Geochemical High-Resolution Isotope Ratio Logging for Subsurface Hydrocarbon Analysis: Emerging Tech for Real-Time Reservoir Analysis

     As someone who has geosteered many horizontal wells, I have always thought that new methods could be combined with it for more thorough zone analysis and stratigraphic interpretation confirmation. Drill cuttings can be utilized in real-time or in near real-time for XRF/XRD analysis for the geochemical analysis of the zones being drilled. This has been used effectively for the correlation of zones and marker beds. Another geochemical technique being utilized more recently is isotope logging. This involves continuous measurements of chemical isotopes during drilling. For dry gas reservoirs, the isotopes logged are methane (C1) isotopes. For fluids and reservoirs with more liquids, C1-C3 isotopes are logged as well as CO2 isotopes. The ratios of the different isotopes can be used to geochemically characterize a reservoir and determine important information such as source, generation, alteration processes, and fluid properties of the hydrocarbons.

     Isotope ratio analysis has been around for a long time and has been used for many scientific studies involving geochemistry. Carbon isotope ratios in CO2 and oxygen isotopes have been used extensively for paleoclimate analysis. Nitrogen isotopes have also been used in studies. Other types of hydrocarbon analysis utilize ratios of alkanes of natural gas composition, such as methane (C1), ethane (C2), propane (C3), butane (C4), isobutane (also C4), Pentane (C5), as well as heavier hydrocarbon molecules (C6+).

 

SLB’s Isotope Logger (C1 and C1-C3)

     According to Carl Symcox, Director of Geosciences at Edge Systems:

“This advanced gas detection tool can identify depletion trends, fracture and fault contributions, fluid mixing, and distinct “line of death” compositional shifts. Because methane isotopic signatures are fundamentally linked to hydrocarbon fluid maturity, they provide a robust geochemical signal that is less susceptible to operational noise and surface contamination than traditional gas ratio analyses.”

     SLB has a 2016 case study that uses isotope logging to address rock and fluid heterogeneity along Marcellus laterals. Real-time logging involved generating a thermal maturity log that matched well with a vitrinite reflectance map, the standard for mapping thermal maturity trends.

“SLB proposed using isotope logging service to continuously measure isotopic ratios of δ13C–CH4 from surface while drilling to produce a continuous thermal maturity log. A continuous thermal maturity log is the first step toward determining hydrocarbon fluid type in place, and providing this data would enable improved while-drilling reservoir management decisions.”

     Below is the thermal maturity log generated along with the vitrinite reflectance map.

     SLB notes that real-time isotope ratio logging data can be integrated with quantitative fluid composition data via its FlairFlex™ advanced real-time fluid logging and analysis service.

     For C1 Isotope logging, SLB emphasizes a better understanding of depth resolution and small-scale features:

“A continuous, quality-controlled measurement log of isotopic ratios provides dramatically improved depth resolution and enables identifying small-scale features that would otherwise be missed.”

     For C1-C3 isotope fluid logging, SLB notes that this works with a gas chromatograph, which is standard in mud-logging units, and an isotope ratio mass spectrometer.

“Isotope fluid logging  C1–C3 service is composed of two main parts, the gas chromatograph to separate the C1, C2, and C3 components and the isotope ratio mass spectrometer (IRMS). Both parts can be installed in a standard mud‑logging rack. The service is always deployed in the mud‑logging unit.”

     In March 2025, a case study of the Ledong Gas Field in the Yinggehai Basin in the South China Sea, analyzed with isotope logging, was published in the journal Energies. This basin produces from high-temperature/high-pressure (HT/HP) reservoirs. Isotope logging has been used in the area since 2014 and is noted for identifying “gas origins, source rock maturity, and gas source type and help judge the sealing quality of overburdened mudstone caps.” These are vital things to know in an HT/HP reservoir.

     Below are the abstract and conclusions of the paper.

    

Advanced Mud Gas Logging Tools 

     According to a 2019/2020 paper in AAPG’s Search and Discovery, the development of advanced mud gas logging (AMGL) tools in the 2010s involved the incorporation of advanced degassing systems and geochemical analyzers. The geochemical advancements include analysis of C6-C8 isomers and a continuous high-resolution carbon stable isotope composition of methane (expressed as δ 13C-C1). This enabled the determination of the mixing of thermogenic and biogenic fluids, biodegradation of petroleum, and phase separation and leakage via cap rocks.

     AMGL can be used to determine the hydrocarbon charge history of a reservoir through fluid fingerprinting. These methods of deeper real-time geochemical analysis do not have the limitations of conventional mud gas analysis. They can be used in combination with more traditional analysis such as gas ratio analysis of alkanes.

 

 

References:

 

Isotope Logging. SLB. Isotope Logging | SLB

At-surface real-time isotope logging assesses lateral heterogeneity in Marcellus Shale. SLB. March 24, 2016. At-Surface Real-Time Isotope Logging Assesses Lateral Heterogeneity in Marcellus Shale | SLB

Application of Carbon-Isotope-Logging Technology in High-Temperature and High-Pressure Wells: A Case Study of the Ledong Gas Field in the Yinggehai Basin. Heng Geng, Xiaojun Xin, Leli Cheng, Jiarong Su, Yitao Hu, Ting Song, Ruike Wang, and Yongkang L. Energies. March 29, 2025, 18(7), 1728. Application of Carbon-Isotope-Logging Technology in High-Temperature and High-Pressure Wells: A Case Study of the Ledong Gas Field in the Yinggehai Basin | MDPI

Charge History Clues from Advanced Geochemical Mud Gas Logging. Alan Keith Fernandes and Dariusz Strąpoć. AAPG. Search and Discovery Article #42509 (2020). Charge History Clues from Advanced Geochemical Mud Gas Logging, #42509 (2020).

Isotope fluid logging C₁–C₃. SLB. isotope_fluid_logging_c1–c3_data_sheet.pdf

Isotope logging C₁. SLB. isotope_logging_c1_data_sheet.pdf

High Resolution Isotopes: A new approach to unlock valuable reservoir insights in Marcellus/Utica. Carl Symcox. Pittsburgh Association of Petroleum Geologists. Meeting. March 19, 2026. (Meeting Abstract).