Multi-scale stratigraphic and statistical analysis of allogenic and autogenic controls on fluvial systems

Abstract

Fluvial systems are important hydrocarbon reservoirs around the globe, including the high net-sand content fluvial reservoirs of the North Sea and the low net-sand content Mungaroo of offshore Australia. Despite their economic significance, fluvial successions are a challenging reservoir type to predict, characterize, and model. Because large amounts of hydrocarbons are stored in subsurface fluvial reservoirs, understanding the stratigraphic expression of external (allogenic) and internal (autogenic) forcing mechanisms at multiple scales is key to predicting reservoir connectivity from the large basin to the small bar scale. At the basin scale this dissertation quantitatively compares and contrasts the influence of lateral boundary conditions on fluvial channel belt stacking patterns. Specifically, how the valley confined Dakota Sandstone is inherently different than the unconfined lower Wasatch Formation in regards to clustering, compensational stacking, and connectivity (Chapter 2). Results from this chapter document that the confined Dakota Sandstone has stronger clustering, lower compensation stacking and higher connectivity than the unconfined lower Wasatch Formation. However, both systems show similar longitudinal trends in these characteristics. At the channel belt scale (Chapter 3), this dissertation puts forth process-based theory coupled with satellite, seismic, outcrop, and numerical experiments to document how the autogenic morphodynamics of the sediment routing system control the planform shape of channel belts. Specifically, that the erosion coefficients of the subjacent and lateral material determine the final channel-belt morphology given long enough residence time on the floodplain. At the smallest spatial scale, the bar scale (Chapter 4), this dissertation uses facies proportions and sedimentary structures coupled with a paleomorphodynamics workflow to document persistence or transience of mean flow velocity. This in turn was used to infer perennial and ephemeral flow conditions. Furthermore, this chapter documents that allogenic signals can either be preserved or shredded by the stratigraphic filter depending on the accretion style of the channel belts. Intra channel-belt signal preservation comes at the expense of basin-scale preservation, where either channel stacking patterns or barforms record the allogenic signal but not both.