Abstract

The Ivar Aasen (IA) oilfield is located on the Gudrun Terrace on the eastern flank of the Viking Graben in the Norwegian North Sea. The field was discovered in 2008. The reservoir is located within a sedimentary sequence of Mid-Jurassic to Late-Triassic age, which consists of shallow marine to fluvial, alluvial, floodplain and lacustrine deposits overlying a regionally extensive, fractured calcrete interval. The sequence exhibits a complex mineral composition and is heterogeneous at a scale below that of a logging sensor. Shale layers, re-deposited shale and what was first believed to be redeposited calcrete fragments present in various forms throughout the sequence. Looking more in depth to XRD and XRF data and contrasting Fe concentration in the dolomite, it is also possible to explain some of the carbonate deposits through other processes.

Extensive data acquisition in the form of advanced wireline logs and coring with analysis performed in “geopilot” wells before production start, enabled a novel thin bed formation evaluation technique based on the modified Thomas-Stieber method (Johansen et al. 2018). The method increased the in-place oil volumes within the Triassic reservoir zone internally named Skagerrak 2. This led to several improvements and a modified drainage strategy of Ivar Aasen. Several good producers were placed in the complex net of the Skagerrak 2 Formation. Results from these producers have encouraged development of an even more marginal and complex net, deeper into the Triassic sedimentary sequence. Therefore, another “geopilot” was drilled into the deeper Triassic sediments, internally named as the Alluvial Fan. This zone exhibits conglomerate clasts in a matrix varying between clay, silt, feldspars, and very fine to very coarse sand fractions, grading towards gravel. Previously, this zone was considered to be mostly non-net. Applying the same interpretation method as for Skagerrak 2, the Alluvial Fan promised economic hydrocarbon volumes. The latest geopilot proved producible hydrocarbons, and subsequently a producer was also successfully placed in this part of the reservoir.

Production data and history matching from the beginning of production have for a long while established the previous increase of IA Triassic oil volumes published in 2018. Advanced studies of mineralogy and spectroscopy (Johansen et al. 2019) have indicated that a significant amount of the previously interpreted dolomite, could be reinterpreted as ferroan dolomite. The latter is a heavier mineral that increases the matrix density, hence also the total porosity.

The additional findings described provided another necessary first-order correction to further enhance the evergreen geomodel. This paper describes this methodology which resulted in improved petrophysics and reservoir properties of the Alluvial Fan, yet again demonstrating the value of advanced wireline logs and detailed analysis that in total impacts the IA reserve volumes in a significant manner. Repeated success with the applied spectroscopy data and the thin bed methodology used today (Johansen et al. 2018), has resulted in even the deeper Braid Plain Formation becoming of economic interest. It is expected to lie within the oil zone in an upthrow block in the northern part of the IA field and could be developed into the next target.

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