Clinoform development and topset evolution in a mud‐rich delta – the Middle Triassic Kobbe Formation, Norwegian Barents Sea

Abstract

Clinoform surfaces are routinely used to mark transitions from shallow waters to deep basins. This concept represents a valuable tool for screening potential reservoir intervals in frontier basins where limited data are available. Variations in the character of clinoform geometries and shoreline and shelf‐edge trajectories are indicators of a range of different factors, such as palaeobathymetry, changes in relative sea‐level and sediment supply. Applications of conceptual and generalized models might, however, lead to erroneous assumptions about the supply of coarse‐grained material to the delta front and basin when superficial similarities between clinoform geometries are not treated holistically. The present study examines the mudstone‐dominated Middle Triassic Kobbe Formation – a potential hydrocarbon reservoir interval in the Barents Sea, where prodeltaic to deltaic deposits can be examined in cores, well logs and two‐dimensional and three‐dimensional seismic data. Despite pronounced acoustic impedance contrast to the surrounding shale, channel belt networks are not observed close to the platform edge in seismic datasets, even at maximum regressive stages. However, sub‐seismic prodeltaic deposits observed on the shallow platform indicate that prodeltaic deposits were sourced directly from the delta plain. Clinoform surfaces with different geometries and scale are observed basinward of the palaeoplatform edge of underlying progradational sequences, correlative to mudstone‐dominated prodeltaic core sections. Results indicate that platform‐edge deltas developed at discrete sites in the basin due to normal regression, but the positions of these deltas are not directly relatable to variations in clinoform geometries. Transitions from third‐order to fourth‐order clinoform geometries record discrete transgressive–regressive cycles but are not necessarily good indicators of sandstone deposition. Because of prolonged periods with high accommodation, channel avulsions were frequent and only very fine‐grained sandstone was deposited in heterolithic units at the delta front. Sandstones with good reservoir properties are predominantly found along basin margins.

Mud‐rich delta‐scale compound clinoforms in the Triassic shelf of northern Pangea (Havert Formation, south‐western Barents Sea)

“…The Ural‐derived system has been interpreted as a very large system draining a vast hinterland and delivering mostly fine‐grained sediments (e.g. Henriksen et al ., ; Klausen et al ., , ), and in this sense it could be compared with modern systems such as the Amazon or the Ganges–Brahmaputra. The Amazon delta discharges mainly fine‐grained sediments (silt and clay) into a highly energetic environment (Kuehl et al ., ), and these sediments accumulate on the inner shelf as a large subaqueous delta (Fig.…”

“…The interpretation of these systems in the rock record can be additionally complicated by the fact that the major facies belts are in some cases dominated by fine-grained heterolithic deposits (e.g. Martinius et al, 2001;Birgenheier et al, 2017;Klausen et al, 2017b;Zhang et al, 2017), which can occur in a wide range of sub-environments (from intertidal flats and marshes to subtidal estuarine and deltaic; Jaeger & Nittrouer, 1995), and with features that can be generated by more than a single depositional process (e.g. Reineck & Wunderlich, 1968;Terwindt, 1988;Rossi et al, 2017).…”

What is the topset of a shelf‐margin prism?

“…Intra‐shelf “platforms” abandonment by the deltas caused subsequent deltaic clinoform heights to increase basinward). Succession is mainly prodelta heterolithic deposits (blue); shoreline (yellow) and delta plain (grey) deposits are thinly developed (from Klausen et al, )…”

“…The flat‐lying topsets of the large clinothems, when seen only on seismic data, are often little described internally compared to the deeper water correlative strata (but there are exceptions, see Ainsworth et al, ; Lobo & Ridente, ; Pellegrini et al, ; Rossi, Paterson, Helland‐Hansen, Claussen, & Eide, ). However, the complexity of facies and sub‐environments of topsets is well known from classical shallow‐marine and deltaic literature, for example, from the North Sea Brent Delta (Graue et al, ; Helland‐Hansen, ), Barents Sea deltas (Glørstad‐Clark, Birkeland, Nystuen, Faleide, & Midtkandal, ; Klausen et al, ), Western Interior Seaway (Mellere & Steel, ), or modern deltas (Olariu, ).…”

The Triassic of Traill Ø and Geographical Society Ø, East Greenland: Implications for North Atlantic palaeogeography

“…Triassic palaeogeographies of the North Atlantic region have focused on the data‐rich regions of the Barents Shelf to Svalbard (Fleming et al, ; Klausen et al, in press ; Klausen, Ryseth, Helland‐Hansen, Gawthorpe, & Laursen, ; Riis, Lundschien, Høy, Mørk, & Mørk, ) and the Northern North Sea (McKie & Williams, ). Reconciling the marine influence recognized in the north with the continental environments found in the south has proved problematic.…”