An improved method for evaluating the TOC content of a shale formation using the dual-difference ΔlogR method

Abstract

Abstract The total organic carbon (TOC) content is the most important geochemical parameter of a shale reservoir; thus, the methods used for its prediction require further study. The ΔlogR method is currently the only available TOC calculation method based on a rigorous petrophysical model derivation. However, because the ΔlogR method does not consider the complex mineral compositions of shales or the effects of hole enlargement, the applicability of the ΔlogR method to the evaluation of an actual shale reservoir is obviously inadequate. In this paper, a targeted and fundamental improvement is made to the ΔlogR method, and a dual-difference ΔlogR (DDΔlogR) method is proposed. First, a dynamic theoretical relation curve is derived, meaning that the ΔlogR method considers the characteristics of a variable theoretical relation curve. Second, two ΔlogR values are calculated using both the acoustic curve with TOC information and the theoretical acoustic curve without TOC information, and the difference between the two is used to characterize the TOC content. Finally, the radioactivity of organic matter is characterized using a log curve that excludes the influence of the clay content. Through the processing of actual data from the Sichuan Basin, the DDΔlogR method effectively improves the ΔlogR calculation accuracy in consideration of rock mineral composition variations and hole enlargement effects, and the prediction results are better than those obtained using machine learning algorithms. The DDΔlogR method proposed in this paper greatly expands the applicability of the ΔlogR method and can effectively aid in the exploration and development of shale reservoirs.