Development of rift basins illustrated by the structural evolution of the Oseberg feature, Block 30/6, offshore Norway

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Abstract
The Mesozoic tilted fault block in Norwegian North Sea block 30/6 containing the giant Oseberg oil and gas field is used to illustrate basin development during the Jurassic and Cretaceous. There is seismic evidence for an early rifting phase in the northern Viking Graben, on which the Jurassic/ Cretaceous rifting phase (early Bathonian to Berriasian) is overprinted. The Oseberg structure is representative of other fault blocks bordering the northern Viking Graben axis. Planar normal faults, and listric faults which were restricted to the rifting phase, dominate. Both fault types exhibit syn-fault sedimentation. Listric faulting is recognized on seismic sections by the differential tilting, reverse drag, antithetic faulting, and a wedge-shaped syn-rift reflector package. The initiation of rifting is dated by baselap onto pre-rift reflectors, and cessation is dated by an unconformity beneath the generally onlapping post-rift sediments. Steeply-dipping planar normal faults, often active along previously established fault zones, played a significant part in the post-rift development of the basin. The last movement on planar normal faults in the Oseberg fault scarp becomes younger away from the rift axis, and this produced the complex fault scarp visible on many seismic lines. The topography, defined by the post-rift unconformity, is a composite surface which formed in response to subsidence processes; it is not the result of orogenic events such as the Kimmerian 'orogeny'. Footwall uplift associated with planar normal faults caused local unconformities in complex fault scarps and on fault block crests.

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