The formation history of Gamma High helps us understand the mechanisms influencing the structural setting. It provides a better understanding of the potential drilling targets, saturation distributions, and potential reservoir management problems.
A lineament is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault.
The Gamma High is supposed to consist of rotated fault blocks with a complex genetic history based on the known stress fields, the observed lineaments and the stratigraphy encountered in the wells in the Sleipner area (Image 1).
Image 1: The Sleipner Area – Structural setting
From the seismic sections (Image 2 and 3), we see that there is reversed movements of some deep faults. The structure is strongly influenced by tectonic movements.
Image 2: Seismic inline 510 through Theta Vest and Sleipner øst
Image 3: Seismic crossline 445 through well 15-9/17
The major faults on the T. Rotliegendes level can sometimes be traced up to the Base Cretaceous level, but then often with reversed displacement (Image 4).
The Top Rotliegendes is locally be uplifted in some low areas, thus forming a pattern of repeated local inversions as evident in the vertical cross-section through Theta Vest and Theta Sør (Image 4). This pattern can not be explained as a result of a single dominating stress field.
Image 4: Structural cross section through Theta Vest and Theta Sør
Models describing the formation of Gamma High structure
Haleokinesis
The study of Halokinesis describes the subsurface flow of salt and the emplacement, structure, and tectonic influence of salt bodies. We also refer to it as “salt tectonics.”
Image 5: Halokinesis model for the formation of the structure.
Step 1: Initial salt deposition up to a thickness at which the salt becomes mobile. This is uncertain at the Gamma High (Image 6).
Step 2: The theory states the formation of the turtleback structure. The clastic Post-Permian sedimentation would first be concentrated in local basins separated by salt doming (Image 6).
Step 3: Upon salt drainage from the area, the former clastic depocenters would become local high areas and supply sediments for deposition in the former salt-dome regions (Image 6).
Depocenter refers to an area or site of maximum deposition, or the geographic location of the thickest part of any specific geographic unit in a depositional basin. Within a basin, different areas receive different amounts of sediment through time, resulting in numerous depocenters.
Image 6: Trusheim’s model for turtle-back structures.
Normal faulting in the core of a turtle-back is a required characteristic. So, this model also cannot explain the frequent reverse faulting penetrating the Rotliegendes.
Shear Faulting
Regarding known shear movements along the Tornquist Zone, this shearing is expected to have some impact on the tectonic development of the Sleipner area.
Shear leads to strike-slip faults. The fault in this rock strata are displaced mainly in a horizontal direction, parallel to the line of the fault. Shear faulting may juxtapose fault blocks with different sediment thickness.
The Post – Rotliegende section is consistent with the Tornquist zone. The thickening and flexuration of the Jurassic and Lower Cretaceous sediments are in agreement with the known extensional stress-field along the Tornquist Zone during that period.
However, restoration of the fault blocks is not possible with this model either, and the extension of the Tornquist Zone (at least the magnitude of shear movements) is uncertain.
Faulting and folding along the NW-SE dome axis confirms Post-Paleocene compressional phase. (Image 7)
Image 7: Structural depth map of Top Heimdal Formation.
Both Jurassic extension and Early Cretaceous and Tertiary compression appear to have been of importance for the development of the Theta Vest structure, as illustrated in the seismic section (Image 8).
Image 8: Seismic Inline 703 in the Sleipner Øst area.
Also, the abundant NW-SE lineaments cut across to the Sleipner Terrace as well and seems to separate distinctive fault compartments. (Image 9)
Image 9: The Sleipner Area–Structural setting.
Conclusion
The “Shear fault model” seems to agree better with the observed data than a purely Halokinetic model. The latter model seems insufficient in order to explain typical observations in the area. While, salt could have secondary importance locally, especially in eastern parts of the Gamma High. In areas surrounding the Gamma High, salt movements are thought to have accompanied movements of major faults and deposition of Triassic clastic sediments (Pegrum and Ljones, 1984).
It is probable that the two NW-SE trending lineaments, bounding Theta Vest, have had a major influence on the development of that structure.
