Structural Framework of the Statfjord Formation (Rhaetian-Sinemurian) in the Oseberg South Field, Norwegian North Sea
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The Statfjord Formation (Rhaetian-Sinemurian) produces from six fields across the North Sea, but no discoveries have yet been made in the 12 exploration wells across the Oseberg South Field. The field has undergone two major periods of rifting in the Permian-Triassic and from the mid-Jurassic to Early Cretaceous. The Statfjord Formation was deposited during the Permian-Triassic post-rift period, but its tectonic influence on the paleogeography of the formation is not well understood. An isopach map produced from seismic interpretation and RMS modelling of the Statfjord Formation showed a westward thickening trend towards the present-day Viking Graben. This study presents results obtained using new, high-quality OBC seismic data that has allowed for faults throughout the field to be mapped in great detail. Supported by stratigraphic correlations and biostratigraphy, the mapping has showed that most faults can be assigned to either of the main rifting phases or their associated post-rift subsidence histories. Large, east-dipping faults are believed to have originated during the Permo-Triassic rifting, with evidence of movement into the Cretaceous. Large thickness increases of the formation over the westward dipping Oseberg and Brage Faults, as well as syn-rift sediments within some grabens in the J structure indicate movements of these faults during deposition. Biostratigraphic data show that the lowermost part of the Statfjord Formation was approximately the same thickness across the field until the Late Triassic, constraining the initiation of the Oseberg and Brage Faults to the Early Jurassic. Interpretations from timelines correlated within the Statfjord Formation suggest that the rate of subsidence along different faults was not consistent through time. Thickness changes along strike of the fault indicate that the movement along the fault was diachronous. This study aims to show that major fault activity influenced the deposition, and possibly preservation potential of sediment in the Statfjord Formation. The second phase of rifting is believed to have initiated many of the faults within the field, as well as reactivated the Oseberg and Brage Faults. Additional NW-SE faults in the Omega structure show no evidence of syn-rift sediments at the Statfjord Formation level, suggesting a mid-Jurassic post-rift origin. Similarly oriented faults were seen in the C structure, however, the presence of syn-rift sediments was difficult to ascertain, and no conclusions about the timing of initiation were made.