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Abstract

While the hydrocarbonfractionof a petroleum fluid hostsvaluable information on mainly source and maturity, the more polar NSO compounds are potential tracers forreservoir processes like filling historyand fractionationdue to production. Due to their polar nature, they interact more strongly with other polar phasesin a petroleum system such as the residual kerogen, formation water, and clay minerals.The giant Valhall oil field, an elongated anticline structurewhich is located in the SW of the Norwegian sector of the North Sea,is a well suited candidate to investigate those interactions. Main reservoir strata are Upper Cretaceous chalks of the Tor and Hod Formations withporosities in the range of30 to 50% that increase from the flanks towards the crest. The early mature black oils sourced from <1000 m thick Upper Jurassic marine shaleswith varying terrigenous organic matter (OM) contribution(Hughes et al., 1985; Cornford, 1994)migrated through thick packages of OM-poor shales.In this communication, we address the chemical heterogeneities of the in-reservoir retained petroleumand the produced fluids to identify migration routes. More importantly,wegive implications on production fractionation,using the compositions of NSO compounds with different polaritiesfrom ultra-high resolution FT-ICR-MS combined with variousionization techniques. Therefore, we sampled9pairs of oils and corresponding reservoir corebitumensfrom identicalwells or nearby subsurface positions in the same reservoir compartmentsat the crestand theflanks of the Valhall field as well asfrom anoil retrieved from the shallow overburden above the Valhall crest.