Oxygen index as an indicator of early organic maturity

Abstract

Most maturity indices can be used during catagenesis and metagenesis but there are only a few indicators to show the degree of organic maturation within the diagenesis zone. A previous study has already demonstrated that the oxygen index (OI) measured by Rock-Eval analysis provides reliable information about the early evolution stages of coals and type III kerogens. The main purpose of this study was to investigate the conditions of OI application in both natural and artificial evolution. The results of experiments presented here revealed that the accuracy of OI in estimating the evolution level depended on the amount and chemical composition of the organic matter. Natural series of low-rank coals showed OI to be a valuable rank parameter not only during diagenesis but in early catagenesis also. OI appeared to be a sensitive indicator during both simulated diagenesis and the entire oil window for gas-prone coal having a substantial humic contribution to its precursor material. In contrast, the OI of oil-prone coal and coal with mixed oil and gas potential proved to be a valuable rank parameter only up to the beginning of the oil window. Though it could be utilized successfully for describing depth profiles in organic-rich intervals of early maturity in wells, its use in determining natural maturity was restricted by the organic-richness of rocks, as well as by the effect of the mineral matrix. Providing humic material is present, OI can be used to follow the artificial thermal maturation of type II kerogen in the absence of mineral matrix.