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Abstract

The Mandal Formation, the principal source rock in the well-explored North Sea Central Graben, has been identified as a locally very inefficient expeller of its generated products, although having charged numerous petroleum accumulations in that basin. This unusual behaviour for marine shales makes the Mandal Formation an interesting natural laboratory to re-assess the factors controlling generated, retained and expelled fluid compositions as a function of maturity and kerogen type, and to unravel processes affecting compositional fractionation during expulsion and migration of different evaporative fractions. Aiming this, bulk chemical methods and compositionally ultra-highly resolving analyses have been applied to source rock samples, their solvent extracts and expelled crude oils. The Mandal Formation seems to be a typical marine source rock containing mainly Type II organic matter (OM) of marine algal origin with up to 12 % TOC content, a generation potential up to 647 mg HC/g TOC and generating a Paraffinic-Naphthenic-Aromatic Low Wax oil. Being mainly a function of maturity, aromaticities of the source rock pyrolysates vary within the same maturity levels. While retention of gaseous hydrocarbons is controlled by the pyrolysate’s aromaticity, i.e. by the amount of its cross-linked monoaromatic sites, oil retention is strongly dependent on the relative proportion of generated high-molecular weight (HMW) bituminous OM. The Mandal Formation behaves untypical in comparison to the marine Posidonia Shale, a source rock with high expulsion efficiency. The HMW OM fraction dominates the retained products and contains elevated oxygen contents. 2 to 6 O-atoms are incorporated in compounds with longer aliphatic chains and/or larger aromatic ring systems than observed in Posidonia Shale extracts. This might be due to variations in the paleogeography and palaeoclimate affecting the composition and relative contribution of microorganisms within the marine system. The dominance of highly polar, large and awkwardly shaped molecules can be seen as a limiting factor causing delayed expulsion until effective bulk fluid migration sets in during the main generation phase. Selected crude oils in the Central Graben contain less amounts of polar compounds than source rock extracts and are composed of lower polar constituents (N1 > O1 >> O2, N1O1). Maturity is the principal process affecting the composition of polar compounds as revealed by a coherent correlation with biomarker data. However, secondary migration promote the loss of most polar benzocarbazolic and phenolic homologues with shortest aliphatic side chains in oils from carbonate reservoirs which overly thick OM-poor shale packages and are charged from deeper source kitchens. Again it might be the activity of polar sites, the size and shape of migrating phase affect the chemical fractionation upon migration by mutual interaction of polar phases and surfaces. This has an impact on density and phase behaviour of crude oils and hence significantly influences the in-reservoir oil quality.