Cross-border petroleum geology in the North Sea: an introduction

The North Sea has entered a phase of infrastructure-led exploration in an attempt to extend the economic lives of the main fields and arrest the overall production decline to a certain extent, while the transition to a future low-carbon use of the basin is also in progress. As the papers in this volume demonstrate, in order to find, appraise and develop the mostly smaller near-field opportunities as well as making sure to grasp the opportunities of the near-future energy transition, a regional understanding of the North Sea is still critical. Even more so, a cross-border approach is essential because 1) some of the plays currently being targeted have a clear cross-border element, 2) it allows the comparison of stratigraphic names throughout the entire basin and 3) it enables explorers to learn lessons from one part of the rift to be applied somewhere else.This volume offers an up-to-date, ‘geology-without-borders’ view of the stratigraphy, sedimentology, tectonics and oil-and-gas exploration trends of the entire North Sea basin. The challenges associated with data continuity and nomenclature differences across median lines are discussed and mitigated. Examples of under-exploited cross-border plays and discoveries are discussed.

Carbon cycle perturbation and mercury anomalies in terrestrial Oceanic Anoxic Event 1b from Jiuquan Basin, NW China

The Oceanic Anoxic Event (OAE) 1b is well documented in western Tethys, however, records in Eurasia are still lacking. Here, we carried out high-resolution organic carbon isotope (δ13Corg), total organic carbon (TOC) contents and mercury (Hg) concentrations analysis of the lacustrine sediments from the Xiagou and Zhonggou formations in the Hanxiagou section, Jiuquan Basin, northwestern China. The lacustrine δ13Corg curve presents three stages of negative excursions above the basalt layer dated at 112.4 ± 0.3 Ma in the lowermost Zhonggou Formation. The three negative δ13Corg excursions, well corresponded with the three subevents (Kilian, Paquier, and Leenhardt) of the OAE1b in Poggio le Guaine (central Italy), Vocontian Basin (SE France) and St Rosa Canyon (NE Mexico) sections, supporting the record of the terrestrial OAE 1b in the Jiuquan Basin. Five mercury enrichment (ME) intervals in Hg/TOC ratios were recognized, indicating that the pulsed volcanism from the southern Kerguelen Plateau likely triggered the OAE 1b. However, the decoupling between NIE shifts and mercury enrichments signifying other carbon reservoir (with no link to mercury) probably contributed to the global carbon cycle perturbation during the OAE 1b period. Our results provide direct evidence to link the OAE 1b and terrestrial ecosystem in the Eurasia.

Sedimentary dynamics and topographic controls on the tidal-dominated Zagra Strait, early Tortonian, Betic Cordillera, Spain

The approximately 350 m-thick stratigraphic succession of the Zagra Strait records an important oceanographic phase of basin interconnection between the Atlantic Ocean (Guadalquivir Basin) and the Mediterranean Sea through the Betic Cordillera (southern Spain) during the early Tortonian. The Zagra Strait developed as a narrow structurally-controlled marine corridor. The sedimentary dynamics of the Zagra Strait was interpreted from the sedimentological features observed in six sections at well-exposed outcrops. Large-scale (>10 m high) compound and compound-dune complexes moved parallel to the strait margins under strong tidal currents generated by tidal amplification at the strait entrance and exit. Dune distribution can be divided in three sectors with different palaeocurrent migration, lithological and topographical characteristics. The northern and central sectors were separated by a deep depression (>75 m water depth) where tidal currents were weaker and dunes were not generated. The southern sector records a relative decrease in current strength compared with the northern and central sectors, and a significant increase in the bioclastic content in the sediment. Terrigenous content generally increases towards the strait margins, and reciprocally, carbonates towards its axis. The closure of the Zagra Strait resulted from tectonic uplift of that part of the Betic Cordillera before the late Tortonian.

Detrital zircon geochronology of Late Cretaceous successions in the Ganzhou basin, South China: Evidence of a major tectonic transition

Situated within the southern segment of the South China Block (SCB), the Ganzhou Basin formed due to subduction of the paleo-Pacific plate beneath to the SCB. Late Cretaceous successions in this basin consist of fluvial and lacustrine facies red beds hosting abundant dinosaur and dinosaur egg fossils. This study reports detrital zircon geochronological data from a crystallized tuff and four sandstones found in the Late Cretaceous Ganzhou Group of the Ganzhou Basin. Age distributions included four major age subpopulations of predominantly Triassic, Devonian-Ordovician, Neoproterozoic and Paleoproterozoic ages. These indicate source material derived from Yanshanian and Triassic granitoids as well as from Kwangsian and Jiangnan orogens. Age signatures generally resemble those recorded in the adjacent Nanxiong Basin but also include distinctive features. Provenance signatures from successive units indicate a tectonic transition from intracontinental extension at ∼120 Ma to compression near the Cretaceous/Paleogene boundary. This tectonic transition was probably driven by continent-continent collision between the Indian and Eurasian plates, as well as by a shift in the subduction direction of the paleo-Pacific plate beneath the Eurasian plate.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5776518

Sedimentation associated with glaciovolcanism: a review

Three discrete categories of sedimentary deposits are associated with glaciovolcanism: englacial cavity, jökulhlaup and lahar. Englacial cavity deposits are found in water-filled chambers in the lee of active glaciovolcanoes or at a locus of enhanced geothermal heat flux. The cavities provide a depocentre for the accumulation of debris, either abundant fresh juvenile debris with sparse dropstones (associated with active glaciovolcanism) or polymict basal glacial debris in which dropstones are abundant (associated with geothermal hot spots). Described examples are uncommon. By contrast, volcanogenic jökulhlaup deposits are abundant, mainly in Iceland, where they form extensive sandar sequences associated with ice-covered volcanoes. Jökulhlaups form as a result of the sudden subglacial discharge of stored meltwater. Analogous deposits known as glaciovolcanic sheet-like sequences represent the ultra-proximal lateral equivalents deposited under the ice. Glaciovolcanic lahars are associated with ice-capped volcanoes. They form as a result of explosive eruptions through relatively thin ice or following dome collapse, and they trigger mainly supraglacial rather than subglacial meltwater escape. Sediment transport and depositional processes are similar in jökulhaups and lahars and are dominated by debris flow and hyperconcentrated or supercritical flow modes during the main flood stage, although the proportions of the principal lithofacies are different.

Artesian conditions in the Chilterns Chalk aquifer (North West of the London Basin) and the implications for surface water - groundwater interactions

The Chalk is a principal aquifer which provides an important resource in Southeast England. For two centuries, it allowed the establishment of a thriving watercress-growing industry, indirectly through diverted stream flow and directly, through the drilling of flowing artesian boreholes. The distribution of artesian boreholes across different catchments, suggests a regional control on vertical groundwater flow within the New Pit and Lewes Chalk units. Interrogation of location-specific information points to the confining role of a few key marls within the New Pit Chalk Formation, which can be traced up-catchment to where they naturally outcrop or have been exposed by quarrying. Evidence is found in geophysical logging of a number of boreholes across catchments, confirming a consistent pattern of the spatial distribution of such key markers. When tectonic stress was applied to the various Chalk Formations, the marl bands would have reacted producing more plastic deformation and less fractures in comparison with rigid rock strata. Such scenario would have created the conditions for secondary aquifer units, giving the Chalk confining or semi-confining hydraulic characteristics on a regional scale. This conceptual understanding helps explain the reasons that the river flow response to reductions in groundwater abstraction varies across the flow duration curve.

Coeval development of extensional and contractional features along transform margins: insights from the Diaz Marginal Ridge

The Diaz Marginal Ridge (DMR), on the southern transform margin of South Africa, is a bathymetric feature parallel to the Agulhas Falkland Fracture Zone (AFFZ) that has long been considered an archetype marginal ridge; and yet its origin and evolution remains unconstrained. Using recently acquired seismic data we present a new structural interpretation of the DMR and its association with the evolution of both the AFFZ and the Southern Outeniqua Basin. In contrast to previous scenarios invoking thermo-mechanical explanations for its evolution, we observe a more straightforward structural model in which the genesis of the DMR results from the structural inversion of a Jurassic rift basin. This inversion resulted in the progressive onlap of latest Valanginian-Hauterivian aged stratigraphic units, important for the formation of stratigraphic plays of the recent Brulpadda discovery.Paradoxically, this contraction is contemporaneous with renewed extension observed in the inboard normal faults. The orientation of the DMR and inboard structures have been demonstrated to be controlled by the underlying Cape Fold Belt (CFB) fabric. The onset of motion across the AFFZ shear system led to east-west orientated maximum stress and north-south orientated minimum stress. We propose this stress re-orientation resulted in strain partitioning across existing structures whereby in addition to strike-slip on the AFFZ there was coeval extension and contraction, the nature of which was determined by fault orientation. The fault orientation in turn was controlled by a change in orientation of the underlying CFB. Our model provides new insights into the interplay of changes in regional stress orientation with basement fabric and localised magmatism along an evolving transform. The application of horizontal strain partitioning can provide an explanation of similar features observed on other transform margins.

Concepts and Revised Models for Phanerozoic Orogenic Gold Deposits

Existing published models for orogenic gold deposits (OGDs) do not adequately describe or explain most deposits of Phanerozoic age, and there are numerous reasons why Phanerozoic OGDs might differ significantly from older deposits. We subdivide Phanerozoic OGDs into four main subtypes, based on a number of descriptive criteria, including tectonic setting, lithological siting, and characteristics of the mineralization in each subtype. The four subtypes are: 1) crustal scale fault associated (CSF) subtype, 2) sediment-hosted orogenic gold (SHOG) subtype, 3) forearc (FA) subtype, and 4) syn- and late tectonic dispersed (SLTD) subtype. Lead isotopic studies suggest that Pb and other metals in all but the FA subtype were likely derived from relatively small source reservoirs in the middle or upper crust. OGDs formed in large, lithologically and structurally homogeneous regions will tend to be of the same subtype; however, in geologically complex orogenic belts it is common to find two or more subtypes that formed at approximately the same time. Based on the synthesis of global OGDs of Phanerozoic age districts containing CSF or SHOG subtype deposits appear to have the best potential for hosting multiple large deposits. FA subtype deposits form in a relatively uncommon tectonic setting (accretionary forearc, possibly overlying a subducting spreading ridge) and are likely to be rare. SLTD subtype OGDs are the most common, but most are small and uneconomic, although they commonly generate substantial alluvial gold deposits.

Early post-breakup kinematic adjustments of continental-oceanic transform fault zones; Cape Range, Coromandal and Romanche transform margin case study

A comparison of transform margins that started their evolution as continental transforms shows differences in their tectonic style, which can be attributed to the variable kinematic adjustments they underwent during the post-breakup continental-oceanic stage of their development. Three end-member examples are presented in detail. The Cape Range transform fault zone (Western Australia) retained its strike-slip character during its entire continental-oceanic stage, as documented by the transform-perpendicular system of spreading-related magnetic stripe anomalies. The Coromandal transform fault zone (Eastern India) adjusted its kinematics to a transtensional one during its continental-oceanic stage, as indicated by the transform-oblique system of magnetic stripe anomalies and extensional component of movement indicated by a narrow zone of crustal thinning. The Romanche transform fault zone (Equatorial Africa) adjusted its kinematics to transpressional, as documented by the changing geometries of magnetic stripe anomalies and transpressional folding during its continental-oceanic development stage. Based on the recognition of the aforementioned adjustments, we suggest a new categorization of transforms into (1) those that experience transpressional adjustment, (2) those that experience transtensional adjustment and (3) those that do not experience any adjustment during their continental-oceanic development stage.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5762388

Contrasting transform and passive margin subsidence history and heat flow evolution: insights from 3D thermo-mechanical modelling

Transform and passive margins developed during the continental rifting and opening of oceanic basins are fundamental elements of plate tectonics. It has been suggested that inherited structures, plate divergence velocities and surface processes exert a first order control on the topographic and bathymetric evolution and thermal history of these margins and related sedimentary basins. Their complex spatial-temporal dynamics have remained controversial. Here, we conducted 3D magmatic-thermo-mechanical numerical experiments coupled with surface processes modelling to simulate the dynamics of continental rifting, continental transform fault zone formation and persistent oceanic transform faulting and zero-offset oceanic fracture zones development. Numerical modelling results allow to explain the first order observations from passive and transform margins, such as diachronous rifting, heat flow rise and cooling in individual depocenters and contrasting basin tectonics of extensional and transtensional origin. In addition, the models reproduce the rise of both marginal ridges and transform marginal plateaus and their interaction with erosion and sedimentation. Comparison of model results with observations from natural examples yield new insights into the tectono-sedimentary and thermal evolution of several key passive and transform continental margins worldwide.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5756555