Shallow-marine reservoir development in extensional diaper-collapse minibasins: An integrated subsurface case study from the Upper Jurassic of the Cod terrace, Norwegian North Sea

Abstract

A B S T R A C TUnderstanding the factors controlling the development of accommodation above collapsing salt diapirs and their influence on reservoir distribution is critical in reducing exploration risk in salt-influenced sedimentary basins. In this study, we use an integrated subsurface data set (three-dimensional and twodimensional seismic reflection, wire-line-log, core, and biostratigraphic data) from the Upper Jurassic of the Cod terrace, Norwegian North Sea, to understand the influence of rifting on accommodation creation and shallow-marine deposition during the initial-stage collapse of salt diapirs. We demonstrate that rifting resulted in the rise and fall of salt diapirs, and the formation of supra-diapir minibasin-style depocenters that became sites for deposition and preservation of up to 500 m (1640 ft) thick nettransgressive shallow-marine sandstone reservoirs. Maximum thickness is recorded in the axis of minibasins with a reduction in thickness of up to 65% noted on their flanks. The stratigraphic architecture of individual minibasins is variable. Proximal-todistal facies variations from shoreface to offshore shelf and commensurate changes in reservoir quality occur over scales larger than individual minibasins. These deposits contain large sand volumes, and are not confined to areas of localized sandstone subcrop. In combination, these features suggest that the minibasins

The stratigraphic record of minibasin subsidence, Precaspian Basin, Kazakhstan

“…Jackson and Hudec, 2017). Thus, depending on their composition (see Wagner and Jackson, 590 2011; Jackson et al, 2014) the development of salt welds is often critical to allow transmission of 591 hydrocarbons from subsalt source rocks into suprasalt, minibasin-hosted reservoirs, or between adjacent 592 minibasins (Rowan, 2004;Jackson et al, 2014Jackson et al, , 2018. The timing of salt welding is also critical (Rowan, 593 2004); for example, if welding occurs after hydrocarbons have been expelled from the source rock, then 594 these hydrocarbons may be either trapped below the salt or may migrate updip into other parts of the 595 subsalt succession.…”

“…Reservoirs in the centres of bowl-shaped seismic sequences may rely on more subtle stratigraphic 626 trapping configurations, or post-depositional structural deformation related to turtle anticline formation 627 or shortening (Mannie et al, 2014). For example, the thickest shallow 628 marine reservoirs in the Ula minibasin (Upper Jurassic), eastern Central Graben, offshore Norway are 629 located at the centre of a broadly bowl-shaped seismic sequence, thinning and onlapping towards the 630 bounding salt-cored structural high ( fig.…”

“…For example, the thickest shallow 628 marine reservoirs in the Ula minibasin (Upper Jurassic), eastern Central Graben, offshore Norway are 629 located at the centre of a broadly bowl-shaped seismic sequence, thinning and onlapping towards the 630 bounding salt-cored structural high ( fig. 12 in Mannie et al, 2014). The aforementioned discussion is 631 predicated on the fact that the basin is underfilled and that at-surface relief is developed during 632 minibasin subsidence; e.g.…”

Impact of normal faulting and pre‐rift salt tectonics on the structural style of salt‐influenced rifts: the Late Jurassic Norwegian Central Graben, North Sea

“…Mandal High) (Ziegler, 1990;Gowers et al, 1993;Fraser et al, 2003). Shallow marine clastics (Ula Formation) followed by deep marine mudstones (Haugesund and Farsund Formations) were deposited in fault-controlled depocentres and in subbasins that developed over collapsed salt walls (Hodgson et al, 1992;Gowers et al, 1993;Smith et al, 1993;Mannie et al, 2014;Wonham et al, 2014). A second phase of rifting is interpreted to have occurred from late Kimmeridgian to early Volgian (Berriasian) and is associated with overall sediment starvation that led to the deposition of organic-rich mudstones (Mandal Formation) (Gowers et al, 1993;Rattey & Hayward, 1993;Erratt et al, 1999).…”

“…5a, b). The mechanisms of interpod development have been discussed by several workers and includes regional extension or salt dissolution (Hodgson et al, 1992;Smith et al, 1993;Clark et al, 1999;Kane et al, 2010;Mannie et al, 2014). One of the key characteristics of interpods is that they are not strongly linked to sub-salt normal faults (e.g.…”