An organic geochemical study of Mesozoic shales from Andøya, North Norway

Abstract

Abstract Andoya, North Norway, is the only locality in Norway in which Mesozoic sediments outcrop onshore. Consequently, Andoya has received special attention with respect to petroleum exploration on the Norwegian Continental Shelf north of 62°N. A series of outcrop samples of Cretaceous and Jurassic age was collected and analysed. The samples produced varying amounts of extractable organic material, ranging from 120 to 320 ppm of the whole rock or 13.7–183.1 mg/g of total organic carbon (TOC) for the Cretaceous samples, and 250–5200 ppm of whole rock or 17.8–258.0 mg/g TOC for the Jurassic samples. Gas chromatographic analyses of the saturated hydrocarbon fractions showed distinct differences between the 2 sets of samples (Cretaceous and Jurassic). The Jurassic samples have a pronounced distribution of high molecular weight (C23+) compounds and high CPI values whereas the Cretaceous samples have a large unresolved envelope in the n-C15n-C20 region. Large variations in the composition of the saturated hydrocarbon fractions are recorded in the Jurassic samples. Some contain rather large quatities of isoprenoids while others contain minor amounts only. A high proportion of polycyclic aliphatic compounds was found in some samples. Various steranes and triterpanes were identified by GC-MS, and the variations in relative abundances of these compounds are discussed relative to the other parameters studied. Rock-Eval pyrolysis, together with visual kerogen and vitrinite reflectance analyses were undertaken on certain samples and showed a higher maturity for the Cretaceous than for the Jurassic samples.