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Structure
and Geometric Evolution of a North Sea
Graben-Diapir Pair as Constrained by Data, Models, and Restoration
KILSDONK, BILL and CHARLES J. SICKING
A linear graben-diapir pair runs north-south through the center of a 3-D
seismic survey in the UK Southern Gas Basin. The complex, faulted geometry of
the structure made an accurate structural interpretation and velocity model
critical to 3-D seismic depth migration. We constrained and revised our
interpretation of this structure by iterating kinematic restoration, structural
analysis, and seismic analysis with successive migration iterations. We
constructed, restored, and revised six serial cross sections to constrain the
3-D model of the structure. The cross sections were based on seismic data, tied
to nearby wells, and guided by structural models. We used shallow, well imaged
structures to constrain the structure of the poorly imaged deeper section. Fault
offsets, clearly imaged in the shallow section, were carried down to the deeper,
poorly imaged top Zechstein Salt. Because the top salt should be displaced by
nearly the same amount as the shallow section, the height and location of the
poorly imaged salt ridge could be interpreted. Extension above the salt was
localized in the graben-diapir pair on conjugate normal faults that intersect
and offset one another. Slip on these faults produced both the graben and the
fault bounded salt ridge below it. The structure initiated as a necking
instability in reaction to East-West extension. Reactive growth continued until
the diapir locally developed sufficient head differential to drive isostatic
folding and pierce the overburden
. Restorations indicate that total extension
along the graben varies from 1 km in the South to 1.5 km in the North.