title: Sedimentology and sequence stratigraphy of tidal sandstone bodies : implications for reservoir characterisation.
Creators: name: Philip. Wonham, Jonathan
Publisher: name: University of Liverpool Repository
Year: 2023

description: Analysis of the facies and sequence stratigraphy of tidally-influenced, siliciclastic-dominated successions from five different areas has allowed a survey and comparison of the sedimentological and architectural characteristics of a variety of tidal sandstone body types. This study is concerned with: (i) describing these different sandstone body types; (ii) analysing the controls on sandstone body internal architecture, external geometry and connectivity, and (iii) outlining the implications of such studies for reservoir characterisation. Facies analysis of the studied successions by previous workers has identified the importance of tidal processes in sandstone body deposition, however, the application of sequence stratigraphy suggests reinterpretation of a number of facies. This applies, in particular, to facies interpreted as shelf sand wave deposits which have previously been recognised in all of the field study areas. These facies are now reinterpreted as incised valley fill deposits following the recognition of: (i) the estnarine facies and facies associations which make up these sandstone bodies, and (ii) the presence of sequence bounding erosional unconformities at the base of these sandstone bodies. The studied basins show contrasting subsidence rates and structural styles. Two successions from foreland basin settings have been studied: the OligO-Miocene Marine Molasse (=IOOOm thick) of the Digne-Valensole basin, and the Eocene Figols Group (",600m thick) of the Ager basin. Two orders of type-I depositional sequence, with thicknesses of lOs and l00s of metres respectively, have been identified from both areas. Passing upwards, both successions show a change from an overall transgressive to regressive setting interpreted to reflect increasing rates of sediment supply through time. Sandstone bodies of the Marine Molasse include: (i) estuarine incised valley fills; (ii) tidal inlets; (iii) delta mouth bars, and (iv) tidal flat channels. These units vary from a few metres thick and few hundred metres wide to several lOs of metres thick and several kilometres wide. Sandstone bodies of the Figols Group, by contrast, are developed within large incised valleys approximating to the scale of the Ager basin itself (6-7km wide, 15km long). These sandstone bodies are sharp-based estuarine mouth bars. The mouth bars themselves may be further compartmentaIised into erosively based sandstone lenticles «6m thick, 200m wide and &lt;600m long) which are interpreted to have formed by the migration of very large-scale dune bedforms. The deposits of the Lower Jurassic Tilje and St0 Formations were developed within extensional basins of the Norwegian shelf and show much slower subsidence rates. The studied successions are dominated by tidal flat, tidal channel and tidal delta mouth bar deposits. Tidal flat parasequences are either coarsening- then fining-upwards or simply fining-upwards in character. Identification of retrogradationally stacked tidal flat parasequences suggests that sandstone-dominated intervals may be identified using sequence stratigraphy. The Lower Cretaceous Woburn Sands succession (up to 120m thick) shows the development of two depositional sequences in the Leighton Buzzard area. The majority of the succession consists of incised valley fill deposits showing a regressive estuarine facies association. New evidence which supports this interpretation includes the recognition of tidal flat deposits at the top of the incised valley fill, overlain by a well developed initial flooding surface. This sandstone-dominated succession shows the development of laterally extensive clay drapes which strongly compartmentalise the sandstone body. These clay drapes are interpreted to develop in the toe of laterally migrating sand banks and form an important and hitherto unreported permeability barrier type. The identification of incised valley fill deposits in a nwnber of basin settings has allowed a survey of the variability of this type of sandstone body. Incised valleys vary according to whether they show: (i) a single or a composite fill; (ii) a siliciclastic or mixed siliciclastic/carbonate infill; (iii) a retrogradational or progradational infill; (iv) presence or absence of an estuary mouth barrier, and (v) infill dominated by very large-/large-scale cross-beds or mediwn-/small-scale cross-beds. The infill character of individual incised valleys is shown to be controlled by the relative importance of rate of relative sea level rise, sediment supply and tide-dominance. Successively developed incised valleys show changing infill character related to the superimposition of low and high order cyclic controls on relative sea level change.