Abstract

Quantitative estimation of total-organic-carbon (TOC) content using well logs is very important for shale-gas reservoir evaluation, especially when core data are limited. Even though many techniques have been developed to calculate TOC from well logs, they all have their limitations and accurate assessment of TOC in organic-rich tight shales is still challenging. We have used an improved log-overlay method for evaluating TOC content in organic-rich tight shale reservoirs by overlying the properly scaled gamma-ray, sonic transit time, and bulk density curves on top of the compensated neutron log (CNL) curve based on the same concept to the original ΔlogR technique. These logging curves are overlapped with the CNL in nonsource intervals, and they are separated in organic-rich shale reservoirs. The separation magnitude increases along with the increase in TOC content of the shale, and this relationship is transformed to calculate TOC. This method was tested and verified by doing a case study using well-log data from the Jiaoshiba tight shale-gas play in the Sichuan Basin. The results illustrated that the new TOC evaluation method is more practical and effective compared with existing TOC evaluation methods.

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