The Barents Sea petroleum system contains multiple source rock intervals, with hydrocarbon generation having occurred over a long geological time. As such the area possibly represents an overfilled petroleum system. Still, there are not many large oil and gas accumulations found till this date. Despite this, there are numerous failed traps with dismigrated oil in the region – sometimes extending over 100-200 vertical meters in dry structures which testify to the immense generative ability of the source rocks in this basin. Early exploration did probably rely too much on exploration models that had worked so well in the Central Graben, in the Viking Graben and in the Norwegian Sea, not taking uplift into account. Many workers had by the mid 80ties written off the Barents Sea as a gas province. They considered several phases of uplift as the main reason for the lack of commercial discoveries. Some workers postulated that oils in the basin had suffered palaeo-pasteurization, invoking directly that the traps filled a very long time ago, long before Tertiary uplift (Wilhelms et al., 2001). Others claimed that uplift and dismigration had rendered the whole region nonproductive for commercial oil discoveries. However, the Goliat and Nucula oil discoveries, and perhaps also the more recently reported gas and oil discovery well 7222/6-1, goes a long way to expose the shortcomings of the knowledge of the petroleum systems in the Barents Sea. Evidence exists to suggest that long-range migration and trap-refill induced by uplift are central elements to consider. The thesis in hand seeks to shed light on processes that affected reservoired oil and gas. Geochemical investigation is used to present evidences, from various sources, of post-entrapment modifications of gas and oil in the Barents Sea. It is concluded in this work that irrefutable evidences exist for massive biodegradation of gas in the Barents Sea, resulting in CO2-rich gas accumulations. Furthermore, evidence is presented for biodegradation of light HCs in the Goliat oil which also contains sign of biodegradation of n-alkanes in the n-C10 range in the same trap system. Loss of benzene from the same Goliat oil could reflect water-washing. The Skrugard core extract examined show clear evidences for biodegradation of specific aromatic HC isomers and the extract shows a pronounced unresolved complex mixture (UCM) which could reflect biodegradation. It is proposed in this thesis that these findings – interpreted as effects of biodegradation – directly contradict the palaeo- pasteurization model of Wilhelms et al. (2001). It is also discussed in this thesis if the biodegradation could mechanistically be related to the last glaciations and meteoric water fluxes. Additionally, several positive effects of uplift on hydrocarbon composition and redistribution are discussed. These effects can positively influence exploration if properly understood. Finally, a model for exploration in uplifted regions is presented.