A case study of mapping igneous sill distribution in coal measures using borehole and 3D seismic data

Abstract

Igneous intrusions in or adjacent to coal seams impact the coal quality directly and cause hazards for both coal mining and coal seam gas extraction. Specifically, sills can be identified at borehole level by wireline logs, e.g. natural gamma-ray, density, and resistivity tools because of their unique petrophysical characteristics. The literature describes also the use of surface seismic surveys as a tool for mapping thick sills. However, there is no precise and efficient way to delineate its horizontal distribution where sills are thin in thickness, e.g. less than 5 m. This paper presents an integrated method to predict igneous sill distribution in the Rangal Coal Measures, which are the Late Permian seams from the Blackwater Group in the Bowen Basin, based on petrophysical logs and seismic attributes. In this study, the mean amplitude, half-energy time, and root mean square amplitude from a post-stack 3D seismic survey have been used and compared. Results show that the sill thickness ranges from 0.5 m to 6.4 m with an average of 2.1 m from the analysed 12 wells; The prediction results of the sill are depending on the time-depth windows for calculating and mapping the half-energy time and their cut-offs; The half-energy time calculated with a time-depth window of about 40 ms with a cut-off of 13.5% yields the highest prediction accuracy of 83%. Compared to the mean amplitude and root mean square, the half-energy time is found the best surface seismic attribute in predicting horizontal sill distribution. The sequential Gaussian simulation is successfully used in mapping the sill distribution for the whole study area.