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Carbon oxygen logs

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Pulsed neutron (PN) Carbon oxygen ratio (C/O) log is routinely run for reservoir saturation monitoring and fluid contact movement during field development, for monitoring reservoir performance, identifying by-passed oil for recompletion or evaluating sweep efficiency for feasibilities of enhanced oil recovery projects. The main limitation of C/O logs is its shallower depth of investigation, often quoted as 6 inches, thus proper environmental correction of borehole conditions, borehole fluids, and well completion such as casing and cement is essential. Since C/O log is insensitive to formation water salinity, the main advantage when comparing PN sigma log and slimhole resistivity log, C/O log is a popular and indispensable reservoir surveillance tool, especially in fields with water injections to maintain reservoir pressure for injected water usually has different salinity than formation connate water, i.e., the mixed salinity environments.

How carbon/oxygen logs work

A typical PN tool has a neutron (n) generator and multiple gamma ray (g) detectors, i.e., an n-g tool, though there is one tool that is based on detecting remaining neutrons by using a neutron detector, i.e., the n-n tool.

Also, a typical n-g PN tool has two running modes; the high energy inelastic Carbon oxygen ratio, C/O, mode and the lower energy thermal capture sigma mode.

A C/O log

  • has a shallower depth of investigation, about 6”,
  • insensitive to water salinity, and
  • can be processed using either a windows method, a yields method, or a combination of windows and yields method (often called alfa method)

A C/O log is based on the facts that C exists in oil, but not in water, while O exists in water, but very little in oil, thus the amount of C detected would be proportional to oil and amount of O detected proportional to water, after environmental correction for effects of wellbore fluids, well completion, and rock lithology.

On the other hand and in comparison, a sigma log

  • Has a deeper depth of investigation, about 12”,
  • Very sensitive to water salinity, and
  • Processing for water saturation is simple and straight forward by using the volumetric mixing rule.

Current (2023) commercial PN logging tools

SLB RST and Pulsar (PNx)

Saturation Monitoring With the RST Reservoir Saturation Tool- Disk 2 (

Pulsar (

Halliburton RMT

Reservoir Monitor Tool 3-Detector™ (

Baker Hughes RPM

Reservoir performance monitor service | Baker Hughes

Weatherford Raptor (CRE)

Raptor Cased-Hole Evaluation System | Weatherford International

Sondex SPN

Sondex-spectral-pulsed-neutron-SPN-service-slsh.pdf (

Probe RAS

RAS™ - Probe | Cased Hole | Wireline | Logging | Evaluation | Monitoring | Oil Well Tools (

Antares PND CORE

5-PND-Brochure.pdf (

ANTARES Datensysteme GmbH » Products (

Hotwell PNN

Pulse Neutron NeutronHotwell US

Noteworthy papers in OnePetro

R. Eyvazzadeh, O. Kelder, A. Hajari, S. Ma, & A. Behair. 2004. C/O logging methodologies: comparing saturation determination techniques, SPE 90339, ATCE 2004, Houston.

O. Kelder, A. Hajari, R. Eyvazzadeh, S. Ma, & A. Behair. 2005. Expanding the operating envelope of Carbon-Oxygen saturation monitoring technology, IPTC-10458, Nov. 2005, Doha, Qatar.

A. Sunbul, S. Ma, A. Hajari, A. Srivastava, & R. Ramamoorthy. 2005. Quantifying remaining oil by use of slim-hole resistivity measurement in mixed salinity environments –a pilot field test, SPE 97489, Int’l IOR Conf. in Asia Pacific, Dec. 5-6, 2005, Kuala Lumpur, Malaysia.

S. Ma, A. Hajari, G. Berberian, & R. Ramamoorthy. 2005. Cased-hole reservoir saturation monitoring in mixed salinity environments –a new integrated approach, SPE 92426, 2005 MOES.

M. Nasser, S. Ma, N. Musharfi, A. Muthana, S. Riley, & A. Geevarghese. 2013. Quantifying gas saturation with pulsed neutron logging- an innovative approach, SPE 166025, Abu Dhabi, UAE, 16-18 Sept 2013.

S. Ma, H. Pfuzner, A. Hajari, N. Musharfi, P. Saldungaray, & M. Hisham. 2014. Resolving the mixed salinity challenges with a methodology developed from pulsed neutron capture gamma ray spectral measurements, SPE 170608, SPE ATCE 2014, Amsterdam.

Y. Al-Taher, S. Ma, Mcllroy, C., & Guergueb, N. 2017. Reservoir surveillance optimization using nuclear modeling integrated with formation sampling, SPE 187972, SPE KSA, 24-27 April 2017.

Y. Al-Taher, S. Ma, & M. AlNasser. 2018. Critical factors affecting pulsed neutron saturation monitoring in carbonate reservoirs, SPE 192216, SPE KSA ATSE, April 2018.

S. Ma, N. Guergueb, W. Guo. & M. Eid. 2021. Intrinsic Carbon-Oxygen Logging for Enhanced Consistency of Reservoir Saturation Monitoring, paper SPWLA-2021-0021, SPWLA 62nd Annual Symposium, 17-20 May 2021.

Y. Eltaher & S. Ma. 2022. In situ oil carbon density characterization for enhanced reservoir saturation monitoring using carbon-oxygen logs, paper IPTC-21976, 21-23 Feb 2022, Riyadh, Saudi Arabia.

Y. Al-Taher & S. Ma. 2023. Carbon/Oxygen Spectral Data Processing, Its Affiliation to Scintillation Detector Selectivity & Their Impacts on Reservoir Saturation Monitoring, Lessons Learnt and Recommendation Workflow, SPE 212,613, SPE Reservoir Characterization and Simulation Conf and Exh, 24-26 Jan 2023, Abu Dhabi, UAE.

M. van Steene, S. Ghadiry, & S. Ma. 2023. Rigless mineralogy logging- a reality for improved evaluation of complex formations, paper SPE 213,525, Middle East Oil, Gas, & Geoscience Show, 19-21 Feb 2023, Bahrain.

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See also

Nuclear logging