The major structural elements of the Sleipner Area shows a lot of tectonic activity. We are interested in the zone close to the Theta Vest area (Volve Oilfield). The structural framework map shows us the Viking Graben, Central Graben and the Norway-Danish basin around the area (Image 1). The faults are oriented in the N-S, NE-SW and NW-SE directions.
Image 1: The present day structural framework map (from Gage & Dore, 1987)
Graben: An elongated block of the earth’s crust lying between two faults and displaced downwards relative to the blocks on either side, as in the case of a rift valley.
Lineaments: A linear feature in a landscape which is a manifestation of an underlying geological structure such as a fault.
Terrace: A step-like surface, bordering a valley floor or shoreline, that represents the former position of a flood plain, or a lake or sea-shore.
Structural High: A general term for an elevated tectonic feature such as a crest, culmination, anticline, dome, horst, ridge or spur. (Ref)
Image 2: Graben and Terrace structure description
Image 3: Structural setting map of Sleipner Area
The Structural Setting Map shows us the presence of a major fault between the Sleipner Terrace and the Gamma High. (Image 3)
These Sleipner Field Major Faults and Lineaments map is plotted on the top of Mesozoic sandstone and is based on the seismic interpretation of the Sleipner vest field in 1993.
Image 4: Main faults and lineaments in the Sleipner Field
Now let’s understand these main structures:
- The Main Fault zone
- The Sleipner Terrace Area, and
- The Gamma High
The Main Fault Zone
The Sleipner Field Major Faults and Lineaments map summarizes the major features of the Main fault zone (Image 5 & 6).
Image 5: Main faults and Lineaments in the Sleipner Field
Image 6: Structural setting map of Sleipner Area
The Sleipner Terrace Area
The Sleipner Vest Field is located west of the Sleipner Terrace. Many of the faults around and within the main fault blocks are composite faults wherein all the three main directions play a role.
The Sleipner Field Major Faults and Lineaments map summarizes the major features of the Sleipner Terrace (Image 7).
Image 7: Structural setting map of Sleipner Area
Gamma High
On the Gamma High, including Sleipner Øst, Loke and Theta Vest, the number of mapped faults are few compared to what is observed on the Sleipner Terrace. This is at least valid in the Post Triassic section.
The similarities between Gamma High and Sleipner Terrace are seen in fault orientation directions which are N-S, NE-SW and NW-SE.
The westward extension of the lineaments from Gamma High, line up directly with many of the major NW-SE faults mapped over the Sleipner Vest Field and are evidently connected (Image 8). These lineaments also have a significant influence on shaping the structures on Gamma High. Many of the highs and lows on Gamma High, thus, have an elongate shapes orientated in the NW-SE direction.
The main NW-SE lineament cutting across the Sleipner Øst Field seems to have a sealing effect. This explains the water-bearing Jurrasic/Triassic reservoirs in the south (wells 15/9-9 and 15/9-16) and hydrocarbon-bearing Hugin Formation in the north (wells 15/9-11 and 15/9-13). (Image 8)
Image 8: Structural setting map of Sleipner Area
The Gamma High area and its structures are strongly influenced by doming during the Cretaceous and Tertiary compressional phases. This may have resulted in reverse movements on faults due to reduction in their throw and extension; thereby masking most of them from seismic resolution and mapping. On the Top Rotliegendes level, however, these faults are larger in extension and throw (Image 9).
Image 9: Structural depth map- Top Rotliegende
Tectonic thinning of Hugin Formation
The Hugin formation is eighteen meters thick and has a high variation in sand quality, leading to a poor seismic resolution. The low thickness of the formation encountered in the well can be due to the high degree of faulting.
Image 10: Structural cross section along the 15/9-19 SR well
A rough estimate of the representative reservoir thickness may be obtained by using a pure-shear or simple-shear analysis based on data from structural core logging. We use information from a core to understand field scale characteristics of the structure.
All natural fractures found in the core were assumed to be shear fractures. A mean displacement obtained from (a limited number of) measurable displacements in the core is applied to all these fractures Also, an initial fault dip of 60 degrees was assumed.
Pure strain model: There is no rotation and the deformation is coaxial (along both axes).
Simple strain model: The object is subject to a uniform shear in a direction parallel to a direction, leading to
Image 11: Representation of Pure strain and Simple strain model
The core (52
For a pure-shear model, tectonic thinning equals sum of the throws on faults. For a simple-shear model, a rotated fault block model (rotation is relative to disturbed bedding surface), including the shear angle, is expresses the thinning. The shear angle is found using a numerical procedure.
The calculations indicate that the pure-shear model results in a thickness of 27m and the simple shear model
This method of using throw and fault measurements from the core is of limited accuracy, because faults with throw greater than the core dimension cannot be accommodated.
