Impact of salt on the structure of the Central North Sea hydrocarbon fairways

Abstract

The Central North Sea lies within the North Permian evaporite Basin, which was largely flexural but with locally significant basement faults. While the main evaporite basin fill is halite, seismically reflective basin margin facies delineate the salt basin shape. The marginal Zechstein strata record Permian salt tectonics including listric faulting, slumping of kilometre-scale carbonate rafts and halite diapirism, all related to localized pre-Triassic gravity-driven collapse of the Zechstein basin margin. Triassic thickness in the Central North Sea (CNS) was directly influenced by salt, the most significant phase of salt tectonics in the history of the CNS in terms of volume of salt redistributed and dissolved. Lower–Middle Triassic ‘minibasins’ subsided into the salt over much of the CNS. Different initiating mechanisms operated around the basin–differential loading was important near sediment input points, and thin-skinned extension on the platforms balanced a pulse of basement extension in the Central Graben. Minibasin subsidence was accommodated by salt wall growth; salt was eroded during periods of base-level fall and redeposited locally and in the Southern North Sea. The minibasins touched down on basement progressively from the basin margins towards the Central Graben, the timing being a function of salt thickness. The diachronous halting of accommodation space generation combined with the effects of erosion led to a variety of field relationships between the Middle–Upper Triassic Skagerrak sands and the underlying salt–minibasin system. Regionally, the Skagerrak sands are thickest near input points and in the Central Graben, where accommodation space was supplemented by basement faulting.The platforms were subaerially exposed and eroded for much of the period between the late Triassic and late Jurassic. On these platforms, the distribution of the relatively thin Late Jurassic section was controlled by a combination of regional tilt and pre-existing topography created by differential erosion (including dissolution) of the salt walls versus grounded minibasins. The late Jurassic Fulmar shallow marine sands are thickest on the differentially eroded salt highs, which constituted several hundred foot-deep palaeovalleys. Rifting occurred in the Central Graben where tilts of detachments on top of major basement fault blocks caused thin-skinned extension in the overlying strata, either for the first time, or reactivating Triassic minibasin systems. The Fulmar sands are thickest where basement-controlled accommodation space was accentuated by detached fault systems (e.g. fallen diapirs) near sediment input points in the Central Graben.The Early Cretaceous was a time of tectonic stabilization following Late Jurassic basement rifting, although the perched detachment systems in the major basement graben continued to evolve. Accommodation space for Upper Jurassic and Lower Cretaceous turbidites was influenced by up-dip salt high collapse and down-dip ‘rim syncline’ growth. By the end of the Early Cretaceous these systems had stabilized and the surrounding basement highs were sufficiently blanketed to shut off salt dissolution. Chalk isopach variations pick out subtle regional tilts and local basement fault reactivations.Major flexural subsidence in the Tertiary tilted the basin again and precipitated local cover slip, e.g. extensional faults balanced by down-dip folds on the West Central Shelf and diapir rejuvenation on the graben margin.A variety of inversion structures are present in the CNS and are dominantly Cretaceous and Tertiary in age. Several styles of inversion structure occur including thin-skinned and thick-skinned compression, diapir rejuvenation, compaction and dissolution-related features. Salt governed the geometry of many of these structures and in turn the geometry of the Tertiary sand fairways.

Tectonostratigraphy of a rift basin affected by salt tectonics: synrift Middle Jurassic–Lower Cretaceous Dutch Central Graben, Terschelling Basin and neighbouring platforms, Dutch offshore

“…2). These minibasins are either due to Early Triassic progradational fluvial systems or to the relative proportion of marginal Zechstein facies compared to the amount of salt (as proposed for the UK sector minibasins by Stewart & Clark 1999). The main period of salt movement in the study area was the Middle and Late Triassic (Geluk 2007), and continued during the Jurassic and Early Cretaceous (van Winden et al 2018).…”

“…Locally, salt tectonics started as early as the latest Permian (Stewart 2007) but became more widespread during the Early Triassic, leading to the formation of minibasins and rafts (Stewart & Clark 1999;Penge et al 1999). In the Dutch sector, such Early Triassic salt-controlled minibasins are rare and only observed in the A Block (insert map, Fig.…”

Fault growth and interactions in a multiphase rift fault network: Horda Platform, Norwegian North Sea

“…53 overpressured mudstone or and salt; e.g. Stewart and Clark, 1999) and the development of 'release' 54 faults (e.g. Destro, 1995).…”

A new stratigraphic trap for CO2 in the UK North Sea: Appraisal using legacy information

“…The exact location of the limit of the halite is uncertain, as there is only some agreement between Glennie et al (2003) and Stewart and Clark (1999). Halokenesis has only occurred in the thicker halite in the north of the area (Stewart and Clark, 1999) so that there is a zone of between the southern limit of the halite, and the edge of the Rotleigend reservoir (Fig. 3) which was deemed to be the optimum location for CO 2 injection (Fig.…”

“…In Fig. 3 the limit of the Rotliegend is shown from Gatliff et al (1994) and Stewart and Clark (1999). Here, the point at which the Rotliegend reservoir thins to zero thickness is referred to as the 'pinchout', the nature of which is discussed in section 4.1.…”