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Title: Depositional character of "syn-rift" deep-water deposits : a case study from the upper Jurassic Buzzard turbidite system
Author: McKinnon, Mark
ISNI:       0000 0004 2745 9823
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2013
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The Buzzard Field lies within outer Moray Firth (Central North Sea) and is part of a Late Jurassic, syn-rift, deep water depositional system that comprises deep-water sandstones interbedded with ‘expanded’ hemipelagic mudstones, and mud-rich mass transport deposits, sourced from basin margin highs. Sandstones are interpreted mainly as the deposits of large volume, sustained high density turbidity currents sourced axially through linear systems aligned with the west-to-east regional structural trend. The extensive core database has formed the basis for the sedimentological interpretation of the field, which has been extended into uncored wells using a new lithofacies recognition technique pioneered within this study. Dipmeter data have been used to supplement lithofacies interpretation of conventional wireline logs. Neural net analysis of these log signatures, calibrated to core description, has revealed the lithofacies present in uncored section and allowed an improved understanding of lithofacies variability across the field. The depositional history of the Buzzard system, based on fieldwide correlation and a regional biostratigraphic framework, is interpreted to consist of seismically triggered flow events, which were initially confined by structurally generated basin floor topography; later, more frequent flows encountered simpler bathymetry and were weakly confined, depositing lobe shaped sandstone bodies. This sequence is summarised by several depositional cartoons. The field is described here as a three-way stratigraphic trap with a down-dip oil-water contact despite being deposited in a tectonically active period. The sedimentological factors that contribute to the trapping mechanism are discussed in detail including: flow divergence; slope angle; and basin margin interaction. This study has implications for the interpretation of flow processes in (and sediment delivery mechanisms to) the deep-water environment, recognition of lithofacies from wireline data, and for future exploration in syn-rift, deep-water settings. Analysis of the extensive core dataset has revealed that sands were transported to the deep basin as part of large-scale flow events and were deposited rapid mainly through fallout from suspension. These infrequent but catastrophic flow events moved large volumes of sediment from a shallow marine staging area to the deeper setting, eventually depositing a characteristically thick-bedded sandstone succession. The coarsest-grained portions of individual event beds have been identified as high permeability layers and are essential to understand from a production perspective. The geometry and distribution of these high permeability zones is controlled by depositional process and are iii located in the axial, updip parts of event beds. Therefore, accurate correlation of thick event beds is particularly important for reservoir characterisation. The updip depositional limit of these sands defines a stratigraphic pinchout margin that corresponds to an Upper Jurassic isochron which may be used to define the limit of Upper Jurassic sandstones in other areas of the North Sea. Furthermore, the pinch out limit is defined at a transition from high structural complexity updip to lower complexity downdip. Crucially, this transition allowed flows that were routed through the complex updip palaeobathymetry to diverge downdip, thus rapidly depositing their sediment load. Well established sediment gravity flow pathways such as those interpreted to feed the Buzzard basin are rare in the Upper Jurassic of the North Sea. Despite that, this combination of flow pathway, shallow marine source area and structural configuration we essential factors in producing a reservoir of the quality of Buzzard;similar tectono-stratigraphic settings may also produce Buzzard-style reservoir and should be considered exploration targets.
Supervisor: Not available Sponsor: Nexen (Firm) ; Suncor (Firm) ; EDG (Firm) ; BG Group
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Geology ; Marine sediments