A Practical Model for Organic Richness from Porosity and Resistivity Logs

ABSTRACT A practical method, the Δ log R technique, for identifying and calculating total organic carbon in organic-rich rocks has been developed using well logs. The method employs the overlaying of a properly scaled porosity log (generally the sonic transit time curve) on a resistivity curve (preferably from a deep-reading tool). In water-saturated, organic-lean rocks, the two curves parallel each other and can be overlain, since both curves respond to variations in formation porosity; however, in either hydrocarbon reservoir rocks or organic-rich non-reservoir rocks, a separation between the curves occurs. Using the gamma-ray curve, reservoir intervals can be identified and eliminated from the analysis. The separation in organic-rich intervals results from two effects: the porosity curve responds to the presence of low-density, low-velocity kerogen, and the resistivity curve responds to the formation fluid. In an immature organic-rich rock, where no hydrocarbons have been generated, the observed curve separation is due solely to the porosity curve response. In mature source rocks, in addition to the porosity curve response, the resistivity increases because of the presence of generated hydrocarbons. The magnitude of the curve separation in non-reservoirs is calibrated to total organic carbon and maturity, and allows for depth profiling of organic richness in the absence of sample data. This method allows organic richness to be accurately assessed in a wide variety of lithologies and maturities using common well logs.