The effects of temperature and food availability on feeding and egg production of the Arctic copepod Calanus hyperboreus were investigated in Disko Bay, western Greenland, from winter to spring 2009. The abundance of females in the near bottom layer and the egg production of C. hyperboreus prior to the spring bloom document that reproduction relies on lipid stores. The maximum in situ egg production (± SE) of 54 ± 8 eggs female −1 d −1 was recorded in mid-February at chlorophyll a concentrations below 0.1 μg l −1 , whereas no egg production was observed in midApril when the spring bloom developed. After reproduction, the females migrated to the surface layer to exploit the bloom and refill their lipid stores. In 2 laboratory experiments, initiated before and during the spring bloom, mature females were kept with and without food at 5 different temperatures ranging from 0 to 10°C and the fecal pellet and egg production were monitored. Food had a clear effect on fecal pellet production but no effect on egg production, while temperature did not have an effect on egg or fecal pellet production in any of the experiments. Analyses of carbon and lipid content of the females before and after the experiments did not reflect any effect of food or temperature in the pre-bloom experiment, whereas in the bloom experiment a clear positive effect of food was detected in female biochemical profiles. The lack of a temperature response suggests a future warmer ocean could be unfavorable for C. hyperboreus compared to smaller Calanus spp. which are reported to exploit minor temperature elevations for increased egg production. KEY WORDS: Calanus hyperboreus · Egg production · Fecal pellet production · Effect of temperatureResale or republication not permitted without written consent of the publisher

The Halfdan Northeast gas field is a widespread, thin accumulation in high porosity, low permeability chalk of Danian age. Successful development relies on implementation of long single- or multilateral horizontal wells in thin reservoir zones and securing the maximum number of reservoir feet for each well bore. Consequently, detailed knowledge of reservoir geometry and properties is essential. This paper describes how the detailed reservoir architecture was resolved by geophysical reservoir characterization based on rock physics understanding using optimized seismic elastic inversion products. The chalk reservoir is difficult to distinguish from the overlying shale on conventional seismic as the effects of porosity and gas/fluid mixture cause the reservoir chalk to exhibit similar acoustic impedance as the shale above. The placement of wells is further challenged by the fact that erosional depressions, ranging in size from ca. 400 to 6,000 metres across, are present in the chalk. Optimized elastic inversion allowed mapping the top of the continuous chalk and the base of the overburden shales, whereby the infill of these structures could be characterized, consisting of marl and disturbed chalk. A well targeting the disturbed chalk infill confirmed this interpretation and found it to be of reservoir quality. The further development strategy is to tap both the laterally continuous chalk and the disturbed chalk bodies to maximize recovery. Introduction Halfdan Northeast is a chalk gas field offshore Denmark, comprising a thin but widespread accumulation with very low relief. The reservoir is coccolith chalk of Danian age (Ekofisk Formation), characterized by extremely small grain size (average around 1 micrometer), high porosities (30–45% in reservoir zones) with a cyclical development and very low permeabilities (typically 0.1 - 1 mD). Figure 1 shows the field location and a typical vertical well log. The gas bearing chalk is generally less than 70 ft in thickness, and overlies water bearing chalk of variable porosity. Target zones for reservoir drilling are typically less than 20 ft thick. The reservoir chalk is capped by marl and shale of Tertiary age. Ensuring recovery from Halfdan Northeast relies on placing long horizontal wells in the thin high porosity reservoir zones. Due to the thinness of the reservoir and the presence of mobile water directly below it, fracturing is not viable. Because of the very low permeability, the production from wells is strongly correlated to reservoir contact, i.e. the amount of reservoir feet (Reference 1). For well implementation, the key to success is therefore a detailed understanding of reservoir geometry and characteristics combined with the ability to optimally place these horizontal wells using geosteering. The interface between Danian chalk and overlying Tertiary marl and shale may often be mapped on seismic as an increase in acoustic impedance. However, if the porosity of the chalk is sufficiently high and the chalk is gas-bearing, the interface may be mapped as a decrease in acoustic impedance2–3. In the case of Halfdan Northeast, the reservoir is gas-bearing, but of a porosity that makes it impossible to map top reservoir from near stack amplitude seismic, because the acoustic impedance of the reservoir is similar to the impedance of the overburden. This paper describes how this challenge has been met by a multidiciplinary, iterative approach comprising rock physics, geophysical reservoir characterization, geological interpretation and modeling and well implementation.

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