Print this pageStructure 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.