Prediction of vitrinite reflectance of shale oil reservoirs using nuclear magnetic resonance and conventional log data

•Based on the petrophysical bulk model of shale and the variation of kerogen density with thermal maturity, a new thermal maturity prediction index UTM is proposed.•The new index UTM is constructed by density log, NMR log and TOC, and it can be predicted by log data continuously.•A linear model is established to predict vitrinite reflectance from the new index UTM, with high accuracy verified by experimental data. With the change of oil and gas exploration and development focus to shale reservoirs, the source rock has become a research hotspot in well logging. It is of great significance to obtain continuous organic geochemical parameters based on log data. In this paper, according to the density-bulk model of hydrocarbon source rocks, a new indicator parameter named UTM, which can characterize the thermal maturity, is constructed based on the variation of kerogen density with thermal maturity in the Jimusar Sag of Junggar Basin. This parameter can characterize the thermal maturity and be calculated by density log, nuclear magnetic resonance (NMR) log and total organic carbon content of core. This parameter has the advantage of avoiding the difficulty of calculating kerogen density. The model was validated using cores and log data that were not involved in the modeling. The vitrinite reflectance predicted by the UTM index was in good agreement with the actual cores demonstrating the reliability and applicability of the method. The method provides a new idea and practical solution for the quantitative characterization of thermal maturity in the shale oil fields.