The petroleum system in the Barents Sea is complex with numerous source rocks and multiple uplift events resulting in the remigration and mixing of petroleum. In order to investigate the degree of mixing, 50 oil and condensate samples from 30 wells in the SW Barents Sea were geochemically analysed by GC-FID and GC-MS to INTRODUCTIONRecent oil discoveries in the Barents Sea, such as Gotha, Alta, Wisting, Johan Castberg (Skrugard and Havis), Nucula and Goliat ( Fig. 1) (Norwegian Petroleum Directorate, 2014), are evidence for the presence of commercial volumes of liquid petroleum in an area which was long considered to be gas-prone (Stewart et al., 1995). These discoveries led to renewed geochemistry-based investigations of the petroleum systems in the Barents Sea (Bjorøy et al., 2009;Killops et al., 2014;Rodrigues-Duran et al., 2013). The presence of multiple potential source rocks may have resulted in the occurrence of mixed petroleum charges (Ohm et al., 2008;Vobes, 1998). Petroleum distribution and composition was influenced by several episodes of Cenozoic uplift and burial (Cavanagh et al., 2006;Ohm et al., 2008). Cenozoic uplift had a negative effect on the petroleum systems because it resulted in the cessation of hydrocarbon generation and expulsion (Henriksen et al., 2011). In addition, uplift resulted in alterations to the physical properties of trapped hydrocarbons, including changes to gas-to-oil ratios and associated expansion of gas columns, causing oil to be forced out of structures below the spill point (Nyland et al., 1992). Remigration/leakage of petroleum phases also occurred as a result of cap-rock failure or fault reactivation (Ohm et al., 2008;Ostanin et al., 2013). Thus geochemical interpretations in the Barents Sea suggest that uplift-related remigration/long-distance migration may be the key to understanding the complex petroleum systems present (Lerch et al., 2016;Ohm et al., 2008;Rodrigues-Duran et al., 2013).While Rodrigues Duran et al. (2013) focused mainly on the gaseous and light hydrocarbon maturation and alteration parameters in samples from the Hammerfest Basin, Ohm et al. (2008) among others investigated isotope signatures of oils from a larger area to identify potential source rocks. Bjorøy et al. (2009) correlated source rock extracts with oils to determine genetic relationships, and Killops et al. (2014) used novel age-related biomarkers to identify possible source rocks. However, until now few studies have attempted to correlate maturity signatures in order to understand regional variations in petroleum composition.The purpose of this study is to investigate the molecular evidence for the presence of mixed or transformed (biodegraded or water-washed) petroleum in the Barents Sea, and to assess regional similarities and/or differences in petroleum. Saturated and aromatic maturity parameters from the medium molecular (C 14 -C 18 ) and biomarker-range (C 20+ ) hydrocarbon fractions were analysed. Results were compared with results obtained by Lerch et al. (2016), who investigat...

Norwegian oils are generally considered sourced primarily from the Kimmeridge Clay equivalent shales such as the Draupne, Mandal, Spekk and Hekkingen formations, with secondary contributions from the mid–lower Jurassic, and also from the Triassic in the Barents Sea (Botneheia Formation). Still, as most of our age inferences concerning source-oil correlation are based on facies-specific biomarkers, a number of proposed correlations have been questioned. Thus, source to oil correlations were frequently made on the basis of facies parameters, and rightfully so, but facies-specific signatures in oils will transgress age – and, in principle, not correlate with the phylogenetic evolution. This means that one could, in principle assign an oil to ‘the wrong’ age – when one is, in fact, linking it to a known source rock signature. A series of 40 oil samples and core extracts, which cover a wide range both stratigraphically and geographically, have been analysed. In this paper, we present for the first time a Norwegian oil-age map based on age-specific biomarkers among the nordiacholestanes and triaromatic steroids parameters, and delineate also where we find Cretaceous- and Palaeozoic-derived oils. The reasons for this distribution pattern, compared to that of Mesozoic oils on the Norwegian Continental Shelf (NCS), are discussed.

This paper investigates the filling history of the Skrugard and Havis structures of the Johan Castberg field in the Polheim Sub‐Platform and Bjørnøyrenna Fault Complex, Barents Sea (Arctic Norway). Oil and gas occurs in the Early Jurassic and Middle Jurassic Nordmela and Stø Formations at Johan Castberg, and both free oil and bitumen are interpreted to be sourced from the Upper Jurassic Hekkingen Formation (Kimmeridge Formation equivalent). The geochemical characteristics of the petroleum from Skrugard and Havis, including the GOR, API and facies and maturity signatures, can be understood within a complex fill history which includes a palaeo oil charge, Tertiary uplift (>2 km), dismigration, in‐reservoir biodegradation, and late‐stage refill with gas. The API and GOR of the Skrugard oil are 31° and 60m3/m3, respectively. The petroleum is geochemically similar to that in the nearby Havis structure, to that in the Snøhvit region to the south of the Loppa High, and also to the petroleum recorded as traces in well 7219/9‐1, approximately 16 km SW of Johan Castberg field. However, the petroleum differs from the oil in the Alta well 7120/2‐1, located in the southern part of the Loppa High, illustrating the complexity of the regional petroleum systems. The Skrugard oil is of medium maturity (ca. 0.8–0.9% Rc), and is significantly biodegraded despite being gas‐saturated. Evidence for biodegradation includes the reduced concentrations of C10‐C25 n‐alkanes and the presence of a prominent unresolved complex mixture (UCM) in gas chromatogram traces. However non‐biodegraded C4‐C8 range hydrocarbons are also present in the reservoir. This suggests a recent charge of gas/condensate into the structure which therefore contains a mixture of palaeo‐degraded and unaltered petroleum. Oil‐type inclusions within authigenic quartz and feldspar from reservoir sandstones at Skrugard were analysed. The results indicate that the structure (present‐day depth 1276–1395m) underwent Tertiary uplift by ca. 2–3km following an earlier phase of oil emplacement. The presence of the oil type inclusions, both in the current gas zone (Stø Formation) and in the oil zone (Stø and Nordmela Formations), indicates that the positions of the oil‐water and gas‐oil contacts have changed over time. This is consistent with a recent gas charge to the upper part of the reservoir, and also with the gas being at dew point. These observations are supported by analyses of core extracts which show an increasing bitumen content towards the OWC, and the oil‐type bitumen in the present‐day gas zone. A charge history model for the Skrugard structure is proposed which integrates both the observations concerning the petroleum inclusions and the biodegraded oil together with observations of seismically‐monitored gas fluxes along the rim of the Loppa High. Improved understanding of the Skrugard structure and its filling history will assist exploration in similar settings in other parts of the Barents Sea and worldwide, particularly where multiple source rocks and a multi‐stage ...

Investigation of petroleum inclusions in carbonate samples from the Senilix well in the Barents Sea reveals petroleum entrapment in Paleozoic carbonates at reservoir temperatures from as low as 87.3°C to more than 130°C. Using corrected bottom hole temperatures, this corresponds to depths of 2800–4100 m, compared to the present-day depth of these samples of only 1965.9–2020.5 m. The oil in the Gohta and Alta discoveries is concluded to be of either Lower Triassic or Paleozoic origin based on the isomer distribution of triaromatic dimethylcholesteroids (TA-DMC). A potential source-rock candidate is the Ørret Formation, which is the time-equivalent to the Ravnefjeld Formation in Greenland. These oils are of a different origin compared to oils in the nearby Skrugard (renamed to Johan Castberg) discovery which contain oil sourced from the Upper Jurassic Hekkingen Formation. Evidence is presented to suggest that the Gohta and Alta oils represent blends of petroleum expelled at maturities ranging from about 1.0% calculated vitrinite reflectance (Rc) to more than 1.3%Rc, and this corroborates the inferences made from the petroleum inclusions. This emerging play is significant to exploration in the karst developed on the Barents Shelf and the Bjarmeland Platform during the Permo-Carboniferous. Karst reservoirs have been linked to eustatic sea-level changes, and analogous karst reservoirs may be present elsewhere in the Circum-Arctic: for example, in the Sverdrup Basin.