Tectonostratigraphy and Fault Analysis of the Øygarden Fault Complex Footwall, Northern Horda Platform, Northern North Sea.

• Main Author: Bjerkeli, Ådne
• Format: Master Thesis
• Language: English
• Published: 2019
• Subjects: Krossfjord*
• Online Access: http://hdl.handle.net/10852/70041; http://urn.nb.no/URN:NBN:no-73162

The evolution of the Øygarden Fault Complex (ØFC) footwall is studied using tectonostratigraphic analysis of 3D seismic reflection data. The footwall is comprised of a North-South trending graben bounded by three reactivated basement involved faults. Within the graben, a network of smaller faults has developed a step-like prominent fault pattern. Two dominant fault sets are recognized, one with a strike in an NNW-SSE direction and the other in an N-S to NNE-SSW direction. The relevance of understanding this poorly understood system is due to its location up dip from a prominent CO2 storage site and potential leakage across the Øygarden fault into the footwall. This study and represents the first attempt to document the fault architecture and the sediments present in the footwall graben of the ØFC. Seismic amplitude signature correlation between the ØFC footwall and hanging-wall, conducted herein, indicate that the footwall graben is filled with Middle to Late Jurassic sediments supported by findings of Jurassic sediments in fault zones in nearby onshore areas. Four prominent reflectors are recognized in the footwall strata which are interpreted to be (or approximate to) the tops of the Brent Group, Krossfjord, Fensfjord and Sognefjord Formation. The Jurassic strata represents a thinner succession on the footwall of the ØFC compared to the hanging-wall as it has been heavily eroded and truncates against the Base Pleistocene erosional unconformity (BPU). This unconformity makes specific dating of the faults challenging as cross-fault stratigraphic correlation cannot be utilized. iii No wedge shaped syn-rift deposits are recognized within the footwall graben, indicating post-depositional faulting. Graben development is envisaged to have occurred during the Late Jurassic or Early Cretaceous. This timing is consistent with the diachronous basin to margin transfer of strain post Jurassic-Cretaceous extension. This study has shown that there is no bias towards one fault set abutting the other, and fault populations are likely to have developed simultaneously. Regional faults on the Horda Platform suggest a SSW-NNE extension direction is likely during the Late Jurassic to Early Cretaceous Phase of rifting, however, faults in the ØFC footwall appears to be dominated by oblique reactivation of N-S faults that formed initially during PermoTriassic extension. Mass transport direction as an indication of extension direction show that the ØFC footwall faults have a WSW direction of movement. However, the smaller faults are clearly dominated by the reactivated N-S trending faults. The prominent basement high located under Mesozoic sediments in the footwall of the ØFC are envisaged to have been previously uplifted, fractured and weathered. If CO2 injected into the Smeaheia subsurface comes in contact with the ØFC, there is a potential for along-fault leakage either to the free surface, or into Jurassic sediments in the footwall described herein. Therefore, knowledge of the footwall sediment and fault architecture is fundamental to de-risking the adjacent CO2 storage prospect.