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Sequence Stratigraphy in a Rift Basin: An Example from the Middle Jurassic Hugin Formation, Southern Viking Graben, North Sea*
By
Atle Folkestad1 and Nicholas Satur1
Search and Discovery Article #40309 (2008)
Posted July 7, 2008
*Adapted from oral presentation at AAPG Annual Convention, San Antonio, TX, April 20-23, 2008
1StatoilHydro, Bergen, Norway ([email protected])
The Jurassic Hugin Formation consists of shallow marine sandstones that belong to a significant hydrocarbon reservoir in the Sleipner Area in the North Sea. The formation encompasses coarsening-upward units of mouthbar and shoreface facies, interpreted as deposits during regression, and fining-upward units with tidal channel, dune, and tidal flat facies interpreted as an estuary environment during transgression. The correlations reveal that the studied part of the Hugin Formation consists of 8 sequences, each with a transgressive and a regressive unit, representing the transgressive systems tract and the highstand systems tract, respectively. The sequences are stacked landward, as a result of rapid tectonic subsidence due to rifting of the Viking Graben that led to an elongated-shaped graben where tidal currents were amplified and wave-action damped. Lowstand and forced regressive systems tracts are not identified, as relative sea-level falls are suppressed in a rapidly subsiding basin as the basin subsidence rate outpaces any fall in eustatic sea level. Thus sequence stratigraphic architecture for subsiding basins can be very different than those sequence stratigraphic models proposed for passive margins. Through facies interpretation and sequence stratigraphic correlations between wells, these regressive and transgressive units show thickness trends in the form of sigmoidal shaped wedges stacked in an offset manner in a dip direction. These thickness trends illustrate sediment partitioning within the sequences and are explained by the relationship between accommodation spaces versus the sediment supply. During regression the focus of sedimentation is pushed basinward, and during transgression it is pushed landward as sediments are trapped there. The mapping of these sequence stratigraphic units serves as input to reservoir drainage management and to identify new exploration targets.
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The Hugin Formation consists of a fining-upward estuary environment deposited during transgression (TST), and a coarsening-upward prograding delta with associated shoreface environment deposited during regression (HST). The Hugin Formation has an aggradational to retrogradational stacking pattern caused by the opening and drowning of the Viking Graben. The Viking Graben was formed by rifting and subsidence had an elongated shape with an impact on the depositional environments. The transgressive (TST) and regressive (HST) units are sigmoidal shaped with a skewed thickness distribution--due to the interaction between accommodation space versus sediment supply. Sequence stratigraphy in rift basins can be different from passive margins and foreland basins. The model is predictive in terms of facies distribution and reservoir properties. This model has been confirmed by new wells in the field. Husmo, T., G.P. Hamar, O. Hoiland, E.P. Johannessen, A. Romuld, A.M. Spencer, and R. Titterton, 2003, Lower and Middle Jurassic, in D. Evans, C. Graham, A. Armour, and P. Bathurst, eds., The millenium atlas: Petroleum geology of the central and northern North Sea: Geological Society (London), p. 129-156.
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