A simplified model of stress-slip relationships: application to the Frøy field, northern North Sea

Abstract

We apply an analytical method, Sortan, to determine fault slip as induced by far-field stress. The Sortan method is based on the Wallace–Bott hypothesis. The method is applied to all major faults of the Frøy oil field located at the eastern margin of the southern Viking Graben, North Sea. The purpose was to quantify the most likely senses of motion experienced by the faults during the Late Jurassic–Early Cretaceous rifting event.Because strike-slip displacement is generally difficult to determine based on classical exploration methods, particular attention was paid to the simulated strike-slip component along the major faults. Depth-converted structural maps were used to determine fault attitudes. The presence at the northern edge of the field of a major syn-rift dextral strike-slip fault, the North Frøy Fault, allowed us to constrain the most likely characteristics of the prevailing stress field during rifting. According to different possible stress states, different models were run. The results from the Sortan modelling indicate that the strike-slip component of motion was in general minor on the major faults within the oil field. The inferred stress state is found consistent with rifting involving fault-related displacement strongly dependent on σ 3 orientation. This stress state prevailed at least during the latest stages of rifting (Latest Jurassic–Early Cretaceous) of the northern North Sea. This case study shows that where paleostress inversion methods cannot be applied in the absence of direct observation, the Sortan method has the potential to constrain the slip behaviour of the brittle structure. It also enables one to rapidly check the overall mechanical consistency of tectonic interpretations.