Abstract

Anticlines along the western margin of the South Viking Graben in the Brae area of the U.K. North Sea form the structural components of large structural and stratigraphic traps within Upper Jurassic Brae Formation submarine fan deposits. Various interpretations of the origin of these anticlines and their attendant inboard synclines at the graben boundary have been previously published, with both gravity-driven processes and inversion being invoked. Based on regional interpretation of 3-D seismic data sets and analysis of thickness variations in uppermost Jurassic and Cretaceous sequences in numerous wells, it is concluded that gravity-driven processes were more important than inversion.

Differential compaction of mudstone-rich slope deposits laterally adjacent to coarse clastic submarine fan reservoirs has resulted in the field reservoirs now being at slightly higher elevations than the finer grained deposits along the length of the anticlines. Compaction of the very thick sandstone- and mudstone-dominated successions in the basin center has also been greater than that of the more conglomeratic successions adjacent to the basin margin, where sequences are underpinned by the slope of the footwall, resulting in over-steepened slopes toward the basin on the outboard side of the anticlines. Movement of Permian salt that underlies the Jurassic (and Triassic) in the basin has also had significant broad effects on the Upper Jurassic structures, creating depressions and underpinning some anticlines. Continued slow subsidence of the basin-fill down the main graben boundary fault system in the under-filled rift during the latest Jurassic and Early Cretaceous, above changes in footwall slope (from eroded slope to graben-boundary fault, or, in the case of East Brae, across a plunging basement nose) is considered to be the primary cause of the anticlines and their inboard synclines. Reversal of movement along the main boundary fault, causing inversion of the graben-margin sequences, is considered unlikely as the primary mechanism for anticline formation. Additional movement down the graben-boundary fault system in the early Maastrichtian may have slightly tightened the anticlines. Final minor fault movement along the graben margin occurred in the mid Eocene, but this is unlikely to have significantly affected the Brae structures. Some of the anticlines provide evidence of the presence of the underlying thick reservoir sequences (due to differential compaction over conglomeratic sections), but not all positive structural features contain coarse clastic sediments.