Seismic section from the SW part of the Loppa High showing the rotated fault blocks where the Gohta and Alta discoveries were made. The discovery wells and the formations in which the discoveries were made are marked. Seismic data are courtesy of MultiClient GeoPhysical ASA, Norway.

The source may have been found

The Ørret Formation of Late Permian age is a good candidate for being the source of oil in the Alta discovery in the Barents Sea.

Investigation of petroleum inclusions in carbonate samples from the Senilix well (7120/2-1, drilled on the Alta structure in 1985, before Lundin made the Alta discovery in 2014) proving oil shows, with “oil bleeding from fractures and vugs”) in the Barents Sea reveals petroleum entrapment in Paleozoic carbonates at reservoir temperatures from as low as 87.3°C to more than 130°C, writes source rock experts from the University of Oslo.

Using corrected bottom hole temperatures, this corresponds to depths of 2800–4100 m, compared to the present-day depth of these samples of only 1965.9–2020.5 m.

The oil in the Gohta and Alta discoveries is concluded to be of either Lower Triassic or Paleozoic origin.

A potential source-rock candidate is the Ørret Formation, which is the time-equivalent to the Ravnefjeld Formation in Greenland. These oils are of a different origin compared to oils in the nearby Johan Castberg field which contains oil sourced from the Upper Jurassic Hekkingen Formation.

Evidence is presented to suggest that the Gohta and Alta oils represent blends of petroleum expelled at maturities ranging from about 1.0% calculated vitrinite reflectance (Rc) to more than 1.3%Rc, and this corroborates the inferences made from the petroleum inclusions.

Migration and filling of the Senilix structure occurred at greater depths than, and prior to, the Late Paleogene uplift event, the authors conclude.

This emerging play is significant to exploration in the karst developed on the Barents Shelf and the Bjarmeland Platform during the Permo-Carboniferous, says Z. Matapour*, D. A. Karlsen, B. Lerch & K. Backer-Owe in their recent scientific paper.

REFERENCE: Petroleum occurrences in the carbonate lithologies of the Gohta and Alta discoveries in the Barents Sea, Arctic Norway, Petroleum Geoscience, https://doi.org/10.1144/petgeo2017-085

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