Tectonic History: Geologic Era

The regional lineaments play an important role in the interpretation of the Structure. The prominent NW-SE lineament may be an extension of the Tornquist Zone.

A lineament is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault. Typically a lineament will appear as a fault-aligned valley, a series of fault or fold-aligned hills, a straight coastline or indeed a combination of these features.

Tornquist Zone, is the crustal boundary between the Precambrian East European Craton and the Phanerozoic Orogens of South-Western Europe.

Image 1: Extent of different crustal types across the contact zone between the East European Craton and Paleozoic platform of central Europe.^​1​

Three regional lineaments are fundamental to the tectonic history of the Theta Vest field:

• Highland Boundary Fault-Ling Graben Alignment
• Viking Graben, and
• Central Graben

Image 2: The present-day structural framework map (from Gage & Dore, 1987)

Image 3: The Sleipner Area – Structural setting

Geologic era

Paleozoic Era

Based on the results of wells 15/9-9 and 15/9-16 where the thickness of Anhydrite Zechstein Group (overlying the Rotliegendes Gp. – shaley sandstone and breccia) was 20 and 56 m respectively, the Gamma high is probably a basin margin area. While the well 15/12-3, to the south, had a thickness of 1154 m for the Zechstein Group, this large difference indicates the presence of the basin margin.

Mesozoic

In the north-western Europe the Triassic was dominated by regional extension and rifting.

The Mesozoic extension led to the reversal of the movements along the Tornquist Zone, from right-lateral/compressional to left-lateral/extensional which later culminated in Late Jurassic time.

The western margin of the Gamma High consists of major normal faults with periodic movements associated with the development of the Viking Graben and Central Graben. There has apparently been a “footwall uplift” yielding a general dip down to the east and with very extensive erosion along the fault margin.

Image 5: The Sleipner Area – Structural Framework map

There was a regional uplift during Early – Mid Jurassic in the central North Sea with erosion in high areas as well as on the Gamma High. Previously deposited sediments on the Gamma High were largely removed by erosion. There are abundant sandy deposits on the Sleipner Terrace of Bathonian – Callovian age and much thinner development on the Gamma High (of Callovian Hugin Formation).

Image 6: The Sleipner Area – Structural setting

During Late Jurassic the vertical movement of the western- and north western margin of the major fault was 400-600m. On the Gamma High there are differences in amplitude between highs and lows for the BCU (Base Cretaceous Unit) reflector. This is interpreted to be an effect of local thinning of the Viking Group on these high areas and consequently the structures were in development at least during Upper Jurassic time.

During the Lower Cretaceous time, the area has been subject to compression with the result that faults were steepened and partly overturned. (Image 7)

Image 7: Seismic line 510 through Theta Vest and Sleipner East.

During Upper Cretaceous there was subsidence and deposition of the marls and chalk of the Shetland Group.

Cenozoic

Laramide tectonism and associated drop in sea level caused large paleogeographic (Paleo = geologic past) changes in the North Sea Area (Ziegler, 1981).

Laramide : The geologic event that lead to the formation of rocky mountains in North America

Shetland Platform was subjected to uplift and erosion, giving rise to eastward directed output of clastic sedimentation. Flexuring and faulting of the Top Shetland Group indicates that faults bounding the Gamma High could have had a minor reactivation in the Paleocene. (Image 9)

Image 9: Structural cross section along the well 19/9-19 SR

Flexuring is the process by which the lithosphere bends under the action of forces such as the weight of a growing Orogen or changes in ice thickness related to (de)glaciations.

The last tectonic event of importance for the Gamma area occurred in the Mid Tertiary in connection with Alpine folding.

This event resulted in a regional compression and probably also reactivation of the Tornquist lineament, causing doming and creating the hydrocarbon trap in the Paleocene

The Alpine orogeny is caused by the continents Africa and India and the small Cimmerian plate colliding (from the south) with Eurasia in the north.