Regional Tectonics, Sedimentary Processes and Global Oceanography in the Formation of a World-Class Reservoir: Insights From the Johan Sverdrup Giant Oilfield, Norway
Vigorito, Mario; Martinsen, Ole J.; Nødtvedt, Ane Birgitte; Skjæveland, Øyvind; Gregersson, Anna-Sofia; Martin, Richard; Windelstad, Jørgen; Jørgenvåg, Sivert; Fjelland, Målfrid; Ferstad, Tone
The Johan Sverdrup is the largest oil discovery of the 2010 (>1 bilion bbls). The discovery was made in a Middle Jurassic to Early Cretaceous clastic wedge (up to 100 m thick) deposited within the 20x5 km Karmsund Graben, located on the inner margin of a major basement high (Utsira High) on the western periphery of the North Sea Rift Basin.
The reservoir consists mostly of late Jurassic-early Cretaceous, poorly to moderately sorted, coarse to very coarse marine sandstones (Draupne Sandstone) which overlie, on top of a regional erosion surface, fluvial to shallow marine Middle Jurassic sandstone that form the lower part of the reservoir section. Most of the Draupne Sandstone is virtually mud-free and exhibits porosities in the range of 0.24-0.32 with permeabilities of 5-30 D. This world-class reservoir sandstone consists mostly of gravity flow deposits laid down along and at the front of coarse grained fan-deltas directly fed from the basement high and reworked by marine currents.
The Draupne Sandstone is part of a regionally-developed overall transgressive sequence and was deposited during a phase of generalized relative sea-level rise associated with the expansion of the Atlantic Ridge and acme of the rifting in the North Sea Basin. This led also to the concomitant rising and tilting of the shoulders (including the Utsira High) of the main rift-system which in turn promoted the erosion of the uplifting basement highs and deposition of large volumes of coarse-grained sediments in the adjacent basins. Tectonics also controlled basin topography and in turn marine current circulation and strength. These currents were important for preventing deposition of finer grain fractions thus enhancing reservoir quality.
Starting from the late Tithonian, the Karmsund Graben was rapidly drowned causing formation of palimpsest glauconite-rich sediments and phosphatic-carbonate hardgrounds which preceded the deposition of deep-water hot shales. At the same time, the shallow marine clastic domains stepped backwards onto the margins of the basement high where coarse-grained sandstones alternate and are capped by fine spiculitic sandstones which represent the younger portion of the reservoir section. The blooming of benthic communities, dominated by siliceous sponges, during the late Jurassic, is a global event and is inferred to record a change in the chemistry of the oceanic waters likely in relation with intensified oceanic spreading.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013