECT. Show me your determination and commitment to learn. PLEASE TRY TO WRITE RIGHT, PROPER AND CORRECT LANGUAGE. DO NOT USE TEXT(SMS)STYLE. Thanks TARIQ


Petroleum Exploration

Petronas Carigali Sdn Bhd

In Pakistani and Asian Petroleum Confertence special Publications

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

©2017 All rights reserved.