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Geology/Well site identification of reservior fluid with oil based mud during drilling



Good Day. I have some confusion regarding differentiating reservoir fluid with oil based mud. Normally in water based mud we can easily identify reservoir fluid through fluorescence but in case of oil based mud how do we identify it.

Also my second question is which is most important thing to monitor during coring operation. What are the necessary quality  control measures to be taken for successful coring.

Thanks and regards

Rahul Raj

ANSWER: Rahul,

I am not a well-site man, so my answer would not be that of an expert in that field.

For fluorescence please check with an expert.

In case of an exploratory well the decision of lowering the core-barrel is the most crucial one. Luck rather than the technique plays a more important role. However,you have to be very cautious when the expected depth of a reservoir is close. The interval in ditch-samples should be decreased gradually, change in ROP and any drilling break should be carefully monitored. Normally the basal part of a cap-rock shows higher pressure that increases with depth as the reservoir is approached. As a general rule it is better to lower the core barrel by keeping your fingers crossed than to miss the crucial range!

However, in a development well the depth prognosis is more precise, markers have been established and hence the decision is easier. Invariably the desired range is successfully cored.

Subject to the strength/type of the rock a suitable core barrel is selected, which is already provided in the 'Depth Data Note'. In a highly fractured carbonate reservoir, or a poorly cemented sand reservoir you have to have special core barrels. I have seen a core from a virtually un-cemented sand Cretaceous reservoir. A sleeved core barrel had to be used to bring the material up in a secured manner.

Preservation of core (reservoir) on the surface is as crucial as coring itself. I have seen that this aspect of coring operation does not get the attention it merits.

Hope it helps.

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


Thanks for the reply.

During coring job what are the most important parameters we have to monitor.

Thanks and regards

Rahul Raj


I think the important aspect is ROP. Is it smoothly rotating and gaining the desired depth and that there are no drilling breaks. Is the core being cut as per design i.e. vertical or following the desired angle. The fishing related symptoms are to be carefully monitored so that to stop at the right time. Check out with others too.


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Nusrat Kamal Siddiqui (Mr)


I am a petroleum exploration geologist with about 40+ years of diversified experience. I can answer questions on stratigraphy, log interpretation, field studies based on set of logs and core results, general geology etc. Can provide guidance for career planning in earth sciences. Can not help in questions about hard rocks, igneous petrology & mineralogy, sequence stratigraphy and seismic.


Petroleum Exploration - field geology, stratigraphy, prospect generation, interpretation of satellite imagery (visual), log interpretation.

Retired in May, 2006 as Senior Manager Exploration from Pakistan Petroleum Limited after serving 26 years. Since then continuing in active service with petroleum exploration companies.A cumulative experience of about 40 years covers 30 years in petroleum exploration, 5 years hydrogeology experience in Libya, and about 5 years of dam geology in Pakistan.

-AAPG Bulletin (Vol.88, July, 2004); paper on Sui Main Limestone (Eocene), a prolific gas reservoir in Pakistan. -Have presented papers on diversified topics ranging from spirituality, flood control, remote sensing and off course geology in France, Nepal, Bangladesh, Sri Lanka, UAE, Kashmir, India and Pakistan that have been published in respective Special Proceedings. -ONGC Bulletin(India) Vol. 44, Number 2, December, 2009; Chapter 19.A & 19.B on basin architecture and stratigraphy of Pakistan, p.402-474.

BSc Honours in Geology and Master of Science in Petroleum and Structural Geology. PGD from ITC, The Netherlands.

Awards and Honors
-Remained Active Member of AAPG from 1990-2006; founding member of PAPG (a local affiliate of AAPG). -Life member of SEGMITE & AGID, and Senior Editor of SEGMITE magazine. -Member of the Board of Studies, i. Department of Petroleum & Gas Engineering (BUITMS, Quetta), and ii. Geology Department, Sindh University, Jamshoro. -Member of Stratigraphic Committee of Pakistan. -Represented Mining Journal, UK in Pakistan till 2007. -Chairman of Annual Technical Conference (ATC)& Oil Show, an SPE/PAPG annual event, in 2003. -Chairman Technical Committee (ATC, 2002).

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