Seismic, petrophysical and petrological constraints on the alteration of igneous rocks in the Northern Carnarvon Basin, Western Australia: implications for petroleum exploration and drilling operations

The Northern Carnarvon Basin (NCB) hosts an extensive record of Jurassic–Cretaceous rift-related igneous activity, manifested by a >45 000 km2 intrusive complex and series of volcanic centres constrained by seismic mapping. However, there are relatively few well penetrations of these igneous rocks (<1% of ~1500 exploration wells) in comparison to other basins that witness extensive magmatism, and thus, their lithological and petrophysical characteristics are poorly understood. Here, we describe the properties of igneous rocks encountered in nine petroleum exploration wells and scientific boreholes in the NCB and evaluate their impacts on exploration and development issues. Igneous rocks in the NCB are characterised by pervasive alteration, with ramifications for seismic imaging and drilling. For example, low acoustic velocities in mafic lavas altered to clays in Toro-1 were mistaken for overpressure, whilst intrusive rocks in Palta-1 were initially unrecorded and only recognised due to subsequent post-drilling thermal history analysis. The alteration of mafic igneous rocks to clays reduces acoustic impedance contrasts relative to sedimentary host rocks, making their identification prior to drilling more challenging. Whilst the preferential emplacement of intrusive rocks in Triassic strata deeper than reservoir targets is primarily responsible for the paucity of well penetrations, our findings of extensive alteration of igneous rocks in the NCB suggests that additional wells may intersect as yet unrecognised intrusive or extrusive sequences.