Petrophysical variations in a syn-rift volcanic sequence on the southwest Australian rifted margin (IODP Site U1513)

Primary lithologic characteristics and subsequent modification by alteration processes control petrophysical properties of volcanic rocks. This study investigates the linkages between physical properties, lithology, and alteration processes. We compile a variety of petrophysical data and correlate these with lithologic, mineralogical, and geochemical analyses, acquired from a submarine volcanic sequence on the Naturaliste Plateau, offshore southwest Australia (International Ocean Discovery Program Site U1513). The plateau is underlain by thinned continental curst displaced from the southwest Australian rifted margin, formed during the early Cretaceous breakup of East Gondwana. The syn-rift volcanic eruption and emplacement occurred in subaerial to shallow water setting. The sequence consists of alternating basalt lava flows and volcaniclastic deposits, intruded by multiple dolerite dikes. Variable alteration intensities from fresh-slight to strong are evaluated using Chemical Index of Alteration (CIA) and alteration mineralogy. Intervals of fresh-slightly altered dikes exhibit low porosity (~13%) and high values of bulk density (~2.7 g/cm3), P-wave velocity (~4900 m/s) and thermal conductivity (~1.7 W/(m·K)). The increase of alteration intensity corresponds to decreases in bulk density to ~2 g/cm3, P-wave velocity to ~2000 m/s, thermal conductivity to ~1.2 W/(m·K) and an increase in porosity up to 50%. Natural Gamma Ray and magnetic susceptibility vary downhole with rock composition and at lithologic boundaries. The distinct variations exhibit a good correlation with primary lithologic characteristics and secondary mineralogical and textural changes attributed to alteration processes by subaerial exposure, seawater interaction and hydrothermal activities. We provide synthesis models of petrophysical variation with alteration intensity. Although differences in primary lithology, alteration type, and petrophysical methodology introduce limitations and uncertainties, there is a reasonable applicability of our results to rapidly characterize the alteration intensity and volcanic stratigraphy in volcanic sequences. Our results will help geoscientists and geoengineers to develop strategies for exploration, drilling, and geophysical research of volcanic terrains.