Rock Physics Based Direct Probabilistic Inversion of Seismic Data for Kerogen Thermal Maturity

Abstract

Abstract A rock physics model is developed that allows forward modelling of the elastic response for the thermal maturity variation of unconventional shale in the seismic domain. The model is calibrated to laboratory core measurements and wireline logs from the Early Cretaceous Toolebuc formation from Queensland, Australia. The model predicts that thermally mature shale should exhibit a significantly different seismic response to shales that are not thermally mature. Significantly, in this paper, the rock physics model is then coupled with a probabilistic seismic inversion method. This allows the rock physics model to be used in the inverse sense, i.e., driven by the seismic data to predict areas with a high probability of thermally mature shale. The use of the specific probabilistic inversion method in this study is also significant as the Toolebuc formation is thin relative to the seismic resolution, averaging 20 m - 45 m across the area of the study, and subdivided into three sub-units (Lithofacies1 (LF1), Lithofacies2 (LF2) and Lithofacies3 (LF3). The elastic contrasts are in some cases very weak. The presented workflow concludes that through the novel combination of a rock physics model of shale thermal maturity, calibration of this model to extensive core measurements as well as wireline logs and a modern probabilistic inversion method, it is possible to infer both the thickness and the thermal maturity of thin shale units despite the bandwidth limitations of the input seismic data.