As the twenty-first Norwegian licensing round approaches, the Barents Sea remains an area of high exploration interest for the oil and gas industry. A large number of nominations have been put forward for the licensing round and interest is particularly strong to the west of the Loppa High. 3D multiclient data were acquired in 2008. Results indicate some obvious flat-spots within the Triassic and frequent high-amplitude shallow reflections believed to be gas or gas hydrates. This article discusses the geological features of West Loppa and the potential implications.
Regional geology
Regional geological features of the western part of the Barents Sea.The West Loppa area forms part of the western margin of the Barents Sea. The main structural elements developed in association with the opening of the North Atlantic in the Late Jurassic and Early Cretaceous. The two main structural features are the Loppa and Veslemøy highs.
The Loppa High is an old,Devonian feature. It is flanked by the Hammerfest, Tromsø and Bjørnoya basins. Sediment cover on this high is thin.
The Veslemøy High sits between the Tromsø and Bjørnoya basins and is a Mesozoic aged feature with a structural trend of WSW-ENE.
The early history (Late Carboniferous-Permian) is one of warm water platform carbonate and reef development on structural high areas with carbonate muds and evaporates in basinal lows. During the Permian, a significant climatic change took place with a cooler more temperate climate leading to the development of cool water carbonates. In the late Permian and into the Triassic, the Uralian fold-belt developed to the east and this came to dominate the sedimentary succession. Erosion of the mountain chain gave rise to significant clastic input, as fluvial and delta deposits, through the Triassic. These sandstones are inter-bedded with marine shales.
A major hydrocarbon source rock of Middle Triassic age is recognized throughout the Barents Sea area. The Steinkobbe has total organic carbon (TOC’s) of 2-5%. Locally other Triassic source rocks are seen. For example, the 7219/9-1 well contains black shale within the Upper Triassic Fruholmen Formation with TOC’s of 1.3-3.6%.
The coarse clastics derived from the east form good quality reservoirs (Klappmyss, Kobbe, Fruholmen and Snadd formations). However, in general these sands become thinner and finer grained to the west, away from their provenance. The location of the West Loppa area means that the understanding of the distribution of these potential reservoirs is crucial. This is demonstrated by the 7321/7-1 well, which contains just two sands in the Snadd Formation of 6 and 11m each, while the 7219/9-1 well has several sands within the Snadd of 5-20 m thickness but with porosity of only 10-15%.
Fluvial and deltaic conditions extended though the Lower Jurassic with the deposition of the Nordmela and Tubåen sandstones. During the Middle Jurassic a transgression gave rise to the development of the Stø Formation, a wave dominated shoreline facies. The 7219/9-1 illustrates these reservoirs, the Tubåen having 64 m of net sand (17% porosity), the Nordmela has 59 m net sand (porosity 16%) and the Stø has 99 m net sand with an average porosity of 18%.
Extension in the Late Jurassic led to the development of deep water conditions in which the Hekkingen Shale was deposited. This forms both a high quality hydrocarbon source rock with TOC’s of up to 10% and a major regional seal when combined with the overlying Cretaceous shales.
The Barents Sea generally underwent steady subsidence, albeit with local uplift, during the Cretaceous and into the Tertiary, but in the west Barents Sea a major period of tectonic activity and rifting affected the Veslemøy High.
Late Tertiary and/or post glacial uplift of the Barents Sea has affected both the present day maturity of source rocks and also allowed for the remigration of oil and gas.
Acquisition parameters for the West Loppa Extension Survey 2009
West Loppa: Recent 3D surveys conducted by WesternGeco.Vessel
Seismic Activity
The survey area was extended to the north and to the west in 2009, covering both blocks awarded in the twentieth round and also unlicensed acreage. The 2009 survey covers an additional area of 2,640 km2. Processing was completed in March 2010. Initial interpretation shows multiple prospects at Tertiary, Cretaceous, Jurassic and Triassic levels. Both Triassic structural plays and Cretaceous stacked submarine fan systems are supported by direct hydrocarbon indicators.Western Geco has conducted several 3D surveys in the West Loppa area. A 915 km2 survey was shot in 2008. The blocks covered by the survey were later awarded in the 20th round. Drilling is expected to commence in the autumn of 2010.
The 2008 survey covers two fault terraces on the west flank of the Loppa High. Stacked reservoirs are probable in several Triassic and Jurassic (Realgrunnen) formations. The seismic example is an east-west line from the 2008 fast-track processing. The data indicates an obvious flat spot on one of the fault terraces with a strong change in amplitude of the reflector above. There is also a possible, less obvious, flat spot in the higher terrace.
In 2010, further acquisition has continued in this exciting area. The survey will extend 3D coverage to the west and southwest of Well 7219/8-1 over an area of 1370 km2, where mapping of existing 2D data shows potential Tertiary, Cretaceous and Jurassic plays.
