Direct and Indirect Effects of Flood Basalt Volcanism on Reservoir Quality Sandstone

Abstract

This thesis describes the direct diagenetic effects on porous clean sand substrate due to the emplacement of basalt lava flows. The thesis also describes the effects of the emplacement of basaltic dykes and sills into clean porous sandstone. The primary dataset comes from the Cretaceous Etendeka Group, NW Namibia, where the Etendeka Flood Basalts (and associated subsurface plumbing system) interacted with the aeolian Twyfelfontein Formation sandstone. Secondary datasets from the recent Rekjanes Peninsular basalts, Iceland; the Miocene Columbia River Flood Basalt province and the Miocene Snake River Basalts, NW USA are used to constrain the direct effects of lava on substrates in a variety of palaeoenvironmental conditions.
The thesis makes use of a number of analytical techniques including: petrography, scanning electron microscopy, image analysis, X-ray diffraction, X-Ray fluorescence, stable isotope spectroscopy (δ18 O and δ13 C) and gas permeability (Hasler and probe).
The findings of this work constrain the degree of porosity reduction in clean sandstones due to intrusion emplacement over a complete range of thermal regimes, controlled by the magma flow pathways and duration. The effects range from mild hydrothermal activity and compaction through to intense pyrometamorphism, sediment melting and segregation. Beneath lava flows, the degree of porosity loss is determined by palaeoenvironment (specifically the availability of free water), the lava thickness and the substrate composition. Together the geometries of the igneous components (intrusions and extrusive flows) of the Etendeka Group compartmentalise the sedimentary components (Twyfelfontein Formation), which can be traced due to their effects on hydrothermal activity. The main compartment forming lithologies are vertical-subvertical intrusions, with the lava flows being a minor contributor. The diagenesis during hydrothermal activity was found to be a natural sequestration mechanism of CO2 derived from igneous activity as well as a highly compartmentalised porosity degradation mechanism.