2012 (October): Effects of the substratum on the formation of glacial tunnel valleys: an example from the Middle Tunnel valleys are elongated incisions formed by meltwater underneath ice sheets that rest on unlithified bed materials. The formation of tunnel valleys is commonly believed to be influenced by the properties of the preglacial bed; however, a detailed analysis of this relationship has not been performed to date. To determine whether tunnel-valley location and morphology are controlled by the substratum, a 3D seismic survey was combined with lithological data from the Wadden Sea area in the Dutch sector of the southern North Sea Basin. This study shows that tunnel-valley floors often coincide with seismic reflectors that mark lithological boundaries in the substratum, and that the location and depth of tunnel-valley incision vary as a function of the properties of the substratum as expressed by lithological and geophysical-log variations. Tunnel valleys are incised preferentially into fine-grained layers, while the top of coarser-grained units commonly coincide with the tunnel-valley floor. These observations indicate that the geometry and orientation of tunnel valleys in the study area are controlled by contrasts in lithological properties of the bed. An explanation for the observed lithological control might lie in large water-pressure differences over fine-grained and impermeable layers along the flow-path of subglacial meltwater flowing through the substratum, from areas with high pore-water pressure towards areas with relatively low pressures in the vicinity of meltwater channels. These pressure differences might have been sufficient for the fracturing and fluidization of these layers. The concepts presented here have implications for existing genetic models and for the prediction of tunnel-valley morphology in understudied areas.

In this study we describe observations of Oceanic Fracture Zones from Albian-Aptian-aged oceanic crust (∼105-115 Ma, ICS-GTS2020 timescale) located approximately 80 km east of the São Tomé and Príncipe islands (STP) in the eastern Gulf of Guinea (Figure 1a). Oceanic fracture zones (OFZs) and Oceanic Transform Faults (OTFs) were recognized from very early on as key tectonic element of oceanic crust, providing important insights on seafloor spreading patterns across nearly all spreading regimes (slow to fast), the plate kinematic evolution of oceanic basins, and relative plate motions (e.g., Gerya, 2012;Le Pichon, 1968; Wilson, 1965 and references therein;Seton et al., 2020). OFZs are the morphological remnants of OTFs which accommodated opposing spreading directions between two offset ridge segments. Extending for tens to thousands of kilometers across ocean basins, they juxtapose segments of oceanic crust with differing ages (Figure 2). Here an 8,000 km 2 Abstract Oceanic Transform Faults are one of manifestations of the three major plate boundaries and a key tectonic feature of oceanic crust. They are broadly considered to accommodate strike-slip displacement along simple vertical faults and to be largely without magmatic addition. We present the first observations from broadband 3D seismic of buried, Cretaceous-aged transform faults in the Gulf of Guinea with complex internal architectures including crustal scale detachments and rotated packages of volcanics within oceanic crust. In the study area, several Oceanic Fracture Zones (OFZ) are described from Top Crust to Moho. OFZ scarps are observed to connect at depth with zones of low angle reflectivity which dip into the OFZ and perpendicular to the spreading orientation. At depth they detach onto the Moho, necking the adjacent crust in the manner of extensional shear zones. Thickly stacked and tilted reflectors, interpreted as extrusive lava flows, are common above the shear zones and infill up to 75% of the crustal thickness. The entire OFZ stratigraphy is overlain and sealed by late-stage lavas that are continuous from the abyssal hills of the trailing spreading ridge. These insights demonstrate complexity previously only predicted in numerical simulations. We propose a model with extension at a high angle to the spreading orientation along a low angle shear zone that acts as a conduit for decompression related melt and volcanism. We conclude that oceanic transforms are non-conservative and not simple strike slip fault zones, contradicting the conventional view.

<p>Oceanic Transform faults are one of manifestations of the three major plate boundaries and a key tectonic feature of oceanic crust. They are broadly considered to accommodate strike-slip displacement along simple vertical faults and to be largely without magmatic addition. We present the first observations from broadband 3D seismic of buried, Cretaceous-aged transform faults in the Gulf of Guinea with complex internal architectures including crustal scale detachments and rotated packages of volcanics within oceanic crust. In the study area, several Oceanic Fracture Zones (OFZ) are described from Top Crust to Moho. OFZ scarps are observed to connect at depth with zones of low angle reflectivity which dip into the OFZ and perpendicular to the spreading orientation. At depth they detach onto the Moho, necking the adjacent crust in the manner of extensional shear zones. Thickly stacked and tilted reflectors, interpreted as extrusive lava flows, are common above the shear zones and infill up to 75% of the crustal thickness within the OFZ. The entire OFZ stratigraphy is overlain and sealed by late-stage lavas that are continuous from the abyssal hills of the trailing spreading ridge. These insights demonstrate complexity previously only predicted in numerical simulations. We propose a model with inside corner extension at a high angle to the spreading orientation along a low angle shear zone that acts as a conduit for decompression related melt and volcanism. Late-stage lavas indicate a second stage of magmatic accretion as dykes propagate through the transform to adjacent crust, as inferred from bathymetric studies. These observations come from a unique 3D seismic dataset and are placed within a kinematic model which combines insights from numerical models. We conclude that these oceanic transforms were non-conservative and not simple strike slip fault zones, contradicting the conventional view.</p>

ADNOC and Shell are currently joining efforts to rejuvenate the exploration portfolio of Abu Dhabi. A country-wide integrated petroleum system study is being carried out to identify new play concepts and opportunities. One of the foundations of this study is the understanding of the structural evolution and its impact on prospectivity. A structural evolution model has been developed using 2D and 3D seismic data and the country has been divided into structural domains. The extent and quality of the seismic dataset provided a unique opportunity to investigate the country wide structural evolution. Special care has been taken to generate seismic attribute volumes that enhance sedimentary features and fault visualization. This allowed the detailed assessment of fault displacement. In addition, mapping of the edge of carbonate platforms through time at the country scale allowed the identification of long wavelength tilts of the Arabian plate in Abu Dhabi. These observations have been linked to the regional phases of deformations. The most important phases of deformation that affected the trap formation are the Jurassic rifting, the Late Cretaceous transtension, and mid Tertiary compression. The country has been divided into specific structural domains using existing structural features. These structural elements comprise NS and NW-SE striking basement features, forced folds associated with basement features, drape folds associated with salt domes, and NW-SE and NNW-SSE conjugate sets of transtensional faults zones associated with pop-up structures. With the help of sandbox experiment analogue models as well as field analogues from Oman, we propose that the Late Cretaceous transtensional faults are decoupled from basement and do not root into any deep basement faults. We also propose a series of conceptual 3D fracture diagrams per structural domain.

The geophysical world has been recording seismic waves for over a century now, with the seismograph seeing first utilization towards exploration of oil and gas in the early 1900s. We started shooting 3D seismic data half a century later, and since then both our acquisition methods and how we reconstruct waves propagating through the earth with hyper-computing capabilities has evolved tremendously. Pushing the envelope of what we can image, in particular, is a major tranche of changes in seismic acquisition that started roughly a decade ago. Some of these recent developments include broader-bandwidth seismic acquisitions, particularly emphasizing low frequencies for both land and marine, and changes to sensors, sensor layouts and patterns used for shooting. These new acquisitions have refocused our emphasis on fundamentals in seismic processing, significantly advancing our ability to see the subsurface. Some of the ideas in seismic processing formulated for these acquisitions, not surprisingly, are not exclusively applicable only for modern acquisitions. Combining some of these newer approaches with pioneering ideas previous generations of geophysicists mastered, has allowed a fresh take on how large archives of legacy seismic, sitting-on-shelves, can be improved to provide fresh insights towards exploration. For the Middle East and North Africa region, where we often deal with ‘difficult’ seismic typically characterized by extremely high noise content, this re-look at older data has resulted in an evolution of workflows for vintage seismic data conditioning, leading to higher quality datasets that increase confidence and reduce uncertainty for the plays, leads and prospects we pursue. The data conditioning workflow involves a number of steps, and are mostly applied post-migration and often post-stack. These are applicable across a spectrum of data types from different sources and geological settings. Typically, these workflows are fine-tuned for each seismic dataset in a matter of days or often ‘on-the-fly’, with implementation on the next generation interpretation platform in Shell, providing extremely rapid turnaround, resulting in dramatic uplift in image quality in many cases. These have demonstrably impacted decision-making in exploration and production, providing the ability to, quite simply, see with significant more clarity what we could not see before. We share examples of utilization of these workflows, contributing towards a number of projects, including an extensive joint Abu Dhabi National Oil Company (ADNOC) and Shell effort to rejuvenate the Exploration Portfolio of Abu Dhabi, working with a country-wide database of multi-vintage onshore and offshore datasets constituting approximately 2500 2D seismic lines and more than 50 3D seismic volumes. Our workflows are grounded in past experience, yet leverage latest signal-processing innovations. They provide a step-change in our ability to rapidly investigate and interpret large volumes of challenging seismic data efficiently, in addition to enabling visualization of geological features indistinguishable on original seismic.