Understanding the nature of the transition between Late Jurassic formations of the Surat Basin through borehole image logs using cumulative dip plots

The Surat Basin located on the border of Queensland and New South Wales is one of the most prominent coal-bearing basins in Australia. Over the past decade, the basin has become one of Australia’s largest and most productive coal seam gas (CSG) provinces, with major reservoirs discovered in the Middle – Late Jurassic Walloon Coal Measures.The Walloon Coal Measures are directly overlain by the Late Jurassic Springbok Sandstone, which has been characterised as a regional aquifer. This raises major environmental concerns regarding the connectivity between the two formations and potential impact of coal seam gas exploration and production from the Walloon Coal Measures on local groundwater reservoirs in the Springbok Sandstone. Therefore, there is a pressing need for a good understanding of the relationship between the two formations, and accurate modelling of reservoir interconnectivity. Despite the growing interest in the Springbok Sandstone, however, the nature of the boundary between the two formations remains unclear. Thus, this study aims to better characterise the Walloon-Springbok contact using highresolution image log analysis in five wells in the north-eastern Surat Basin. Measured bedding-plane orientations are presented on cumulative dip plots that allow precise location of shifts in dip directions and angles related to unconformities, sequence boundaries and structural features.The analysis revealed the presence of a distinct inflection point on the cumulative dip plot between the Walloon Coal Measures and the Springbok Sandstone in only one of the analysed wells. This inflection point reflects a shift in bed orientation that may indicate an unconformity. Conversely, the remaining four wells did not show a significant change in dip magnitude and azimuth at the base of the Springbok Sandstone, suggesting that there is no major or regional unconformity, contrary to previous studies. Additionally, several significant inflection points were observed in the Walloon Coal Measures in all five wells, indicating possible major time gaps in sediment accumulation within the formation, rather than at the formation boundary. These findings largely support recently published depositional ages from U-Pb zircon dating of tuffs that suggest the presence of an unconformity within the Walloon Coal Measures.The image log analysis used in this study provides new constraints on depositional continuity and unconformity distribution in the CSG-producing formations of the Surat Basin. A more extensive application of this approach across the basin in the future will allow assessing the magnitude and regional extent of the identified unconformities.