Quality Isotope Analysis at the Wellsite: Two Case Studies that Validate GC-C-IRMS Mud Gas Isotope Logging for Deepwater Exploration and Development

Abstract

Abstract Geochemical analysis of gases produced during the drilling process is a common study on oil and gas exploration and development wells. This process typically includes the use of gas sample containers or other vessels that allow for single point samples to be collected for shipment to an offsite laboratory. Laboratories use high precision devices to obtain valuable information for reservoir characterization including stable carbon isotope ratios. In recent years there have been efforts to provide similar analyses during the drilling process, using ruggedized equipment suitable for wellsite deployment. This paper demonstrates that a Gas Chromatograph-Combustion-Isotope Ratio Mass Spectrometer (GC-C-IRMS) analyzer, using similar technology to what is most widespread in offsite laboratories (Dashti et al, 2018), can be successfully deployed to the rig site. This type of advanced gas analysis, commonly known as Mud Gas Isotope Logging (MGIL), provides continuous sampling of stable carbon isotopes of methane (δ13C1), ethane (δ13C2), and propane (δ13C3). The service, performed with a GC-C-IRMS analyzer, was proven and validated for an operator through two case studies. 98 The first case compares real time data with discrete gas sample tubes analyzed in an offsite laboratory. It shows how accurate results are possible, even with the presence of artificial gases generated by drill bit metamorphism (DBM) (Wenger et al, 2009). This example also demonstrates how the service enabled immediate analysis for operational decisions by indicating the presence of biodegraded thermogenic fluid. The second case study demonstrates how this wellsite service could corroborate the geological prognosis in a complex field influenced by salt tectonics. In this basin an upthrown reservoir changed the typical behavior observed in conventional wells of increased oil maturity with depth. Stable carbon isotope readings obtained in real time, integrated with cuttings analysis, indicated the presence of out of section lithology. This information allowed for estimating the thermogenic fluid maturity of reservoirs and diagnosis of geological formations that were out of sequence in terms of age (uplifted).