Occurrence of Mid Jurassic, organic-rich, lacustrine deposits on the southern Utsira High

Abstract

A detailed biostratigraphic and organic geochemical investigation was undertaken of an organic-rich, lacustrine interval at ∼2622–2639 m in well 16/4–10, in the Fosen Prospect on the Utsira High in the Norwegian North Sea. A Bajocian to Aalenian age is suggested, similar to that for the immediately overlying Sleipner Fm, whereas the underlying strata were effectively barren and could not be dated with confidence. The dark grey claystone of the lacustrine unit is dominated by Botryococcus contributions, as indicated by palynofacies analysis. The kerogen exhibits a typical algaenan signature with a dominant single activation energy value of ∼54.5 kcal/mol for its bulk kinetics. Total organic carbon (TOC) increases from ∼2% at the base to ∼11% at the top, although the hydrogen index (HI) varies throughout (∼400–∼800‰ HC/Corg). The unit is on the verge of oil generation (Tmax ∼446 °C; estimated vitrinite reflectance 0.5–0.6%; spore colour index 5). Overall, it is similar to lacustrine intervals reported in the Møyfrid Discovery (North Sea wells 2/2-1 and 2/2–4) and Kap Stewart (Jameson Land, East Greenland). Freshwater/brackish lacustrine environment indications include a high tetracyclic polyprenoid (TPP) ratio and particularly abundant 4-methyl-24-ethylcholestanes. Specific botryococcal markers are absent, but the abundant 24-ethylcholestanes probably derive from this organism. Algal contributions increase upwards (at the expense of woody higher plants), as do 3-methylhopanes, whereas the pristane/phytane ratio (Pr/Ph) decreases (from ∼4 to 2), which may reflect increasing methanogenic/methylotophic activity. The immediately overlying Sleipner Fm has much lower TOC and HI values and a greater woody higher plant contribution, in what appears to be a proximal marine setting. Beneath the lacustrine facies is ∼10 m of dominantly sandstone, underlain by brown claystone, exhibiting the characteristics of a highly oxic marine environment, with very low TOC and HI values, but high oxygen index (OI) and Pr/Ph. The Fosen lake seems to correspond to one of numerous mini-basins in the area attributed to Zechstein salt withdrawal during the Jurassic. Like the Møyfrid lake, it appears to lie near the periphery of the sub-aerial, central North Sea volcanic dome. It is possible that organic-rich lacustrine facies developed in other, deeper mini-basins and have expelled some oil. Their subtle contributions to accumulations bearing other Jurassic charges may have been overlooked in the absence of obvious biomarker characteristics.