Hierarchical stratigraphic cycles in the non-marine Clair Group (Devonian) UKCS

Abstract

Abstra~. The Devonian lower Clair Group (c. 300-800 m thick) is dominantly composed of fluviatile sandstones and conglomerates, aeolian sandstones and minor floodplain and lacustrine shales. These facies are organized into stratigraphical cycles which have a threefold hierarchy: an unconformity bounded cycle representing first order retreat and advance of the fluvial drainage system; three subsidiary second order cycles bounded by minor unconformities, and numerous third order cycles. These stratigraphical cycles exhibit common characteristics: they are bounded by sharp, commonly erosive surfaces; they are composed of a fining-upwards succession overlain by a variably developed coarseningupward element; the fining-upward element records a progressive decrease in the fluvial component, and increasing proportion of aeolian, floodplain or lacustrine facies (this trend is reversed in the upper, coarsening-upward section). The grain size of the fluvial facies, and nature of the non-fluvial facies (aeolian vs floodplain/lacustrine) within any order of cycle is dependent on it's position within a lower order (larger scale) cycle.The cycles are areally extensive, and are present in all wells within the Clair Basin, although spatial changes in internal facies make-up occur. The second order cycles show progressive onlap followed by ofliap of the basin margin.These hierarchical cycles are interpreted to be the product of changes in the accommodation:supply ratio of the depositional system. An increasing accommodation:supply ratio provided space to preserve the record of waning fluvial influence, enhanced aeolian reworking, preserved floodplain fines or lacustrine mudrocks. In contrast, a reducing accommodation:supply ratio resulted in filling of accommodation space and sediment bypassing. Accommodation space variations were dictated by tectonic variations in strain rate, which caused incremental uplift of the basin margin and subsidence of the basin. Sediment supply changes were influenced by source area uplift, which changed sediment yield, and climate change which affected runoff. Climatic changes are detectable within the non-fluvial elements of the Clair Group by stratigraphic variations between aeolian, floodplain and lacustrine facies. These vary systematically within and between second order unconformity bounded units, suggesting a tectonic influence on climate.Within the second and third order cycles, those which were evolving towards, or were occurring within, semi-add to arid climatic conditions are commonly (but not exclusively) biased towards fining-upward successions, whilst the more humid systems show a better developed coarsening-upward element. This may be the result of the changing impact of discharge versus sediment supply variations when one of these parameters became suppressed or enhanced under varying climatic conditions.

Sequence stratigraphy of the Westphalian in the northern part of the Southern North Sea

“…Cross et al 1993;O'Byrne & Flint 1993;Wehr & Brasher 1996). However, sequence stratigraphy has rarely been applied to alluviallacustrine successions with the notable exception of Legarreta et al (1993) and McKie & Garden (1996). The model developed here for Westphalian alluvial-lacustrine sequences helps explain certain unusual aspects of the lithostratigraphy such as the widespread occurrence of retrogradational fluvial reservoirs and the absence of shoreface sandstones.…”

Fluvial Systems in Desiccating Endorheic Basins

“…Published sedimentological studies have been by Allen & Mange-Rajetzky (1992), who carried out a palaeogeographical synthesis and an analysis of the heavy mineral suites in the sandstone units, McKie & Garden (1996), who recognized climatically controlled stratigraphic cycles, and Nichols (2005b), who presented a model for the Lower Clair Group in terms of an evolving depositional system.…”

“…10b), the deposits are mainly aeolian sands with some coarser river deposits, representing a period of drier conditions in the basin. A return to a more humid climate led to the formation of a unit (IV), dominated by fluvial channel and overbank facies, with some aeolian reworking of the waterlain deposits (McKie & Garden, 1996). Unit V is gradational with the unit below, but the fluvial channel-fill successions are thinner (2.0 -2.5 m thick) and generally composed of finer grained sediment; much of the deposition may have been from unconfined flows (Nichols, 2005b).…”

“…The primary control on facies in the Clair Basin was climate (McKie & Garden, 1996;Nichols, 2005b). During periods when the water supply was at its maximum most of the basin was covered by a lake (units I and VI), and at the other extreme there were times (unit III) when the basin was relatively dry and aeolian processes were dominant.…”

Correlation of reservoir sandstones using quantitative heavy mineral analysis

“…(Coney et al 1993). In the Core Area of the Clair Field, these requirements are met by Unit V, because this unit has the best reservoir quality due to strong aeolian influences (McKie & Garden 1996) and because there is an open fracture system concentrated in the interval from upper Unit IV to lower Unit VII. Successful exploitation of the Clair Field is therefore dependent on drilling horizontal wells targeted on Unit V. Heavy mineral analysis was considered a possible option to assist with geosteering of these high-angle wells , in view of the variations in mineralogy recognized by Allen & Mange-Rajetzky (1992).…”