Fault reactivation and oblique-slip in the Southern North Sea

Abstract

The structural evolution of the Southern North Sea region is controlled by a number of important tectonic phases. The dominant NW-SE structural grain first became evident during dextral wrenching in the Stephanian/Autunian. A second fault direction trends NNE-SSW, and probably also originated in pre-Permian times. Extensional faulting in the Triassic preceded major Middle Jurassic to Early Cretaceous rifting. Subsequent cooling subsidence was interrupted by inversion in the Late Cretaceous, the mid-Paleocene, and again in the mid-Tertiary. During these periods Jurassic basins were uplifted and, locally, subjected to deep erosion.Despite the multitude of tectonic events, a high degree of fault parallelism can be observed in the Southern North Sea. Fault patterns and directions are strikingly similar, whether viewed on a regional scale or at prospect level. The general structural model is, therefore, one of repeated reactivation of basement faults, which continue to control the structural grain despite changes in tectonic regime. Only rarely and by coincidence do the main structural features have orientations that conform to the regional stress. Consequently the predominant mechanism is considered to be one of oblique-slip.This phenomenon is particularly well observed on structural highs, such as the Cleaver Bank High, straddling the median line between the Netherlands and the UK. Here, the fault trend at Base Zechstein level runs essentially NW-SE. Repeated fault reactivation has led to anomalously high length-to-throw ratios. Locally short N-S and E-W link-ups accommodate oblique-slip. At higher stratigraphic levels en-echelon faults illustrate the strike-slip nature of the latest movements along the pre-existing NW-SE structural grain.The presence of salt in the overburden causes faulting at higher levels to be decoupled from basement faulting. Sandbox experiments show that reverse faults and extensional grabens can develop in the postsalt sequence in response to oblique-slip along the pre-salt structural grain. The location of such grabens is invariably offset towards the footwall of the basement fault, the amount of offset dependent on the thickness of the salt.These sandbox experiments provide valuable analogues for the structural mechanisms that are believed to have operated in the Southern North Sea. Applied to both 2D and 3D seismic interpretation, they allow fault mapping with a greater degree of confidence. Recognition of fault reactivation and oblique-slip will lead to a better understanding of the tectonic history, timing of structuration and trap development.

Reservoir leakage along concentric faults in the Southern North Sea: Implications for the deployment of CCS and EOR techniques

“…Oblique-slip normal faulting predominated after the Variscan Orogeny, with the largest faults cutting through the Variscan fold belt and propagating along older NW-SE trending basement faults. The present-day structural grain of the Southern North Sea follows horst and graben structures formed by these faults (Duin et al, 2006;Oudmayer and De Jager, 1993;Van Wijhe, 1987;Ziegler, 1990) (Figs. 1 and 5).…”

“…The Laramide inversion in the Cenozoic reactivated Subhercynean faults, creating a prominent Cretaceous-Tertiary unconformity (De Lugt et al, 2003;Oudmayer and De Jager, 1993). Major basin inversion events are recorded in the Cenozoic, the largest of which coincided with the Pyrenean Orogeny (Oligocene) and created an unconformity at the base of the Miocene (Oudmayer and De Jager, 1993;Verweij and Simmelink, 2002). It separated the Lower North Sea Group from the Middle North Sea Group (Wong et al, 2001).…”

“…It separated the Lower North Sea Group from the Middle North Sea Group (Wong et al, 2001). The boundary between the Middle and Upper North Sea Groups is based on a break of sedimentation resulting from regional uplift and global low in sea level, associated with the Alpine Orogeny (Savian unconformity, Middle Miocene) (Oudmayer and De Jager, 1993;Wong et al, 2001).…”

“…In this work, high quality 3D seismic data has unveiled some of these concentric faults within, and proximal to, a salt-withdrawal basin generated in the Broad Fourteens Basin, Southern North Sea . They have a net normal separation, have reactivated obliquely and formed during a period of compression, demonstrating a compound history (Maione and Pickford, 2001;Nalpas et al, 1995;Oudmayer and De Jager, 1993). Their occurrence may present an important caveat for the implementation of carbon capture and storage (CCS), and enhanced oil recovery (EOR) techniques, in strata flanking salt structures in the North Sea and in equivalent salt-rich basins (Cawley et al, 2015).…”

Lough Neagh: the site of a Cenozoic pull-apart basin

“…The nature and mode of formation of pull-apart basins have been the subject of study by many workers, and examples have been described worldwide (for example, Aydin & Nur 1982;Mann et al 1983;Sylvester 1988;Bartholomew et al 1993;Oudmayer & de Jager 1993;Dooley et al 1999;Reijs & McClay 2003). Pull-aparts are generally defined as zones of extension produced at bends or step-overs in a strike-slip fault system.…”