--> Alluvial Reservoir Development and Drainage Distribution in Large Igneous Provinces

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Alluvial Reservoir Development and Drainage Distribution in Large Igneous Provinces

Abstract

This study aims to produce a predictive model for subsurface hydrocarbon exploration and development in Large Igneous Provinces (LIPs). This project focuses on the waning phase of volcanism, defined here as following the main phase of eruptive activity. During this time, fluvial systems may aggrade tens of meters of sediments during periods of volcanic quiescence or incise hundreds of meters into the underlying flows. The two ancient field examples studied are the Skye Main Lava Series (SMLS) in NW Scotland, and the Columbia River Basalt Group (CRBG) in NW USA. These represent two significantly different expressions of LIP volcanism. The SMLS covers around 1,500km2. It forms one of a number of lava fields developed within the North Atlantic Igneous Province, and was emplaced by several eruptive centers in the form of overlapping flow fields. Fluvial systems were strongly influenced by gradient changes associated with these local centers. In contrast the CRBG was sourced from up to two main fissure zones and covers approximately 164,000km2, which means that potential correlation distances within fluvial systems are longer and the influence of volcanism on the drainage was simpler. By studying both lava fields, this research intends to discover which aspects of these drainage systems are common to LIPs, and which are unique to separate provinces. During the waning phase of eruptive activity, periods of volcanic quiescence are sufficiently long for basins to form in tectonically active areas, allowing the aggradation of thick sedimentary interbeds in the deepest parts of the basin. Small lava flows which only cover part of the basin deflect fluvial systems by creating a step in topography. In the Lewiston Basin in Washington and Idaho, the interbeds between CRBG flows are up to 60m thick and dominated by siliciclastic fluvial channel and proximal overbank deposits. These sediments thin laterally over ~10km to a paleosol 50cm thick. During the late stage of both field examples, canyons are incised up to ~250m into the underlying lava flows and interbeds, and are subsequently filled by intracanyon flows. This compartmentalizes each interbed into two main sandstone bodies. The conclusions from this work will be used to further develop the model of drainage systems in LIPs to allow more accurate sand body prediction in areas such as the Faroe-Shetland Basin where sedimentary interbeds in LIPs are reservoirs for hydrocarbons.