Seismic VSP tie for the formations

The information about each formation within the discovery well, 15/9-19 SR, is of great importance for understanding the reservoir. Understanding the VSP ( Vertical Seismic Profile) helps us improve seismic interpretation and analyze the mistakes/errors made in the discovery well.

The major formations discussed here are:

• Heimdal Formation
• Shetland Group
• Blodøks Formation
• Viking Group
• Hugin Formation
• Upper Permian and Triassic Formation
• Rotliegendes Gp.

Image 1: Structural cross-section along the 15/9-19 SR well path

Top Heimdal Formation

Discovering hydrocarbons in this formation was the primary target for the well 15/9-19 SR. However, no hydrocarbon deposits were encountered.

The Top Heimdal Formation reflector, representing the top of the Heimdal sandstone, is interpreted regionally in the Sleipner Øst area. It is generally picked in the minimum reflection zone (Image 1: white trough) since there is a marked increase in acoustic impedance from the overlying Shales in the well-consolidated sandstone reservoir.

In the 15/9-19 SR well, however, the Top Heimdal Formation tie is on the onset of a black peak according to the VSP. This means that the formation was encountered, 50 m vertically below the expected depth.

For the VSP to match the seismic data, a negative shift of 5-10 milliseconds is necessary in the VSP plot and the seismic lines along the well path. (Image 2)

Image 2: Composite plot of VSP and random seismic line along the well path

In the synthetic seismogram and the seismic line, we notice that the black peak of the Top Heimdal formation is below the interpreted line. (Image 3)

To further confirm the shift, the VSP log time on several horizons were compared with the seismic data. A shift for the Top Heimdal and Top Blodøks formation should suffice, without requiring a general shift.

Image 3: Synthetic seismogram – Random line along wellpath

The shift is also encountered because of the relatively high velocity in the overlying sediments as compared to the adjacent wells. This led to the velocity map to be adjusted, allowing us to adjust the reflector.

We notice that the Top Heimdal formation is picked at the black peak in the well while it is picked close to the white peak (1/4 cycle higher) in the white trough. The black peak is weak and hard to define in this structure. (Image 3)

Shetland Group

This group is said to be the best-defined reflector in the Sleipner ⌀st Area, indicating the beginning of the Cretaceous.

Image 4: Structural cross-section along the 15/9-19 SR well path

It is picked as a strong white trough. As we move from sandstones & shales in the Tertiary to chalk in the Cretaceous there is a significant increase in the acoustic impedance that allows us to pick this formation. (Image 5)

Image 5: Synthetic seismogram – Random line along wellpath

Blodøks Formation

It is a marl formation near the base of the Shetland group. In most wells, it ties to the top of the black peak.

Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and silt.

However, for the 15/9-19 SR well, the Synthetic seismogram and random line along well path image indicates that a shift in Top Blodøks formation is required for the VSP to match the Seismic.

Image 7: Synthetic seismogram – Random line along wellpath

Base Cretaceous Unconformity (B.C.U)

It represents a fall in acoustic impedance on top of the Jurassic Draupne Formation in the Viking Group. The horizon is therefore picked on the maxima of a generally strong black peak. This horizon is important because it reveals the Jurassic structuring, both regarding the fault block delineation and the dips. The Hugin formation is too thin for the seismic resolution, so the B.C.U horizon is used to understand the structural features. (Image 8)

Image 8: Structural cross section through the well path 15/9-19SR

In the Seismic cross-line 491 through the 15/9-19 SR well image, we notice that the seismic tie based on the synthetic seismogram should be on two-way time 2.547 sec. This is the maxima of a black peak according to the synthetic seismogram. However, the B.C.U. was originally picked one cycle higher at 2.512 sec. (Image 5)

The new interpretation placed the Jurassic section in a down-faulted position (Image 9). The new seismic tie will not affect the Jurassic/Triassic interpretation in areas outside the down-faulted block.

Image 9: Seismic cross line 491 through the 15/9-19 SR well.

The B.C.U. reflector is seen to have a lesser amplitude near the crest of the structures (Image 10). Probable reasons for this can be the thinning of the Draupne Formation and/or the Heather Formation.  These two formations are a part of the Viking Group and their thinning is clearly visible.

Image 10: Amplitude map – B.C.U

Top Mesozoic Sandstone

The oil reservoir of the Hugin formation belongs to this group. As the Top Mesozoic Sandstone reflector is the top sandstone with generally higher acoustic impedance than the overlying shales, this should be picked at a minimum white trough. The top of the Hugin Formation is picked 7 ms below the BCU reflector.

The horizon is hard to tie and interpret consistently because:

• The Viking Group in most Sleipner Øst wells is relatively thin, beyond the resolution of the seismic data. In the 15/9-19 SR well the Viking Group is only 9.5 m thick
• There is a great variation in sand quality and sand thickness throughout the Sleipner Øst area

Other Horizons picked

In the deepest parts of the seismic dataset, the following two horizons were picked which were deeper than TD in 15/9-19 SR:

• Top Smith Bank Formation (interpreted on selected lines) and,
• Top Rotliegendes Group

In the shallow section, the following events are picked:

• Top Pliocene
• Top Utsira Formation
• Near Top Oligocen
• Base Utsira Formation
• Top Balder Formation and,
• Top Lista Formation

Image 11: Table of Litho-stratigraphy, well 15/9-19 SR

Based on the tie from the VSP the Grid Formation was picked as well. This appears to be eye- shaped; consistent with the sedimentological interpretation of this being a channel deposit.

Image 12: Composite plot of VSP and random line along well path

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