Genesis of an over‐thickened shoreface sandstone tongue: The Rannoch and Etive formations of the Middle Jurassic Brent delta, North Sea

Abstract

Progradational, wave‐dominated shoreface sandstones are typically reported to have thicknesses of less than 30 m. The shoreface sandstones of the Rannoch and Etive formations in the North Sea Brent Group, however, comprise far greater thicknesses (in places exceeding 100 m). The genesis of these successions have not been well understood, and the purpose of this study is to investigate within the framework of facies geometries in both modern and ancient depositional systems how the Rannoch‐Etive sandstones could have been formed, on a local‐regional scale. The facies analysis concludes that the Brent delta advance represents a single progradational event, not punctuated by transgressions. In such a scenario, the vertical sandstone thickness would be determined by: 1) shoreline trajectory angle, 2) the horizontal (dip‐directed) length of shoreface sand (typically 1–2 km in modern systems), and 3) shoreface sand pinch‐out depth (typically 5–12 m in modern systems). Within a framework of modern facies geometries, a 100 m thick vertical shoreface sandstone succession could result from a regression characterised by a shoreline trajectory of 2.6–5.4° (implying 80–95 m rise in relative sea level per 1–2 km of progradation), a 5–12 m deep shoreface sand pinch‐out depth, and a 1–2 km shoreface sand length. If, however, one applies such a model for the entire Brent delta progradation in the study area (200 km) this implies a rise in relative sea level of c. 5000 m, which clearly would be associated with large amounts of deposits accumulating behind and in front of the advancing shoreline. Such deposits are not present, which implies that modern facies geometries do not apply and that rising sea level was not of major importance in defining stratigraphic architecture. It is more likely that the regional overthickening of the Rannoch‐Etive sandstones resulted from progradation with a deep shoreface sand pinch‐out depth.

Role of forced regression in controlling Brent Group reservoir architecture and prospectivity in the northern North Sea

“…It seems possible, therefore, that high-frequency sequence boundaries are either obscured or have been overlooked in the three wells reviewed by Helle & Helland-Hansen (2009). The net level of climb of the shoreface observed here (about 0.1%) is similar to that observed by Helle & Helland-Hansen (2009). Significantly, however, within the study area an alternation of periods of erosion and progradation allow for the development of a thick compound shoreface without the need for an anomalously deep delta front.…”

“…This model contrasts with a recent interpretation of the Rannoch-Etive made further to the south and in the Norwegian sector of the Brent province, which emphasizes the role, under steady sea-level conditions, of a single episode of steady progradation towards a deep delta front (Helle & Helland-Hansen 2009). The study by Helle & Helland-Hansen (2009) was largely based on three, widely spaced wells, whereas this study and that of Hampson et al (2004) were able to rely on closely spaced wells to identify candidate high-frequency, sequence boundaries with confidence.…”

“…The study by Helle & Helland-Hansen (2009) was largely based on three, widely spaced wells, whereas this study and that of Hampson et al (2004) were able to rely on closely spaced wells to identify candidate high-frequency, sequence boundaries with confidence. It seems possible, therefore, that high-frequency sequence boundaries are either obscured or have been overlooked in the three wells reviewed by Helle & Helland-Hansen (2009). The net level of climb of the shoreface observed here (about 0.1%) is similar to that observed by Helle & Helland-Hansen (2009).…”

“…This suggests that the observed sequence was generated by relatively modest sea-level change. Modern shorefaces typically show a submarine relief of 15-25 m across a 1-2 km zone parallel to the shoreline (Stive & de Vriend 1995;Howell et al 2008;Helle & Helland-Hansen 2009). If we surmise that the base of the lower shoreface is likely to have been at a depth of around 20 m and the base of the upper shoreface at 7 m (cf.…”

The Shallow-Marine Architecture Knowledge Store: A database for the characterization of shallow-marine and paralic depositional systems

“…11a), which may be interpreted in terms of a control by the source and supply rate of sand to the nearshore (cf. Niedoroda et al 1984;Cowell et al 1999;Helle & Helland-Hansen 2009), whereas the down-dip lateral continuity of nearshore sandstones tend to be highest under conditions where the ratio between the rate of accommodation generation and sediment supply is closest to zero ( Fig. 11b-d).…”