Ole J. Martinsen

Strike variability of clastic depositional systems: Does it matter for sequence-stratigraphic analysis?

Current sequence-stratigraphic models are two-dimensional in nature and predict variability in depositional-dip sections. Strike and dip variability of sediment supply to clastic depositional systems causes significant changes to sequence architecture so that contrasting yet coeval stacking patterns result. These changes ultimately affect prediction potential based on fixed systems tract stacking patterns and relation to relative sea-level curves. Identification of key surfaces in sequences rather than stacking patterns is the most reliable indicator of relative sea-level history.

Cretaceous and Palaeogene turbidite systems in the North Sea and Norwegian Sea Basins: source, staging area and basin physiography controls on reservoir development

The Cretaceous and Palaeogene succession in the North Sea and Norwegian Sea basins show widely variable deep-water sedimentary systems in terms of processes, facies, geometries, scale and distribution. The primary controls on the large-scale variability are considered to be source area size, basin and basin margin physiography and bathymetry, tectonic history and resulting morphology of drainage and delivery systems of sediments to deep-water areas, and the rate of sediment delivery. The North Sea and Norwegian Sea basins were comparable during the earliest Cretaceous, but thereafter developed in widely different ways as a response to proximity to oncoming North Atlantic seafloor spreading. In the North Sea Basin, the Cretaceous and Palaeogene turbidite systems were controlled by an inherited structural template from Late Jurassic rifting, and by source area size. Poorly developed or small drainage systems on the Norwegian margin and the broad Horda Platform gave little sand supply from the east to the Viking Graben area. Sand-rich systems were sourced from a relatively large hinterland and shallow marine staging area on the East Shetland Platform. North of the Horda Platform, sand supply was abundant in very discrete periods, particularly in the Early Eocene. In the Norwegian Sea basins, the Late Jurassic structural template controlled Early Cretaceous deep-water sedimentary systems in a manner similar to the North Sea Basin. Generally small and poorly developed drainage systems caused development of mud-rich systems. In contrast, in the Late Cretaceous, onset of precursor tectonic activity to sea-floor spreading led to increased sand supply from the west into the Voring Basin. A relatively narrow palaeoshelf and a large source area contributed to forming sand-rich systems. Smaller turbidite systems developed along the Norwegian margin, which were sourced from the east from smaller drainage areas, and partially across broad shelves, such as the Trondelag Platform. Both in the Cretaceous and Palaeogene, the sandiest systems are found only to the south and the north of the inherited structural features.

Reconstructing morphological and depositional characteristics in subsurface sedimentary systems: An example from the Maastrichtian–Danian Ormen Lange system, More Basin, Norwegian Sea

Understanding large-scale sediment distribution patterns and morphological characteristics in subsurface sedimentary systems is highly challenging and generally requires regional seismic and well coverage. Here, we test a method that aims to predict first-order morphological characteristics and type of sedimentary transport system in ancient source-to-sink systems based on trends observed in submodern (Pliocene–Holocene) depositional environments. An example from the Paleocene Ormen Lange system (More Basin, Norwegian Sea) demonstrates the application of the method, and several descriptive parameters are estimated for this ancient subsurface system. In the Ormen Lange system, basin-floor fan and distal-slope parameters are well constrained from seismic and well control. However, knowledge of the morphology and relationships between upper slope, shelf, and catchment characteristics and their relationships to deep-water systems is poor, and these are the parameters that are discussed in this study. Estimated parameters of catchment size derived from this technique are in good agreement with preserved remnants of fluvial valleys located onshore. Predicted sediment transport characteristics are also comparable to the depositional mechanisms interpreted from cores and well logs, suggesting a small tectonically active system with high fluvial discharge and low sediment storage potential in the catchment and shelf subenvironments. The discussed method is thus capable of predicting first-order segment characteristics in subsurface sedimentary systems with an uncertainty of one to two orders of magnitude. This information can be used to increase the understanding of unexplored basins or to add data and uncertainty ranges to well-known petroleum systems.

Submarine channel response to intrabasinal tectonics: The influence of lateral tilt

Lateral tilting is a common deformation style in extensional basins; its influence on subaerial channels is, to a degree, understood and may be significant, controlling the style of channel development and the resultant sand-body architecture. Growth faulting and lateral tilting in turbidite channel systems have been demonstrated from three-dimensional seismic data, but the resultant architecture of channels within these settings has not yet been documented. In the Carboniferous of northern England, a sand-rich slope channel, developed within a basin undergoing late-stage extension, underwent progressive and unidirectional migration toward a topographic low on a laterally tilting block. The resultant sandstone body is wedge shaped in cross section and composed dominantly of sigmoidal lateral accretion deposits. The channel returned to an axial course before undergoing lateral migration in the same direction, creating a multistory, multilateral channel sandstone body. The repeated unidirectional migration combined with evidence of syndepositional deformation suggests that active tectonism strongly influenced channel evolution and deposition. A model of submarine channel evolution in extensional basins is presented; in systems where large displacements occur, the channel system may avulse, creating isolated sand ribbons, which are connected updip; where the lateral dip is always more influential than the regional dip, the system may pond in the hanging-wall syncline. The model is compared to a subsurface channel within the Pliocene of the Nile Delta slope, which was influenced by syndepositional fault movement; application of the outcrop-derived model allows some simple architectural interpretations to be made.

Paleogene tuffaceous intervals, Grane Field (Block 25⧸11), Norwegian North Sea: their depositional, petrographical, geochemical character and regional implications

Abstract Paleogene volcaniclastic rocks from four wells from the Grane Field (Block 25⧸11), Southern Viking Graben on the western flank of the Utsira High (North Sea), have been studied. The tuffaceous intervals consist of the volcanic phase 1 (58–57 Ma) in the Vale and Lista Formations, and the volcanic phase 2 (55–52 Ma) in the Sele and Balder Formations. Tuff beds of phase 1 are interpreted as having been redeposited by turbidity currents, whereas those of phase 2 reflect normal settling of fall-out pyroclastics in water. Two types of fragments occur, the most abundant being vesicular pyroclasts with irregular shape and dark, microlithic, partly vesicular granules, representing shallow submarine, Surtseyan-type eruptions. The other type of pyroclastic fragments are vitric shards, which originate by quenching granulation during subaqueous eruptions. The tuffaceous intervals have been subdivided into (1) identified pyroclastics based on textural interpretations, and (2) assumed pyroclastics based on geochemical interpretations. The majority of the samples are sub-alkaline basalts and basaltic andesites. The phase 1 tuffs evolved from basalts to rhyolites upwards in the Lista Formation, and phase 2 tuffs consist of sub-alkaline basalts and basaltic andesites of within-plate origin, similar to the contemporaneous Lower Basalts in East Greenland, the Rockall Trough and the Middle Series on Faroes, all linked to the opening of the North Atlantic Ocean.

Source-to-sink systems on passive margins: theory and practice with an example from the Norwegian continental margin

Abstract Source-to-sink system analysis involves a complete, earth systems model approach from the ultimate onshore drainage point to the toe of related active deepwater sedimentary systems. Several methods and techniques have evolved in recent years, from experimental and numerical modelling through analysis of modern and recent systems, to analysis of ancient systems. A novel method has been developed, bridging between the previous approaches and dividing and analysing source-to-sink systems based on linked geomorphic segments along the source-to-sink profile. This approach builds on uniformitarian principles. The method is driven by the need to understand ancient, subsurface systems and still has high uncertainty but is an original, first-order approach to source-to-sink system analysis. In modern systems, entire onshore-to-offshore systems can be analysed with a higher degree of confidence than in ancient systems and semi-quantitative relationships can be established. Application in ancient systems is much more challenging but, in some cases, antecedent morphologies have been preserved onshore that can be matched with offshore known occurrences of, for instance, sandy submarine fan systems. Along the Norwegian North Sea and Norwegian Sea margins the Paleocene deep-marine reservoir of the giant Ormen Lange gas field is such an example. There, antecedent onshore drainage patterns which formed the feeder system to the offshore, deepwater fan system can be interpreted and aligned with onshore palaeogeomorphological evidence. Understanding the palaeogeomorphic development of basement regions such as the Fennoscandian shield is of high importance for understanding the offshore presence of deepwater sandstones.

Linking offshore stratigraphy to onshore paleotopography: The Late Jurassic–Paleocene evolution of the south Norwegian margin

The temporal link between onshore topography and offshore stratigraphy is of key importance for understanding the long-term development of continental margins. The Mesozoic and Cenozoic landscape topography of southern Norway has long been a matter of debate due to the absence of concrete data reflecting ancient onshore relief. We here present quantitative estimates of paleotopography in southern Norway during three key time intervals (Late Jurassic, Late Cretaceous, and Paleocene) based on the observed volume of sediment stored in point-sourced depocenters along the margin. The aim is to determine the syndepositional paleolandscapes that are in best agreement with the observed distribution, geometry, lithology, and volume of different offshore depositional units. Probability ranges of estimated paleorelief are based on Monte Carlo simulations of a relief prediction model. The results suggest that the late Phanerozoic sedimentary successions along the margin best reflect: (1) a Late Jurassic landscape with a maximum relief of ∼1.6 km; (2) a more subdued Late Cretaceous topography of <0.5 km in the south and up to ∼0.5 km in the north; and (3) a Paleocene topography of up to ∼1.1 km in the north and ∼0.5 km in the south. The temporal and spatial variability in offshore deposition and the linked onshore topography strongly suggest that relief must have been rejuvenated several times during the late Phanerozoic in response to tectonic activity along major fault systems. This quantitative assessment not only sheds new light on the topographic evolution of the south Norwegian margin, it also provides a potential tool with which to investigate the topographic evolution of continental margins and the close coupling between onshore relief and offshore stratigraphy in nearby sedimentary basins.

The maastrichtian and danian depositional setting, along the eastern margin of the Møre Basin (mid-norwegian shelf): implications for reservoir development of the Ormen Lange Field

The reservoir interval of the Ormen Lange Field (More Basin) consists of two units, a lower heterolithic unit mainly of Maastrichtian age (Jorsalfare Formation) and an upper sandstone unit of Danian age (Egga member, Vale Formation). The Cretaceous-Tertiary boundary is located in the uppermost part of the unit. The Egga member has long been known as a potentially prolific reservoir interval in the Slorebotn Subbasin area, where it is up to 150 m thick, consisting of thick, amalgamated medium- to coarse-grained sandstone turbidites. The base of the Egga member in the Slorebotn Subbasin and in other areas close to the eastern margin of the More Basin is a significant unconformity, with the Danian succession usually overlaying strata of Campanian age. This unconformity developed during Maastrichtian times with shelf erosion and sediment bypass. The bypassed coarse clastic sediments were redeposited as turbidites further out in the More Basin, and represent the lower, heterolithic reservoir unit in the Ormen Lange Field. In early Danian time uplift and rotation of the Fennoscandian provenance area in the east led to extensive erosion and redistribution of sandy sediments that prograded westwards into the More Basin and gave rise to amalgamated turbidites of the Egga member. These turbidites probably filled a shallow intraslope basin or depression in the Sloreboth Subbasin and laterally bypassed into the deeper part of the More Basin, where relatively thick turbidites accumulated to form the upper reservoir interval of the Ormen Lange Field. During latest Cretaceous and Palaeogene times the location of the Ormen Lange Field was an area of relatively rapid subsidence, probably with the development of a temporary basin floor depression where deep water sediments were deposited. Inversion and development of the Ormen Lange structure took place in latest Eocene-Early Oligocene time. A considerable change in mineralogy and ichnofabric occurs at the Cretaceous-Tertiary boundary, indicating significant changes in the depositional environment.

Sequence stratigraphy, three dimensions and philosophy

Three-dimensional control and a solid data base are essential factors for making a reliable sequence stratigraphic interpretation of a data set. Three theoretical case studies of varying spatial and temporal scale are provided to illustrate the importance of three-dimensional control. In all the cases, contradictory sequence stratigraphic patterns are created relating either to local sedimentary controls or to large-scale tectonic mechanisms. It is a demanding task for the explorationist to make three-dimensional interpretations, since one-or two-dimensional data sets are most common. However, three-dimensional sequence stratigraphie effects must be considered to improve model-building and the quality of interpretations also in exploratory studies. On a philosophical level, data are interpreted based on perception and experience. There seems to be a paradox in that although geoscientists continuously interpret data based on perceiving what is “behind” (i.e. the interpretations) a data set (i.e. the observations), presently available sequence stratigraphie models are two-dimensional with few assumptions about three-dimensional effects. There is a need to extend the models based on cases considering three-dimensional variability.

Deep-water clastic systems in the upper carboniferous (upper mississippian-lower pennsylvanian) shannon basin, western Ireland

The Upper Carboniferous Shannon Basin of western Ireland contains a more-than-2300-m-thick (7540 ft) basin-fill succession, shallowing upward from deep-water to deltaic and incised fluvial deposits. The deep-water basin floor and slope succession is world renowned as an analog for hydrocarbon-bearing deep-water sandstones on several continental margins such as the basins in East and West Africa, South America, the Gulf of Mexico, and not least offshore northwest Europe. The Shannon Basin is frequently visited by both academia and industry for research and training purposes. A series of behind-outcrop research boreholes reveals the subsurface expression of the deep-water rocks and is complemented by seismic-scale cliff exposures. The succession is interpreted as a first-order basin-scale linked sedimentary system. This system can be analyzed using the principles of source-to-sink analysis and leaves the visitor with a complete picture of the basin fill, enhanced by spectacular sedimentological and stratigraphic detail.