Failure to predict igneous rocks encountered during exploration of sedimentary basins: a case study of the Bass Basin, Southeastern Australia

The Bass Basin, a Mesozoic-Cenozoic intra-continental rift basin along the southern Australian continental shelf, offers an excellent natural laboratory for examining igneous rocks in the subsurface. Igneous material within the basin is manifested as a mixture of predominately mafic extrusive and intrusive rocks, mainly Cretaceous-Palaeocene and Oligocene-Miocene in age. Igneous rocks have been encountered in 20 out of 36 (55.6%) exploration wells drilled within the Bass Basin, but the presence of these has historically been poorly predicted; of the first 11 exploration wells to penetrate igneous rocks, their presence was not predicted in pre-drill interpretations. We present a series of case studies from wells that unexpectedly encountered igneous rocks. The first of these wells (Bass-1) targeted a carbonate reef structure which instead penetrated the flank of a submarine volcano. In another notable example (Flinders-1 well), a relatively discontinuous high-amplitude seismic reflection thought to be a clastic reservoir was found to be an igneous intrusion with a relatively unusual composition. A number of these incidents, where igneous rocks were unexpectedly encountered, can be accounted for by human factors, such as a sparsity of good quality data and a lack of knowledge transfer as companies entered and left basin during different phases of exploration. However, a number of examples, particularly the unexpected occurrence of igneous intrusions, appear to have been caused by anomalously low acoustic impedance contrasts between igneous rocks and surrounding sedimentary sequences. Our findings have generic implications for other sedimentary basins impacted by magmatic activity, such as the importance of integrating available outcrop data in the absence of nearby well control, and the value of fully appraising previous exploration results.Douglas Watson, Simon Holford, Nick Schofield, Niall Mar