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Title: Characterising pre-vegetation paralic sedimentary systems and developing improved architectural reservoir models
Author: Bradley, Ginny-Marie
ISNI:       0000 0005 0290 0683
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2019
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In modern siliciclastic environments, terrestrial vegetation binds substrate, controls weathering and erosion rates, influences run-off, sediment supply, composition and has an important influence on depositional architecture. This study assesses pre-vegetation paralic systems by integrating traditional outcrop sedimentology with Unmanned Aerial Vehicle (UAV) technology. Results show that these ancient sedimentary systems have unique characteristics and highlight the need for tailored models for both fluvial and tidal environments. This thesis examines the Tumblagooda Sandstone of Western Australia to assess the sedimentary characteristics and geobody architectures of pre-vegetation paralic systems. Many statistical datasets exist for fluvial geobody dimensions, mainly derived from modern rivers or Mesozoic outcrops, where deposition was influenced by terrestrial vegetation. Previous authors have noted that pre-vegetation successions have a unique preserved sedimentology and suggested that the depositional architecture is dominated by sheet-braided systems. This comprises planar basal erosion surfaces and aspect ratios of greater than 20:1. Recent work has suggested that this may be an over simplification of the sedimentology and architectures. Detailed outcrop analysis and acquisition of high-resolution 3D digital outcrop models have been used to map the large-scale geometry of the preserved fluvial geobodies. Fluvial facies are dominated by two types of architectural elements: isolated and amalgamated sheets and lenticular shaped geobodies. The overall depositional environment has been identified as a channel-braided system which was dominated by repeated avulsion, reworking and cannibalisation of previously deposited sandstones. These amalgamated units are interpreted to have formed during periods of reduced accommodation space. The overlying isolated fluvial units are encased in tidal sandstones and are interpreted to record periods of transgression. The system does not only display sheet-braided architectures. Geobody have a wide range of architectures extending above and below the previously defined 20:1 aspect ratio and this environment preserves both sheet and wide ribbon geometries. Geobody statistics extracted from the digital reconstructions have allowed the generation of improved conceptual models for pre-vegetation successions. Stochastic modelling compares the sheet model vs the channelised model to determine which better represents the fluvial geometries observed in the outcrop model. These systems are best modelled using object-based methods, as the sheet-like pixel-based model grossly overestimates fluvial facies and does not accurately reflect the geometries and distributions observed in the outcrop analogues. This study also analyses the interbedded marine units in the Tumblagooda Sandstone, interpreting them as tidal successions. Ancient paralic pre-vegetation mature sandstones typically lack mudstone preservation which is an important criterion for identifying tidal facies. Notably they lack structures such as mud-draping and flaser bedding, which may be related to limited delivery and preservation of muds to the shallow marine realm prior to the evolution of deeply rooted plants on land. There is a paucity of pre-Devonian tidal successions reported in literature, and this thesis summarises that this is possibly due to the lack of characteristic tidal indicators. In such cases, application of modern analogues to interpret the environment may lead to erroneous environment designation. This study identifies key criteria required for identification of tidal facies in the absence of preserved mudstone features prior to the evolution of deeply rooted plants. The use of the correct analogues to interpret depositional environment and during reservoir modelling has a profound effect on prediction of subsurface facies distribution, connectivity between wells, fluid flow rates and production strategy. Using the sheet model implies that facies are laterally continuous for 100's km and inter-well connectivity would be likely. This would result in increased fluvial reservoir volume and reserves predictions. Sweep would be more efficient as homogeneous facies are less tortuous than their channelised counterparts. The channelised model implies that fluids would have to flow through complex bounding surfaces at all scales, from cross-bedding foresets to channel margins. Facies would also vary laterally and vertically within channels and fluid flow would be enhanced along higher permeability pathways along channel axis. Production rates may be lower, and the strategic placement of wells would be required to target connected isolated elements and gain effective sweep.
Supervisor: Hodgetts, David ; Redfern, Jonathan Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Digital outcrop models ; UAV ; Paralic ; Tidal ; Sedimentology ; Fluvial ; Pre-Vegetation