The structure is adjacent to gas-condensate structures: Sleipner Vest, Loke and Sleipner Øst, justifying the need to understand the source of the oil reserves.
Gas condensate is present in both Jurassic/Triassic Sandstone and the Heimdal formation of the Sleipner and Loke Structures. Oil was discovered only in the Heimdal formation of the Theta Vest Structure.
The Theta Vest structure and the oil trap is a dome-shaped structure. This trap has a well-defined spill point to the east.
Based on geochemical analysis of all available hydrocarbon samples from the Sleipner structure, three different source rocks are involved in generation of hydrocarbons:
- Type 1: Coal type source rock or shale with terrestrial organic matter
- Type 2: Shale type source rock
- Type 3: Mixed type source rock (terrestrial/marine-mixture of source from Type 1 and 2)
However, geochemical analysis of oil in the Theta Vest Structure indicates a different source rock type:
- Type 4: Non-clastic marine source rock (carbonates?)
The geochemical analysis of gas in the Theta Vest Structure cannot be typed due to its unusual isotopic composition.
It is concluded from geochemical analysis that compositions of the liquid hydrocarbons in the reservoirs of Theta Vest, Loke and the Sleipner Øst structures are in general not related, and do not have common source rocks or migration paths.
Source Rock
Most of the organic carbon comes from algae, but there can also be contributions from terrestrial plants that get deposited in marine basins.
The organic carbon is preserved for petroleum generation when deposited in anoxic environment, such as deep seas or sometimes very large deep lakes, or it needs to be buried in a place that has an extremely large sedimentation rate, so that the organic carbon gets buried and compacted faster than it can be oxidized.
Image: Base Cretaceous depth map indicating Source rock
This basin area terminates towards south-west against a prominent NW-SE lineament that can be followed across the central part of the Sleipner Øst structure and further north-west towards the Sleipner Vest Field.
The deepest part of this basin area, approximately 3550 meters (base Cretaceous level), is situated right next to the Theta Vest-Loke area.
The basin area extends towards north-east out of the map area. Probably, it is connected with the Viking Graben area north of the Sleipner Vest and Dagny Field
The type of kerogen depends on the type of organic material that went into making it.
Image: Source rock and Migration paths in the Sleipner area
This source area II is the most probable source area for the Theta Vest oil. Long range migration from more distant source areas (like the Viking Graben) would lead to mixing with hydrocarbons from other sources and therefore can be excluded.
This source area II is also less mature than source area I, and thus a more likely source area for the Theta Vest oil, which has a low maturity.
Migration
Hydrocarbon migration from the most probable area Source area II requires movement from the Sleipner terrace to the Gamma High, via the main fault zone.
In general, the throw at base Cretaceous level is in the range of 200-300 meters across this main fault which makes the presence of sand to sand contact for fluid movement almost impossible across the fault over large areas.
However, in the area west and north of the Theta Vest – Loke area, where the strike of the main fault turns from N-S to NE-SW, the throw of the main fault reduces significantly to 70 meters on the base Cretaceous level. This area is one of the most likely area for migration from the Sleipner Terrace to the Gamma High area.
Image: Main faults and lineaments in Sleipner Field
A more gentle slope between the Gamma High and the Sleipner Terrace exists. At the base of this slope area, is the source area basin II. Up side of the slope, the Theta Vest and the Loke area is situated.
Image: Base Cretaceous depth map
A second possible migration route from the Sleipner Terrace to the Gamma High, is found
at the extreme southern area of the Sleipner Øst Field itself. Here the base Cretaceous
depth map shows a fairly gradual transition from the Sleipner Vest Field area and to the
southern area of the Sleipner Øst Field.
In summary, there exist two probable areas for migration from the Sleipner Terrace and
into the Gamma High.
Spill routes
None of the wells drilled on the Theta Vest, the Loke and the Sleipner Øst structures, have
proved hydrocarbons below the spill points of each structure. It is therefore possible that
a fill-spill relation exists between the structures.
Spill point is in a trap, the low point under which hydrocarbons will escape when the trap is full.
Based on the relative depths of the identified spill points, the hydrocarbons cannot be spilled from other structures to Theta Vest.
Theta Vest must have been sourced/charged by oil by direct migration through the northern migration area. Migration of gas has not taken place through the Theta Vest structure.
There exist separate corridors for migration in the northern migration area; a corridor for oil charging Theta Vest and a corridor for condensate and gas charging Loke.
Image: Source rock and Migration paths of Sleipner field
From a closer examination of the Base Cretaceous and Top Mesozoic Sandstone maps,
it is found that the basinal area on the downfaulted side of the main fault is divided into
several sub basins by horsts structures.
The oil may have also been generated in the sub-basins south of the horst structure and the
lineament. It may have migrated along the crest of the Theta C structure and into the
Theta Vest structure.
Image: Pressure comparison across structures
The reservoir pressure in the Theta Vest Hugin Formation has been compared to the
pressures in equivalent formations in the adjacent wells 15/9-11 (Sleipner Øst) and 15/9-
17 (Loke).
The pressure data excludes direct continuity and contact between the hydrocarbon
accumulation in the Theta Vest structure and those in the Sleipner Øst and the Loke
structures at present.
