Direct TOC Quantification in unconventional Kerogen-rich Shale Resource Play from Elemental Spectroscopy Measurements: A Case Study from North Kuwait

Joshi, Girija (Kuwait Oil Company) | Acharya, Mihira Narayan (Kuwait Oil Company) | Al-Azmi, Mejbel Saad (Kuwait Oil Company) | Dashti, Qasem M (Kuwait Oil Company) | Van Steene, Marie (Schlumberger Oilfield Eastern Limited) | Chakravorty, Sandeep (Schlumberger Oilfield Eastern Limited) | Darous, Christophe (Schlumberger Oilfield Eastern Limited)


The deep organic-rich calcareous Kerogen of North Kuwait, a continuous 50ft thinly alternating carbonate - organic-rich argillaceous sequence, is not only a source rock but has gained importance as potential reservoirs themselves of typical unconventional category. Kerogen characterization relies on quantifying total organic carbon (TOC) and estimating accurate mineralogy. This paper describes an attempt to directly measure TOC of the Limestone-Kerogen sequence.
For the present study, empirical estimations of TOC have been carried out based on various conventional log measurements and also nuclear magnetic resonance. The introduction of a new neutron-induced capture and inelastic gamma ray spectroscopy tool using a very high-resolution scintillator and a new type of pulsed neutron generator for the deep unconventional kerogen resources have provided a unique opportunity to measure a stand-alone quantitative TOC value using a combination of capture and inelastic gamma ray spectra. In this process, Inorganic Carbon Content (ICC) is estimated by using elemental concentrations measured by this logging tool in addition to measuring Total Carbon (TC). The difference between TC and ICC gives direct TOC.
The advanced elemental spectroscopy tool measurements were first used to determine accurately the complex mineralogy of the layered carbonate and organic-rich shale sequence. The petrophysical evaluation and heterogeneity seen on borehole image logs were calibrated with extensive laboratory measurements of core / cuttings data. The final results are considerably improved compared to conventional empirical estimation. Once the mineralogy is properly determined, the log-derived TOC matches very well with core measured TOC.
This technique has provided a new direct and accurate log-derived TOC for Kerogen characterization. The application has a potential to be used for CAPEX optimization of the coring in future wells. This technique can also be applied in HPHT and High-angle horizontal wells, which can overcome challenging coring difficulties in horizontal wells.