Characterising alluvial architecture using physical models, subsurface data and field analogues

Abstract

This thesis presents an integrated subsurface, field analogue and scaled physical modelling approach to the investigation of the autocyclic and allocyclic controls on braided river morphology and preserved alluvial architecture at both the channel-belt and within channel-belt scales. The prediction of inter-well alluvial architecture in the continental Barren Red Beds of the Upper Carboniferous, Lower Ketch Member, Southern North Sea, UK, presents difficulties. Subsurface data from the Tyne Field is subdivided into coarse-grained sandbodies (60%) alternating with siltstone lithofacies (40%). A depositional model is proposed representing an aggrading alluvial plain traversed by a series of 100 to 1100 m wide channel-belts with palaeochannel depths of between 1.5 and 3.5 m, and supplied with a fine grain size distribution (D50 = 0.08 mm). A seasonal climatic regime probably promoted crevasse splay deposition and development of extensive floodplains. The Tyne model can nest within a regional depositional setting where: (1) the predominant sediment supply was from the north; and (2) proximal to distal trends exhibit a grain size diminution, and a lowering of net-to-gross and connectivity. This thesis provides the first complete comparative study between the Tyne Field and an onshore analogue, the Eocene Escanilla Formation, Southern Pyrenees, Spain. New evidence is presented to suggest a fluvio-lacustrine setting for the Escanilla Formation. A marked qualitative and quantitative similarity between the two sedimentary sequences gives confidence in the analogue and permits the use of geometrical datasets, including sandbody width/thickness ratios, frequency distributions, channel-belt orientation and connectivity to be used in the construction of geological reservoir models of the Lower Ketch Member. Empirical estimates of channel-belt width in the prediction of sandbody geometry overestimate -75 % of the channel-belt sandbody widths measured from the outcrop, -because estimates fail to account for: (1) different channel patterns, (2) any autocyclic and allocyclic controlling mechanisms, and (3) vertical and lateral channelbelt stacking. Grain size is a key control on alluvial architecture at both the within channel-belt and channel-belt scale. Physical modelling of the Ashburton River, Canterbury Plains, New Zealand has allowed the impact of grain size to be isolated from other controlling parameters. Three reach-scale, generic Froude-scaled modelling experiments, incorporating a fourfold change in grain size have been undertaken in a flume facility that permits aggradation. Comparisons between model and field studies suggest grain size is not the most influential control on channel geometry. The modern Ashburton River exhibits no change in channel geometry downstream. However, a halving then quartering of the grain size distribution in the Ashburton River model resulted in: (1)channel geometries first deepening then widening, (2) the proportion of overbank sheet flow increasing, and (3) the duration of channel occupancy remaining constant. Under the boundary conditions imposed in the experimental study, grain size limits are recognised for scaled physical models, which suggest that physical models cannot reproduce Lower Ketch Member grain size distribution ranges. The preserved alluvial architecture from the Ashburton model is classified and quantified into six architectural elements. Architectural elements from the Ashburton River model show that alluvial architecture is dominated by channel fill deposits. An increase in primary channel fill width and thickness, an increase in splay widths, a higher proportion of preserved splay deposits, and a decrease in the size and frequency of occurrence of fine-grained architectural elements may all be attributed to a quartering of the grain size distribution. A methodology is presented to link permeability to the architectural elements of the Ashburton model, in order to supply data from the preserved stratigraphy of physical models to serve as inputs for the construction of both static and dynamic (fluid flow) reservoir models. In addition, the potential of an aggrading physical model to examine the impact of sampling frequency and up-scale averaging in the construction of reservoir models is explored