Logging evaluation method and its application for total organic carbon content in shale: A case study on the Lower Cambrian Niutitang formation in Cengong block, Guizhou Province

Shale gas resources are abundant in China. Recently, China has achieved a breakthrough on the Lower Silurian Longmaxi shale in Sichuan basin and its surrounding areas. Compared with the Longmaxi shale, the Lower Cambrian Niutitang shale, which has a greater deposition thickness and wider distribution area, is a significant strata for China's shale gas. The feature of source-storage integration makes the evaluation of the total organic carbon (TOC) content as a significant research content in shale gas exploration and development. In this paper, the Δlg R, natural gamma ray spectrometry, density and multiple linear regression logging methods were used to calculate the TOC content of the Niutitang shale in the Cengong block on the basis of logging response and the geological features of overmatured marine organic-rich shale in the complex structural area in southern China. The calculation accuracy and applicability of each method were compared and analyzed. As a result, the calculation accuracies of the Δlg R, natural gamma ray spectrometry, density and multiple linear regression logging methods increase successively. The Δlg R method is not applicable for overmatured marine shales, which locate in the complex structural areas and have large differences of preservation. The natural gamma ray spectrometry and density logging methods have relatively high accuracy and applicability. Furthermore, the multiple linear regression logging method, which uses logging parameters that correlated with TOC content, can further improve the accuracy and applicability. Besides, some logging parameters such as resistivity and acoustic travel time (AC) are sensitive to gas content, fluid properties and preservation, which can be used as the indicators of shale gas content and preservation. © 2015, China Coal Society. All right reserved.