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Geology/Petrophysics: Saturation and Porosity Relationship

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Question
Sir,my question is regarding petrophysics which involves evaluation of water saturation.  " According to the Archie Equation  why does saturation Sw or So varies inversely with porosity in reservoir zone if the Rt values remain same"
I mean if saturation increases, it means porosity decreases. What is the reason that saturation decreases with increase in porosity.
Can you please explain the reason. I'm quite confused about it.
Looking forward to your reply.

Regards,
Niaz

Answer
Niaz,

You are right that Sw and So are inversely related. Increase in one means equivalent decrease in the other (1-Sw=So)and vice versa. Just solve the Archie equation with variables, like changing the Rt values and keep others constant. Do the same exercise by changing the porosity values only.  You may also solve the equation with variable hypothetical Sw values.You will get a feel how the equation works.

Why do you say that increase in Sw means decrease in porosity for the same Rt value in a reservoir? Water and hydrocarbon saturation is a function of how forcefully the hydrocarbons had displaced the water during the charge. A strong charge may displace water in a low porosity reservoirs to a larger extent, and a weak charge may not be able to largely displace water in a high porosity reservoir. In Sui gas field the porosity is high and the Sw is low (favors your point). In Adhi gas/condensate field, porosity is low (Khewra, Tobra)but the formation water production is not a problem.

The best thing for you is to discuss it with a logging expert of a Service Company.

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Nusrat Kamal Siddiqui (Mr)

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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.

Experience

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

Organizations
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.

Publications
-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.

Education/Credentials
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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