Abstract The Browse Basin is one of Australia's major hydrocarbon provinces, where significant discoveries have been made in recent decades including the Ichthys and Prelude fields, which accounted for ∼15% of the cumulative Australian liquified natural gas (LNG) production in 2019–20. This rift basin hosts extensive Mesozoic intrusive and extrusive igneous rocks, having been identified from both well and seismic data, and which are recognized as one of the key challenges for exploration and production activities in this region. Their impact on petroleum exploration is demonstrated by the number of wells which encountered unpredicted or thicker than expected igneous rock units both within and adjacent to target sections. This study therefore aims to document the reasons of such unexpectedness, and to develop capability to predict the occurrence of igneous rock units prior to drilling in the Browse Basin and other rift settings that contain igneous rocks. Multiple case studies of uncommercial exploration wells are developed by integrating petrophysical and seismic reflection data, focusing in particular along the outboard part of the basin where igneous rocks are most prevalent. Our study highlights the importance of understanding petrophysical, spatial and chemical heterogeneities of igneous rocks in basins to explain their emplacement and distribution, and thereby predict their occurrence prior to exploration and development activities.

Abstract Palaeoformation water trapped in quartz cements in sandstone during diagenesis is typically of interest for constraining the temperature history, cementation and timing of hydrocarbon charge. Recent progresses in developing methods for salinity measurement, gas detection (CH 4 , CO 2 , N 2 , H 2 S) and fluid modelling of the CH 4 –H 2 O–NaCl system by combining conventional microthermometry techniques with Raman spectroscopy provide powerful tools for investigating formation water and its evolution in gas-bearing basins. Samples from the aquifer, in the Plover Formation and in the Brewster Member in the Upper Vulcan Formation, underlying large gas accumulations in the Caswell Sub-basin provided an opportunity to test these new techniques and generate data on formation water evolution. Temperature of homogenization, salinity and gas content of water inclusions show that the salinity of the palaeoformation waters decreased with increasing methane content and temperature. Detection of CO 2 shows, however, that water inclusions with dissolved CO 2 , often in association with CH 4 , do not follow the same salinity trend. These inclusions are often associated with higher trapping temperatures. The salinities associated with water reaching methane saturation (coeval with free gas) are between 8500 and 24 000 ppm eq. NaCl (0.8–2.4 wt%). An influx of meteoric water from the Ashmore Platform in the north is presented as a hypothesis on the origin of the low salinities of the formation water in the Plover Formation in the Browse Basin, supported by the distribution of the lowest palaeowater salinities, but still remains problematic.