Quantitative progradation dynamics and stratigraphic architecture of ancient shallow‐marine clinoform sets: a new method and its application to the Upper Jurassic Sognefjord Formation, Troll Field, offshore Norway

Abstract

AbstractThis article presents a new numerical inversion method to estimate progradation rates in ancient shallow‐marine clinoform sets, which is then used to refine the tectono‐stratigraphic and depositional model for the Upper Jurassic Sognefjord Formation reservoir in the super‐giant Troll Field, offshore Norway. The Sognefjord Formation is a 10–200‐m thick, coarse‐grained clastic wedge, that was deposited in ca. 6 Myr by a fully marine, westward‐prograding, subaqueous delta system sourced from the Norwegian mainland. The formation comprises four, 10–60‐m thick, westerly dipping, regressive clinoform sets, which are mapped for several tens of kilometres along strike. Near‐horizontal trajectories are observed in each clinoform set, and the sets are stacked vertically. Clinoform age and progradation rates are constrained by: (i) regionally correlatable bioevents, tied to seismically mapped clinoforms and clinoform set boundaries that intersect wells, (ii) exponential age–depth interpolations between bioevent‐dated surfaces and a distinctive foreset‐to‐bottomset facies transition within each well, and (iii) distances between wells along seismic transects that are oriented perpendicular to the clinoform strike and tied to well‐based stratigraphic correlations. Our results indicate a fall in progradation rate (from 170–500 to 10–65 km Myr−1) and net sediment flux (from 6–14 to ≤1 km2 Myr−1) westwards towards the basin, which is synchronous with an overall rise in sediment accumulation rate (from 7–16 to 26–102 m Myr−1). These variations are attributed to progradation of the subaqueous delta into progressively deeper waters, and a concomitant increase in the strength of alongshore currents that transported sediment out of the study area. Local spatial and temporal deviations from these overall trends are interpreted to reflect a subtle structural control on sedimentation. This method provides a tool to improve the predictive potential of sequence stratigraphic and clinoform trajectory analyses and offers a greater chronostratigraphic resolution than traditional approaches.