Incised valley fill sandstone bodies in Upper Carboniferous fluvio–deltaic strata: recognition and reservoir characterization of Southern North Sea analogues

Abstract

Multistorey fluvial sandstone bodies in Upper Carboniferous fluvio–deltaic strata are proven reservoirs in the Southern North Sea. Several of these bodies have been interpreted as incised valley fills. Analogous valley fill sandstone bodies have been studied in outcrop and subsurface datasets from Upper Carboniferous successions in the onshore UK and Germany. Using techniques and tools applicable to Southern North Sea datasets, we review the diagnostic criteria of these valley fills: (1) stratigraphic context, characterized by deep erosion into underlying strata, lateral correlation to an interfluve surface and intimate association with an initial flooding surface in overlying strata; (2) basinward facies shift at the base of the sandstone body; (3) erosion of underlying time markers; and (4) distinctive internal architecture, which reflects increasing accommodation space during valley filling.Several features of onshore valley fill sandstones have been characterized in order to constrain the properties of analogous valley fill reservoirs in the Southern North Sea. (1) Valley geometry and extent. Studied onshore valley fills generally have a uniform thickness of 20–25 m, thickening locally to 30–45 m, and widths of 5–25 km. A small number of valley fills, associated with turbidite-fronted deltas, thicken to 50–80m in the 2–5 km adjacent to their mouths. Sheet-like fluvial sandstones overlying sequence boundaries have thicknesses comparable to conventional valley fills, but exceed 35–70 km in width. (2) Valley connectivity; typically, valley fills occur as discrete sandstone bodies, but they may amalgamate laterally, to form sheet-like sandstones, and vertically, producing thick (up to 200 m) sand-prone stratigraphic packages. The internal geometry of these sand-prone packages is complex. (3) Valley fill character. Valley fills generally lack an overall fining-upwards trend and are dominated by coarse- to medium-grained sandstone of low-sinuosity (braided?) fluvial facies throughout. Where present, fine-grained facies are preferentially preserved in the upper part of a valley fill. (4) Detrital mineralogy and provenance. At least some valley fill sandstones exhibit a different composition and provenance to surrounding strata. Such valley fill sandstones are likely to be anomalously feldspathic in the Southern North Sea.

Modelling ancient tides: the Upper Carboniferous epi‐continental seaway of Northwest Europe

“…10-100 Collinson (1988), Ziegler (1990), Cope et al (1992), Hampson et al (1997Hampson et al ( , 1999, Guion et al (2000) Mainland Northwest Europe Isolated continental basins with marine influence restricted to highest sea-level stands (goniatite-bearing marine bands).…”

“…Tidal amplification in narrowing, shallowing water bodies below model resolution may be approximated using . Incised valleys in the Upper Carboniferous of Northwest Europe range from 1 to 25 km in width (Hampson et al. , 1999).…”

Sequence stratigraphy of the upper Millstone Grit (Yeadonian, Namurian), North Wales

“…Sequence boundaries, defined as unconformities developed during falling relative sea-level (Posamentier and Vail 1988;Van Wagoner et al 1990), are most clearly expressed in Millstone Grit strata as deeply incised erosion surfaces at the base of valleys, which were subsequently infilled by multistorey fluvial and/or estuarine sandstones (Hampson et al , 1999a. Incised valleys are recognized on the basis of four criteria: (1) their stratigraphic context, as defined by an underlying high-relief, regionally extensive, erosion surface and an overlying transgressive surface, (2) a 'basinward facies shift' at the base of the valley, which places fluvial deposits directly above deposits representing significantly deeper water, (3) an identifiable time gap at the base of the valley, such as the erosion of an underlying marine band(s) and (4) the multistorey infill of the valley, which reflects an upward increase in accommodation (Hampson et al , 1999a.…”

“…Incised valleys are recognized on the basis of four criteria: (1) their stratigraphic context, as defined by an underlying high-relief, regionally extensive, erosion surface and an overlying transgressive surface, (2) a 'basinward facies shift' at the base of the valley, which places fluvial deposits directly above deposits representing significantly deeper water, (3) an identifiable time gap at the base of the valley, such as the erosion of an underlying marine band(s) and (4) the multistorey infill of the valley, which reflects an upward increase in accommodation (Hampson et al , 1999a. Two candidate incised valley fills are identified in the study area.…”

“…However, the sequence boundary at the base of the Aqueduct Grit can be correlated to an equivalent surface at the base of the Rough Rock braided-fluvial complex farther north and east in the Central Province Basin (Maynard 1992;Church and Gawthorpe 1994;Hampson et al 1996). The uniform, braided-fluvial character of the Aqueduct Grit, and the absence of fine-grained and/or tidally-influenced deposits in its upper part, suggests that it constitutes part of a lowstand systems tract (Van Wagoner et al 1990;Hampson et al 1997Hampson et al , 1999a.…”