North of Troll area delivers againWest Hercules (Photo: Ole Jorgen Bratland)

North of Troll area delivers again

Following on from the success of Røver North, this time Equinor and partners announce the discovery of 75-120 MMboe in Blasto.

Where Tullow Oil failed to prove hydrocarbons in two nearby wells drilled a few years ago, and Shell only found oil shows in the 1980’s, Equinor and partners Vår Energy (25%), Idemitsu (15%) and Neptune Energy (15%) made a sizeable discovery through completing well 31/2-22S in PL090 I.

Blasto is the fourth and potentially biggest discovery in the area north of Troll in less than two years, after Swisher, Echino South and most recently Røver North. Recoverable resources are estimated between 75 and 120 MMboe.

Based on a cross-section published by Tullow Oil a few years ago (see below), it is worthwhile looking at the geological setting of the discovery, and compare it to neighbouring wells.

Lomre Terrace

The area the Blasto well was drilled in can be characterised as a series of Middle and Upper Jurassic fault blocks that step downwards into a northwest direction from the Troll field Horda Platform in the south via the Lomre Terrace into the Viking Graben in the northwest along a complex series of NE-SW trending faults.

With the Troll field being one of the ultimate traps for oil generated in the Viking Graben, the area of the Lomre Terrace – especially the northeastern part – has been an area where migration of oil has probably not been an issue. This is also why Migris – the company that specialises in modelling hydrocarbon generation and migration, earmarked the Blasto wells as “class A”, having an 84% probability of having experienced oil charge.

However, the main risk seems to be trapping.

Mantra and Gotama

As the cross-section shows, the 31/3-4 (Mantra) well drilled by Tullow in 2013/2014 seems to have a similar structural setting to the northern part of the Troll field. Yet, the well was dry.

The Gotama 31/2-21S well was probably the riskiest drilling attempt in this section. Even if Intra-Draupne sandstones would have been present, the prospect must have relied heavily either on stratigraphic trapping towards the east, or effective fault sealing. Now, it seems that the oil migrating through the Gotama prospect area partly ended up in the Sognefjord sandstones of the Blasto discovery.

Interpreted E-W section through the Gotama and Mantra wells drilled by Tullow Oil. The Blasto wells were projected onto the line, so care has to be taken that the actual setting drilled by these wells may differ from what is shown here. The cross-section is a re-draft of a version presented by Tullow Oil (2013 full year results).

Sognefjord reservoir

As schematically illustrated in the cross-section, well 31/2-22S encountered a total oil column of about 30 metres in the upper part of the Sognefjord Formation, 25 metres of which was sandstone with very good reservoir quality. It also encountered an oil column of about 50 metres in the lower part of the Sognefjord Formation, about 40 metres of which was sandstone with good to very good reservoir quality.

The oil/water contacts were proven at 1,860 metres and 1,960 metres, respectively, below sea level.

Well 31/2-22A encountered about 55 metres of water-bearing sandstone with good to very good reservoir quality in the upper part of the Sognefjord Formation and about 25 metres of water-bearing sandstone with poor to good reservoir quality in the lower part of the Sognefjord Formation. The well is classified as dry.

It would be interesting to see how well 31/2-8 drilled by Shell in 1982 ties to the Blasto discovery. Seemingly located in the same fault block, this well penetrated the Sognefjord Fm sandstones at a very similar depth as the Blasto well, but Shell didn’t find commercial quantities of hydrocarbons.

The Blasto wells were drilled by the West Hercules drilling facility, which is now drilling well 34/6-5 S and chasing Cook Formation sandstones north of the Brent pinch-out line.