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How is Detrital Clay Content Distributed in Shallow-Deep Marine Settings? Detailed Insights From Two Highly Active Modern Analogues

Abstract

Primary composition and texture dictate the quality of clastic reservoirs. Depositional mineralogy and spatial-temporal distribution is poorly understood, therefore modern analogues are necessary to address this challenge and aid in advancing reservoir models. In this study, determination of detrital mineral distributions, depositional characteristics and provenance (based on LPSA, XRD, XRF and QEMSCAN) from two modern analogues is assessed to better predict the quality of oil and gas reservoirs. The Bute Inlet study will be the first study to investigate the overall surface distribution of detrital clay content within a modern turbidity system in an attempt to understand initial sediment characteristics and controls on early reservoir quality. The Bute Inlet is an 80km long and 4km wide fjord system along the southwestern coast of B.C, Canada in water depths of up to 660m. The depositional system is dominated by a single sinuous channel 45km long that can be up to 90m deep and shaped by quasi-monthly turbidity currents. The submarine channel is the first where sedimentation and flow properties of turbidity currents have been measured and analyzed from their initiation at the delta front to their dissipation across the terminal lobe. Surface grab samples were acquired from the supplying rivers, along the submarine channel thalweg and at the terminal lobe. The calibration of high-resolution seafloor surveys with such a high spatial density of seafloor samples means we are able to characterise changes in sediment type and detrital clay across the turbidite system in unusual detail.Initial results indicate mineralogy (in decreasing abundance) comprises of detrital feldspars, quartz, trioctahedral mica, Mg-rich amphiboles and chlorite. Trace amounts (<1%) of vermiculite are present but confined to proximal deltaic settings, disappearing in the mid and distal channel. Detrital chlorite is consistent throughout the system at between 4%-6% but its habit and grain relationships have yet to be confirmed.A second study on Ravenglass Estuary; located in the Lake District National Park, UK, focuses on Holocene cores, assessing lateral and vertical variability in mineralogy, lithofacies and spatial distribution to understand early reservoir development. The estuary itself is fed by three main rivers meeting at a point of confluence creating a single channel around 500m wide that is connected to open ocean. Estuarine facies are notoriously complex, poorly understood and documentation related to depositional characteristics is scarce. Preliminary results suggest strong correlations between mean grain size, mineralogy, detrital clay content and facies within estuarine tidal bars.Both modern analogue studies will help facilitate the prediction of sandstone composition and reservoir quality of shallow-deep marine reservoirs.