LAUSR

Abstract The estimated in-place hydrocarbon volume of the giant Johan Sverdrup field is 3.5 billion barrels of oil, the bulk of which is contained in the Upper Jurassic intra-Draupne Formation sandstone. The intra-Draupne Formation sandstone is composed of unusually coarse grained siliciclastic sediments, with an average net/gross of 97% and average porosity of 28%. The median core permeability ranges from 0.5 to 40 Darcies in individual wells, and may reach even higher values based on drill stem testing. The reservoir is interpreted to be a time-transgressive sheet sand, ranging from 2 to 38 m in thickness and covering an area of more than 200 km2. Vertical and lateral facies and grain size trends, combined with biostratigraphy and palaeocurrent indicators, are interpreted to demonstrate a westward onlapping, transgressive shoreface depositional system, with local fan delta input. Reservoir properties vary significantly across the field and relative to facies association. In terms of permeability, the upper and lower shoreface associations in the Avaldsnes High area in the east show the highest median values of 25–27 Darcies. The upper and lower shoreface associations in the Augvald Graben area to the west show median values of 15 and 21 Darcies, respectively. The lowest values are observed in the transgressive shoreline, fan delta and spiculitic shoreface associations, i.e. 8 Darcies, 2 Darcies and 0.1 Darcies, respectively. The latter two facies associations are restricted to local areas in the west and northwest. The coarse grain size, lack of fines, scarcity of cementation and extensive sheet-like distribution of siliciclastic sediments make the intra-Draupne Formation sandstone an ideal reservoir. The depositional model presented in this paper may be used as an analogue when exploring for similar, high quality, shallow marine reservoir sands.