Selection of logging-based TOC calculation methods for shale reservoirs: A case study of the Jiaoshiba shale gas field in the Sichuan Basin

Various methods are available for calculating the TOC of shale reservoirs with logging data, and each method has its unique applicability and accuracy. So it is especially important to establish a regional experimental calculation model based on a thorough analysis of their applicability. With the Upper Ordovician Wufeng Fm-Lower Silurian Longmaxi Fm shale reservoirs as an example, TOC calculation models were built by use of the improved ΔlgR, bulk density, natural gamma spectroscopy, multi-fitting and volume model methods respectively, considering the previous research results and the geologic features of the area. These models were compared based on the core data. Finally, the bulk density method was selected as the regional experimental calculation model. Field practices demonstrated that the improved ΔlgR and natural gamma spectroscopy methods are poor in accuracy; although the multi-fitting method and bulk density method have relatively high accuracy, the bulk density method is simpler and wider in application. For further verifying its applicability, the bulk density method was applied to calculate the TOC of shale reservoirs in several key wells in the Jiaoshiba shale gas field, Sichuan Basin, and the calculation accuracy was clarified with the measured data of core samples, showing that the coincidence rate of logging-based TOC calculation is up to 90.5%-91.0%.