Although normal isotropic hummocky cross‐stratification is commonly interpreted to be the deposit of large‐scale ripples, there are many reasons why this may not usually be the case. These reasons include: (i) that the stratification produced by large‐scale ripples does not particularly look like isotropic hummocky cross‐stratification; (ii) that it is difficult reconciling the abundance of HCS with the restricted hydraulic stability of large‐scale ripples in silt to fine sand (i.e. the grain sizes in which hummocky cross‐stratification is usually found); (iii) that the distribution of hummocky cross‐stratification within ancient storm beds is not the distribution that would be expected from large‐scale ripples; (iv) that the flows calculated to have formed ancient examples of hummocky cross‐stratification would be expected to generate an upper stage plane bed rather than ripples; and (v) that it is difficult to explain why large‐scale ripples would predominate in the proximal parts of storm beds when modern storm flows commonly exceed the threshold for entrainment. In contrast to the various hypotheses which propose that isotropic hummocky cross‐stratification is generated by ripples, an alternative hypothesis which suggests that it is generated by instabilities, does seem to adequately explain the origin of hummocky cross‐stratification. However, it is difficult to accept this hypothesis given that the origin of the proposed instabilities is unproven. These conclusions highlight the continued uncertainty regarding the process, which generates hummocky cross‐stratification.

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The relatively fine-grained Late Devonian to Early Carboniferous 'Cork Beds' succession of the South Munster Basin includes continuous sections of paralic facies that are over 1000 m thick and individual sandstone units over 300 m thick. However, the succession does not reflect prolonged phases when facies belts were stationary, but rather multiple stacking of small-scale, highfrequency sequences, each associated with pronounced migration of shorelines. What seems to have been unusual about the South Munster Basin succession was that the geographical positioning of these high-frequency sequences was fixed. This resulted from an unusual combination of tectonics, shelf hydrodynamics, sedimentation rates and the textural maturity of the sediment within the basin. Of these, tectonics was probably most critical, particularly the juxtaposition of rapid subsidence in basinal areas and a basin margin zone (to the north) that was sufficiently up-standing to pin the maximum extent of transgression during repeated highstands of sea-level, yet not so upstanding as to have diverted the major regional drainage system. The embayed palaeogeography of the area may also have been influential.

Since production began in the HP/HT Kristin Field off mid-Norway, reservoir pressure in each of the three mid to late Jurassic reservoir units (the Garn, Ile and Tofte formations) has declined significantly more rapidly than was initially predicted. In the Garn Formation, the Tofte Formation and to some extent also the Ile Formation, this has occurred at least partly because an unusual distribution of reservoir properties led to bias in the four-well appraisal dataset and this in turn resulted in an overestimation of reservoir properties. Of particular importance to this bias was the fact that very good but unrepresentative reservoir properties were encountered in all three reservoir zones in the discovery well located in the centre of the field. These, it is now realized, are not even typical of most of the central part of the field but are, instead, restricted within one, small, anomalous area. Study of cores and thin sections indicates that in each reservoir unit this directly reflects a concentration of more energetic depositional facies in the area while less energetic facies are present on three sides. This pattern was not predictable from the original dataset and seems to have arisen because there was structural control upon facies positioning during accumulation of the reservoir section. This influenced the distribution of cleaner, coarser grained, more proximal depositional facies and, ultimately, reservoir quality distribution and pressure development. What is interesting about Kristin Field is that the structural influence upon sedimentation is observed within the footwall stratigraphy of a major relay structure where the primary provenance direction was on the hanging-wall side. This pattern is the reverse of what is normally reported in tectono-stratigraphic studies.

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The HPHT Kristin field is located offshore Norway in 350 m water depth and is developed with four sub-sea templates. Since production began in November 2005, reservoir pressure has dropped around 4 times faster than predicted. The main Kristin field statistics include: an initial pressure of 910 bar, a reservoir temperature of 170°C, a dew point of around 400 bars (rich fluid accounts for 50% of income) and in-place volumes of around 100 GSm3 of gas and 100 MSm3 of condensate. The Kristin reservoir section comprises three separate Jurassic sandstone units and the field is segmented by faults. This paper describes the process of continuously history matching pressure decline during the first two years of production in Kristin using: production data, RFT pressures from wells drilled following production start-up, shut-in wellhead pressures, and data from two bottom-hole gauges. Close multidisciplinary cooperation between geophysicists, geologists and reservoir engineers has been key to 're-understanding' the Kristin reservoirs since production start-up and has facilitated the process of tuning the simulation model to obtain a better history matched model. The most important modifications have been the introduction of horizontal pressure barriers to match RFT pressures, and a significant reduction of pressure support from the eastern part of the field. The poor pressure support is interpreted to result from enhanced fault seal in one of the main reservoir units, and a rapid easterly decrease in reservoir properties in the other (main) reservoir unit. These changes have been directly input into the simulation model and have resulted in a good match of the pressure history from the 11 producing wells. Directly input into the simulation model has shortened the updating cycle to a few minutes and saved several months in the history matching process. Work is currently ongoing to refine a more detailed geological model based on these results, and the additional knowledge gained by testing different solutions directly in the simulation model. This improved understanding of the reservoir behaviour also gives valuable input to the uncertainty study required before updating official reserves. Introduction Kristin is a large, high-pressure, high-temperature (911 bar, 170°C) gas-condensate field in the Norwegian Sea (65° N, 6.4° E), with the main reservoir section buried at approx. 4600 to 4850 m MSL (Figure 1). The field was discovered in 1996 when the operator, Saga Petroleum, tested a new exploration concept. Most experts at the time believed that porosity and permeability would be too low for commercial success at such reservoir depths, but the exploration team in Saga thought otherwise and the well came in successfully; with gas in three separate formations, and with high porosities down to almost 5000 m. Three appraisal wells were subsequently drilled, including one side-track, and these proved a substantial discovery. Statoil took over as operator in January 2000, and the plan for development and operation (PDO) for the Kristin field was approved in December 2001. Kristin started production in November 2005 and had by February 2008 produced around 8 GSm3 of gas, 8 MSm3 of condensate and only 350 kSm3 of water. Estimated total in-place volumes in the Jurassic Garn, Ile and Tofte formations are around 100 MSm3 of condensate and 100 GSm3 of gas. Around 10% of the in-place volumes are in the Tofte reservoir and the rest is split equally between the Ile and Garn reservoir. The field is being produced by pure depletion from 11 wells: 4 Garn Fm. producers, 1 Ile Fm. producer, 5 co-mingled Garn & Ile Fm. producers and 1 Tofte Fm. producer. There are further plans for 2 to 3 additional wells to target reserves away from the main crest of the structure. The dedicated producers are located in the central area, while commingled producers are situated towards the northern and southern flank. No communication is expected or observed between the three reservoirs. The highly undersaturated gas in the reservoir makes depletion a fairly efficient recovery mechanism. Gas injection to recover more condensate has been evaluated but found too expensive and challenging with 600 bar required injection pressure.