Sedimentology and sequence stratigraphy of tidal sandstone bodies : implications for reservoir characterisation.
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
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 <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.