Oxygen index as an indicator of early organic maturity

Most maturity indices can be used during catagenesis and metagenesis but th ere are only a few indicators to show the degree of organic maturation with in the diagenesis zone. A previous study has already demonstrated that the oxygen index (OI) measured by Rock-Eval analysis provides reliable informat ion about the early evolution stages of coals and type III kerogens. The ma in purpose of this study was to investigate the conditions of OI applicatio n in both natural and artificial evolution. The results of experiments pres ented here revealed that the accuracy of OI in estimating the evolution lev el depended on the amount and chemical composition of the organic matter. N atural series of low-rank coals showed OI to be a valuable;rank parameter n ot only during diagenesis but in early catagenesis also. OI appeared to be a sensitive indicator during both simulated diagenesis and the entire oil w indow for gas-prone coal having a substantial humic contribution to its pre cursor 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 we lls, its use in determining natural maturity was restricted by the organic- richness of rocks, as well as by the effect of the mineral matrix. Providin g humic material is present, OI can be used to follow the artificial therma l maturation of type II kerogen in the absence of mineral matrix. (C) 1998 Elsevier Science Ltd. All rights reserved.