Total Organic Carbon Evaluation Using Geophysical Well-Logs in Lorraine Coal Basin (NE France).

The Lorraine Coal Basin, recognized as the most promising coal bed methane basin in France and Europe, holds significance for the energy transition after more than a century of coal mining until 2004 and renewed interest in oil and gas exploration in the 80s and 90s. New data from an on-going exploration campaign within this basin facilitates the assessment of gas resources embedded in sedimentary deposits. This evaluation is achieved by determining Total Organic Carbon (TOC) along wells, using well-log data. The study employs a methodology encompassing Schmoker’s method, Δ log R, and multivariate regression, focusing on two reference boreholes, namely Fols 1A and Dbl S1. Schmoker’s method provides the most accurate TOC values, consistent with Rock-Eval data. However, the Δ log R method underestimates TOC values, particularly for coal lithologies, for which logarithmic equations are proposed for proportional corrections. Moreover, new equations based on multivariate regression of gamma-ray, sonic, and resistivity logs are developed for TOC estimation. Schmoker’s method proves to be the most reliable with available density logs in the Lorraine Coal Basin. Alternatively, the modified Δ log R equations or those derived from the multivariate regression of gamma-ray, sonic, and resistivity logs can be used. Based on TOC, lithologies of the investigated sedimentary series may be classified from lean to fair source-rocks for shaly lithologies to excellent for the coal layer.