Tectonic, provenance and sedimentological controls on reservoir characteristics in the Upper Triassic–Middle Jurassic Realgrunnen Subgroup, SW Barents Sea

Abstract

The most prolific reservoir package in the SW Barents Sea is currently the Upper Triassic–Middle Jurassic Realgrunnen Subgroup, comprising the main hydrocarbon accumulations in the Goliat, Snøhvit and Johan Castberg fields and the Wisting discovery. The interval continues to be the main target as hydrocarbon exploration ventures further into this region. However, the package varies considerably in thickness and reservoir quality throughout the basin, and it is therefore very important to understand how this package developed and what has affected it in the time since it was deposited. Here we review controls on the tectonostratigraphic evolution and facies distribution within the Realgrunnen Subgroup, and exemplify the variability in reservoir characteristics within the subgroup by comparing some key wells in relation to their depositional environment and provenance. New provenance data that record a turnover from reworked Triassic- to Caledonian-sourced mature sediment support facies observations which suggest temporal changes in the depositional environment from marine to fluvial. Much of the variability within the subgroup is attributed the tectonostratigraphic development of the basin that controlled accommodation, facies transitions and sediment distribution. This variability is reflected in subtle differences in reservoir quality important both for exploration and production in the remaining underexplored basin.

Distinct petrographic responses to basin reorganization across the Triassic–Jurassic boundary in the southwestern Barents Sea

“…Zircon spectra from the southern basin margin support continued sedimentation from the Baltic shield, thereby linking coarser particles to the southern provenance. On the remaining platform areas, the absence of the Rhaetian–Sinemurian succession represents a period of subaerial exposure and erosion of underlying Triassic strata (Klausen, Müller, Poyatos‐Moré, Olaussen, & Stueland, 2019; Müller et al, 2019).…”

“…Although textural maturity might be related to tidal influence (Figure 2b,d), a facies‐dependent relationship alone cannot explain the absence of ferric minerals in the western and N2 assemblies. As large parts of the Norian–Rhaetian succession are missing across the northern basin interior due to subaerial exposure and denudation (Klausen et al, 2019), upper sections of the Fruholmen Formation in the western basin interior could contain recycled detritus from exposed strata on northern platform areas. A recycled origin could also account for the portion of Triassic zircons in the western petrographic assembly (Figure 7e).…”

The Middle Jurassic to lowermost Cretaceous in the SW Barents Sea: Interplay between tectonics, coarse‐grained sediment supply and organic matter preservation

“…The absence of sequence 1 in the southern part of the study area (the south-eastern part of the Hammerfest Basin and the eastern part of the Finnmark Platform; Figure 5a) is interpreted as subaerially exposed conditions (Figure 13a). This is supported by a widespread near-base Bathonian ravinement surface in the Barents Sea and Svalbard (Berglund et al, 1986;Bugge et al, 2002;Klausen et al, 2019;Koevoets et al, 2018;Olaussen et al, 2018;Rismyhr et al, 2019). The relatively constant thickness and lack of growth strata during the Bathonian-Oxfordian imply that the Hammerfest Basin experienced minor or no fault activity.…”

“…The western margin and southern part of the SW Barents Sea has experienced different extensional events from the late Palaeozoic to the Tertiary (Berglund, Augustson, Faerseth, Gjelberg, & Ramberg-Moe, 1986;Faleide, Vågnes, & Gudlaugsson, 1993;Gernigon et al, 2014;Sund, Skarpnes, Jensen, & Larsen, 1986 Clark et al, 2014;Faleide et al, 1993;Kairanov et al, 2019;Marín et al, 2018b;Rojo, Cardozo, Escalona, & Koyi, 2019;Serck et al, 2017;Sund et al, 1986). The initiation of this extensional phase has been interpreted as Bathonian to Callovian, marked by a regional unconformity between the Stø and Fuglen formations boundary ( Figure 3b; Faleide et al, 1993;Klausen, Müller, Poyatos-Moré, Olaussen, & Stueland, 2019;Mulrooney, Leutscher, & Braathen, 2017). However, the timing of the initiation of this event is difficult to constrain in the different basins of the SW Barents Sea due to the lack of age control, and because the Middle to the lowermost Cretaceous succession is thin and some of the formations can be below seismic resolution (Figures 2 and 3).…”

“…The sequence boundary surface 1, SB1, is marked by a hiatus in most of the Barents Sea (e.g. well 7120/2‐3s located in the Hammerfest Basin; Figure 4), and as in Svalbard represents a subaerial unconformity followed by a transgressive bed (Klausen et al., 2019; Rismyhr, Bjærke, Olaussen, Mulrooney, & Senger, 2019 and references therein).…”