Technical Paper: Determination of Formation Organic Carbon Content Using A New Neutron-Induced Gamma Ray Spectroscopy Service That Directly Measures Carbon

Society: SPWLA
Paper Number:
Presentation Date: 2013


Over the past few decades, many techniques have been developed for the log evaluation of organic-rich rocks (ORR). More recently, ORR have gained economic interest across the globe, not only as the source rocks for conventional reserves, but also as potential reservoirs themselves. Quantifying total organic carbon (TOC) and the complex mineralogy of these rocks are two key components for a robust characterization.

A number of well-log-derived TOC indicators based on measurements such as gamma ray, spectral gamma ray, resistivity, density, sonic, neutron, and nuclear magnetic resonance (NMR) have been developed over the years with different levels of success depending on the specific ORR and where it occurs within a basin. An accurate TOC estimation faces two main challenges: 1) the necessity for a correlation of log data with core to utilize the most representative model and 2) extensive log interpretation. The process of an empirically derived TOC has been hindered by the lack of a consistent direct organic carbon measurement.

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 allows us to measure new elements which, in turn, increases our ability to properly interpret lithologically complex formations. In addition to measuring elements derived from capture spectroscopy, this new service is capable of measuring reliable inelastic yields, including carbon, at reasonable logging speeds. This carbon measurement allows the calculation of a stand-alone quantitative TOC value that does not require local calibrations or multitool interpretation. This is possible because of the successful combination of capture and inelastic gamma ray spectra. Inorganic carbon in carbonate is estimated by using other elements from this logging tool (Ca, Mg, Mn, Fe) to estimate total carbonate, and this value is subtracted from the measured total carbon to give TOC.

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