Egersund Basin Research Articles

Paleozoic-Mesozoic tectono-sedimentary evolution and magmatism of the Egersund Basin area, Norwegian central North Sea

This study focuses on the tectonic evolution of the greater Egersund Basin in the Norwegian central North Sea, with special emphasis on Late Paleozoic extensional tectonics following Caledonian collapse and the Variscan Orogeny, and the impact of the basement structural grain on this evolution. The Caledonian collapse likely resulted in development/rejuvenation of the deepest E-W trending structures/depocenters by Late Devonian time. Thus, a late Devonian-?early Carboniferous age can be assigned to the initial extension, which was associated with the development of an E-W striking basin system, to be overprinted by N–S extensional structures of similar age. A phase of regional magmatism at the Carboniferous-Permian transition (≈300 Ma) may be associated with a large igneous province centred on the Skagerrak area. Faulting during late Carboniferous-early Permian was minor within the study area as reflected by uniform sedimentary thicknesses of the uppermost Carboniferous and lower Permian sequences. Major normal faults, mainly trending N–S, were active during a late Permian-Early Triassic rift phase affecting large parts of the central and northern North Sea area. A later phase of extension was initiated in late Middle Jurassic time and the Egersund Basin proper formed during the Late Jurassic-Early Cretaceous. The depocenters that developed during this phase was influenced by the deep Late Paleozoic (sub-salt) structural grain, including strike-slip movements along the Sorgenfrei-Tornquist Zone. Later events include mild inversion along the northern flank of the Egersund Basin, possibly as a Late Cretaceous response to far-field Alpine compression, and Cenozoic regional tilting.

Reservoir assessment of Middle Jurassic sandstone-dominated formations in the Egersund Basin and Ling Depression, eastern Central North Sea

Abstract Reservoir quality assessment was conducted from petrophysical analysis and rock physics diagnostics on 15 wells penetrating Middle Jurassic sandstone reservoir formations in different regions of the eastern Central North Sea. Seismic interpretation on available 3D and 2D seismic reflection data was utilized to map thickness variations and to draw broad correlations to structural features such as salt structures and faults. In the central Egersund Basin, the Sandnes Formation shows good reservoir properties (gross thickness = 107–147 m, N/G = 33–53%) while the Bryne Formation exhibits poorer reservoir quality (N/G

Burial and exhumation history controls on shale compaction and thermal maturity along the Norwegian North Sea basin margin areas

Abstract The North Sea area has been subjected to significant erosion and subsequent deposition of sediments in the basin margin and deeper basin areas, respectively, during the late Neogene. A large amount of Cretaceous-early Quaternary sediments have been removed below the angular unconformity along the west and southwest coast of Norway and deposited in the huge North Sea Fan at the mouth of the Norwegian Channel. At the same time, a considerable thickness of early Quaternary-Paleocene sediments was also eroded towards the east in the central North Sea and subsequently deposited in the deeper basin areas to the west. This study seeks to estimate exhumation from compaction and thermal maturity based techniques by using sonic velocities of shales/carbonates and vitrinite reflectance data in a large number of boreholes in the central, eastern and northern North Sea. The results indicate no or minor exhumation in the Central Graben and flanking high areas, whereas more than ∼1 km sediments are eroded in the basin margin areas towards the Norwegian coast. More than ∼500 m sediments are eroded in the Egersund Basin and Stord Basin areas. A similarity of exhumation estimates from the Early Cretaceous-Early Miocene shales and Late Cretaceous-Early Paleocene carbonates indicates maximum burial sometime after the Early Miocene in most of the central and northern North Sea areas. However, the maximum burial throughout the North Sea Basin may be diachronous. Seismostratigraphic analysis indicates maximum burial sometime during the Oligocene in the Sorgenfrei-Tornquist Zone area in the eastern North Sea. Maximum burial in the Norwegian-Danish Basin varies from Miocene-Pliocene in eastern parts to early Pleistocene in western parts, whereas sediments are currently at their maximum burial in the Central Graben and southern Viking Graben areas. Restoration of surface elevations to their original position before the onset of erosion indicated large subaerially exposed areas in the Norwegian-Danish Basin and along the southwest coast of Norway. This is also supported by predominantly coastal and/or deltaic environments in the Norwegian-Danish Basin area during the late Neogene. These subaerially exposed areas may be linked to the regional tilting and erosion of the basin margin areas to the east and progressive basinward migration of deposition centres to the west since the Oligocene. The exhumation had significant effects on the petroleum system in the basin margin areas by cooling down the source rock. However, the deeper burial of sediments may also have changed the rheological properties of sediments from more ductile to brittle due to compaction and diagenetic processes which makes them more failure prone during exhumation leading to hydrocarbon leakage or seal failure in case of CO2 injection.

Supra-salt normal fault growth during the rise and fall of a diapir: Perspectives from 3D seismic reflection data, Norwegian North Sea

Normal faulting and the deep subsurface flow of salt are key processes controlling the structural development of many salt-bearing sedimentary basins. However, our detailed understanding of the spatial and temporal relationship between normal faulting and salt movement is poor due to a lack of natural examples constraining their geometric and kinematic relationship in three-dimensions. To improve our understanding of these processes, we here use 3D seismic reflection and borehole data from the Egersund Basin, offshore Norway, to determine the structure and growth of a normal fault array formed during the birth, growth and decay of an array of salt structures. We show that the fault array and salt structures developed in response to: (i) Late Triassic-to-Middle Jurassic extension, which involved thick-skinned, sub-salt and thin-skinned supra-salt faulting with the latter driving reactive diapirism; (ii) Early Cretaceous extensional collapse of the walls; and (iii) Jurassic-to-Neogene, active and passive diapirism, which was at least partly coeval with and occurred along-strike from areas of reactive diapirism and wall collapse. Our study supports physical model predictions, showcasing a three-dimensional example of how protracted, multiphase salt diapirism can influence the structure and growth of normal fault arrays.

Effect of diagenesis on pore pressures in fine-grained rocks in the Egersund Basin, Central North Sea

Effect of diagenesis on pore pressures in fine-grained rocks in the Egersund Basin, Central North Sea

Compaction processes and rock properties in uplifted clay dominated units– the Egersund Basin, Norwegian North Sea

A thick succession of fine-grained sediments in any sedimentary basin can serve as source (if organic-rich) and top seals. Both sealing and petroleum generation capacity of any fine-grained succession is dependent on compaction history (mechanical and chemical compactions) of the sediments. In the Egersund Basin, a thick succession of fine-grained sediments of Upper Jurassic-Lower Cretaceous age overly Middle Jurassic reservoir sands. The Egersund Basin, however, has experienced a complex depositional and tectonic history, particularly, late exhumation (Neogene) which complicates the overall burial, thermal and pressure history.In this study, petrophysical and acoustic properties (bulk density and P-wave velocity) of 10 exploration wells from two neighboring blocks in the Egersund Basin were utilized to investigate compaction processes in the area. In addition, X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses from selected wells and available total organic carbon (TOC) data were used as guide to interpret the compaction processes and resulting rock properties.The results show that the transition from mechanical to chemical compaction is not sharp. The onset of chemical compaction imposes a measurable deviation in the burial depth trends of the petrophysical properties expected from purely mechanical compaction. The continuation of the depth related trends however, indicates that chemical compaction may occur in concurrent with continuing mechanical compaction. The observed compaction varies between the different clay dominated stratigraphic units; indicating that the major control on both mechanical and chemical compaction is the primary sediment composition.

Structural controls on the stratigraphic architecture of net‐transgressive shallow‐marine strata in a salt‐influenced rift basin: Middle‐to‐Upper Jurassic Egersund Basin, Norwegian North Sea

AbstractIn this study, we integrate 3D seismic reflection, wireline log, biostratigraphic and core data from the Egersund Basin, Norwegian North Sea to determine the impact of syn‐depositional salt movement and associated growth faulting on the sedimentology and stratigraphic architecture of the Middle‐to‐Upper Jurassic, net‐transgressive, syn‐rift succession. Borehole data indicate that Middle‐to‐Upper Jurassic strata consist of low‐energy, wave‐dominated offshore and shoreface deposits and coal‐bearing coastal‐plain deposits. These deposits are arranged in four parasequences that are aggradationally to retrogradationally stacked to form a net‐transgressive succession that is up to 150‐m thick, at least 20 km in depositional strike (SW‐NE) extent, and >70 km in depositional dip (NW‐SE) extent. In this rift‐margin location, changes in thickness but not facies are noted across active salt structures. Abrupt facies changes, from shoreface sandstones to offshore mudstones, only occur across large displacement, basement‐involved normal faults. Comparisons to other tectonically active salt‐influenced basins suggest that facies changes across syn‐depositional salt structures are observed only where expansion indices are >2. Subsidence between salt walls resulted in local preservation of coastal‐plain deposits that cap shoreface parasequences, which were locally removed by transgressive erosion in adjacent areas of lower subsidence. The depositional dip that characterizes the Egersund Basin is unusual and likely resulted from its marginal location within the evolving North Sea rift and an extra‐basinal sediment supply from the Norwegian mainland.

Physiography of the NE margin of the Permian Salt Basin: new insights from 3D seismic reflection data

Three-dimensional seismic reflection data from the Egersund Basin, offshore Norway image geomorphological features that record mid-Permian footwall degradation of basin-bounding fault systems. This ancient landscape was subsequently flooded during the pan-European, Late Permian transgression of the North Permian Salt Basin and was fossilized beneath Zechstein Supergroup evaporites. We provide the first conclusive evidence for pre-Zechstein normal faulting in the Egersund Basin, indicating that extensional strain was shared between Permian and Late Jurassic–Early Cretaceous rift events. Supplementary materials: Uninterpreted versions of the seismic sections shown in Figure 2 are available are available at www.geolsoc.org.uk/SUP18678 .

Growth of normal faults in multilayer sequences: A 3D seismic case study from the Egersund Basin, Norwegian North Sea

We investigate the structural style and evolution of a salt-influenced, extensional fault array in the Egersund Basin (Norwegian North Sea) through analysis of 3D reflection seismic and well data. Analysis of fault geometry/morphology, throw distribution and syn-kinematic strata reveal an intricate but systematic style of displacement and growth, suggesting an evolution of (1) initial syn-sedimentary fault growth contemporaneous with salt mobilization initiated during the Late Triassic, (2) cessation of fault activity and burial of the stagnant fault tips, and (3) subsequent nucleation of new faults in the cover above contemporaneous salt re-mobilization initiated during the Late Cretaceous, with downward propagation and linkage with faults. Stage 3 was apparently largely controlled by salt mobilization in response to basin inversion, as reactivated faults are located where the underlying salt is thick, while the non-reactivated faults are found where salt is depleted. Based on the 3D-throw analyses, we conclude that a combination of basement faulting and salt (re-) mobilization is the driving mechanisms behind fault activation and reactivation. Even though the sub- and supra-salt faults are mainly geometrically decoupled through the salt, a kinematic coupling must have existed as sub-salt faults still affected nucleation and localization of the cover faults.

Salt-influenced normal fault growth and forced folding: The Stavanger Fault System, North Sea

Displacement ratio (Dr) is the ratio between salt thickness (Tv) and sub-salt normal fault displacement (D) (Dr = Tv/D), and it is typically used to predict the degree of geometric and kinematic linkage between sub- and supra-salt fault populations, and the overall structural style in salt-influenced extensional settings. However, we currently lack natural examples of how Dr and the underlying geological controls vary, and how these may control the three-dimensional geometry and evolution of salt-influenced normal fault systems. Furthermore, it is currently unknown if kinematic coherence in salt-influenced extensional settings can be maintained over relatively long length-scales (101–103 m) and for relatively long timeframes, and how this may impact the growth and geometry of large-throw (>500 m), salt-influenced normal fault systems. In this paper we use a 3600 km2, high-quality 3D seismic reflection dataset and borehole data from the Stavanger Fault System (SFS), Egersund Basin, eastern North Sea Basin to investigate; (i) how pre-rift salt thickness (Tv) and sub-salt fault throw (T) control the structural style and evolution of a basin-bounding, salt-influenced normal fault system; and (ii) the role salt plays in maintaining kinematic coherence in normal fault systems. We demonstrate that; (i) pre-rift salt distribution (Tv), specifically its presence in the proto-footwall (i.e., when Tv > 0), is the primary control on partitioning of faulting and (forced) folding along the fault system, and the style of linkage (i.e., hard- or soft-linkage) between sub- and supra-salt fault populations; and (ii) sub- and supra-salt fault populations represent brittle elements of a single, geometrically and kinematically coherent structure, the geometry and evolution of which is related to the ductile translation of strain on a scale (up to 8 km) and duration (c. 65 Myr) that believe is significantly greater and longer than previously documented.

Structural style and early stage growth of inversion structures: 3D seismic insights from the Egersund Basin, offshore Norway

High-quality three-dimensional (3D) seismic reflection and borehole data from the Egersund Basin, offshore Norway are used to characterise the structural style and determine the timing of growth of inversion-related anticlines adjacent to a segmented normal fault system. Two thick-skinned normal faults, which offset Permian clastics and evaporites, delineate the north-eastern margin of the basin. These faults strike NNW-SSE, have up to 1900 m of displacement and are separated by an ESE-dipping, c. 10 km wide relay ramp. Both of these faults display exclusively normal separation at all structural levels and tip out upwards into the upper part of the Lower Cretaceous succession. At relatively shallow structural levels in the hangingwalls of these faults, a series of open, low-amplitude, fault-parallel anticlines are developed. These anticlines, which are asymmetric and verge towards the footwalls of the adjacent faults, are interpreted to have formed in response to mild inversion of the Egersund Basin. The amplitude of and apparent shortening associated with the anticlines vary along strike, and these variations mimic the along-strike variations in throw observed on the adjacent fault segments. We suggest that this relationship can be explained by along-strike changes in the propensity of the normal faults to reactivate during shortening; wider damage zones and lower angles of internal friction, coupled with higher pore fluids pressures at the fault centre, mean that reactivation is easier at this location than at the fault tips or in the undeformed country rock. Seismic-stratigraphic analysis of growth strata indicate that the folds initiated in the latest Turonian-to-earliest Coniacian (c. 88.6 Ma) and Santonian (c. 82.6 Ma); the control on this c. 6 Myr diachroneity in the initiation of fold growth is not clear, but it may be related to strain partitioning during the early stages of shortening. Anticline growth ceased in the Maastrichtian and the inversion event is therefore interpreted to have lasted at least c. 20 Myr. This study indicates that 3D seismic reflection data is a key tool to investigate the role that normal fault segmentation can play in controlling the structural style and timing of inversion in sedimentary basins. Furthermore, our results highlight the impact that this structural style variability may have on the development of structural and stratigraphic hydrocarbon traps in weakly-inverted rifts.

Origin of an anhydrite sheath encircling a salt diapir and implications for the seismic imaging of steep-sided salt structures, Egersund Basin, Northern North Sea

The ingress of meteoric fluids into the crests of salt structures typically results in the formation of anhydrite-dominated caprock. The migration of connate fluids up the margins of salt structures has also been documented, although the products related to this are rarely sampled in the deep subsurface. We use 3D seismic reflection and borehole data to document the geometry and lithology of a salt diapir. Seismic data suggest that a weld is developed along the diapir stem, although borehole data indicate that the stem consists of an inner, c . 1500 m thick, halite-dominated zone, and an outer, c . 250 m thick, anhydrite-dominated ‘sheath’. The anhydrite may have formed early in the basin history, as a depositional anhydrite or crestal caprock. An alternative interpretation is that the anhydrite represents ‘lateral caprock’, which formed late in the basin history in response to the migration of NaCl-poor fluid up the margins of the diapir and dissolution of halite. The possibility that lateral caprock may form adjacent to salt structures has implications for understanding the patterns and vigour of groundwater flow in sedimentary basins. Furthermore, our study shows the margins of steep-sided salt structures may be misidentified by several hundred metres if time-migrated seismic reflection data are used.

Mesozoic rift magmatism in the North Sea region: 40Ar/39Ar geochronology of Scanian basalts and geochemical constraints

More than 100 volcanic necks composed of basanites and melanephelinites occur in Scania, southern Sweden, at the junction of two major tectonic lineaments, the Phanerozoic Sorgenfrei-Tornquist Zone (STZ) and the Proterozoic Protogine Zone. New 40Ar/39Ar isotope analyses of whole rock fragments of nine selected basalt necks suggest that the Mesozoic alkaline volcanism in the Scanian province commenced earlier than previously reported and comprised three separate volcanic episodes that span a total period of ca. 80 Myr: a first Jurassic (191–178 Ma), a second at the Jurassic/Cretaceous boundary (ca. 145 Ma), and a final middle Cretaceous episode (ca. 110 Ma). The new results allow for precise time correlations between eruption events in the Scanian and those in the North Sea volcanic provinces. The older, early Jurassic event in Scania is largely synchronous with that in the Egersund Basin and the Forties field whereas the event at ca. 145 Ma is correlated with activity in the Central Graben. These volcanic episodes also correlate in age with Kimmerian tectonic activity. Volcanic activity in the middle Cretaceous period has also been dated in the triple junction in the North Sea and offshore in the Netherland Sector. The correlation of basalt volcanism in Scania with the Egersund nephelinites strongly suggest that volcanism was triggered by repeated tectonic activity along the STZ. Geochemical data of alkaline mafic rocks in the Scanian and the North Sea volcanic provinces imply that different provinces have largely unique geochemical signatures in favour of a heterogeneous mantle in the North Sea volcanic region. However, basalts of different generations in one and the same province cannot be readily separated on the basis of geochemistry, suggesting that the same lithospheric mantle was the source of repeated volcanism over time in each province. The data suggest a low degree of melting of a volatile-bearing mantle lherzolite enriched in incompatible elements with the exception of the Forties basalts in the rift centre, produced by larger degree of melting and evolved by fractional crystallization.

Mesozoic mafic alkaline magmatism of southern Scandinavia

More than 100 volcanic necks in central Scania (southern Sweden) are the product of Jurassic continental rift-related mafic alkaline magmatism at the southwest margin of the Baltic Shield. They are mainly basanites, with rarer melanephelinites. Both rock groups display overlapping primitive Mg-numbers, Cr and Ni contents, steep chondrite-normalized rare earth element patterns (LaN /YbN = 17–27) and an overall enrichment in incompatible elements. However, the melanephelinites are more alkaline and have stronger high field strength element enrichment than the basanites. The existence of distinct primary magmas is also indicated by heterogeneity in highly incompatible element ratios (e.g. Zr/Nb, La/Nb). Trace element modelling indicates that the magmas were generated by comparably low degrees of melting of a heterogeneous mantle source. Such a source can best be explained by a metasomatic overprint of the mantle lithosphere by percolating evolved melts. The former existence of such alkaline trace element-enriched melts can be demonstrated by inversion of the trace element content of green-core clinopyroxenes and anorthoclase which occur as xenocrysts in the melanephelinites and are interpreted as being derived from crystallization of evolved mantle melts. Jurassic magmatic activity in Scania was coeval with the generation of nephelinites in the nearby Egersund Basin (Norwegian North Sea). Both Scanian and North Sea alkaline magmas share similar trace element characteristics. Mantle enrichment processes at the southwest margin of the Baltic Shield and the North Sea Basin generated trace element signatures similar to those of ocean island basalts (e.g. low Zr/Nb and La/Nb) but there are no indications of plume activity during the Mesozoic in this area. On the contrary, the short duration of rifting, absence of extensive lithospheric thinning, and low magma volumes argue against a Mesozoic mantle plume. It seems likely that the metasomatic imprint resulted from the earlier Permo-Carboniferous rifting episode which affected the entire study area and clearly was accompanied by plume activity (Ernst and Buchan in American Geophysical Union, pp 297–337, 1997). Renewed rifting in Jurassic times triggered decompression melting in the volatile-enriched lithospheric mantle and the alkaline melts generated inherited the earlier “stored” plume signature.

I-S precipitation in pore space as the cause of geopressuring in Mesozoic mudstones, Egersund Basin, Norwegian continental shelf

The role of clay diagenesis in pressure-ramp (overpressure) development is evaluated from Mesozoic mudstone well cuttings in the Egersund Basin, Norwegian Continental Shelf. Major changes in the mineral assemblage over the range of increasing geopressure include -50% decrease in detrital kaolinite in bulk material, and increases in the proportions of illite, I-S, and percent of nonexpandable illite-like layers in I-S. SEM and TEM observations confirm that pore space is defined primarily by intersecting clay-mineral packets at the scale of tens or hundreds of nanometers (0.01-0.1 μm). TEM data document the reactions of kaolinite and detrital mica to form I-S. The principal reaction that occurred over the geopressure ramp was dissolution of kaolinite and precipitation of neoformed I-S in pore space of these mudstones. These changes also correspond to changes in <0.1 μm particle populations, as determined by Pt/C shadowed TEM measurements, as well as high-resolution TEM lattice-fringe observations. The neoformed I-S has dimensions similar to those of the pore space in the mudstones. The results support a mineralogical model of precipitation of diagenetic clay, resulting in severe permeability reduction, which can be related to the observed pressure ramp. These observations and results provide an integrated petrologic framework for modeling pressure-ramp development, which had been difficult to achieve using conventional basin models of porosity and permeability evolution based on mechanical compaction.

Cretaceous subsidence and inversion along the Tornquist Zone from Kattegat to the Egersund Basin

The Tornquist Zone from the Baltic Sea to the Egersund Basin (Fig. 1) has been studied by several workers, who have made use of field information, well data, and deep reflection and refraction seismic data as well as commercial seismic reflection data (Baartman and Christensen 1975; Pegrum 1984; Symposium on Tornquist Zone geology 1984; Liboriussen et al. 1987; Ziegier 1987; Aubert 1988; Eugeno-S Working Group 1988; Ro et al. 1990; Michelsen and Nielsen 1991; Jensen and Schmidt 1992; Mogensen and Korstgard 1992; Mogensen 1992).

On the accuracy of modelling hydrocarbon generation and migration: The Egersund Basin oil find, Norway

Quantitative estimates of kerogen transformation and hydrocarbon charge to the γ-structure of Block 9 2 , Norwegian North Sea, were obtained by integrated basin modelling. A probability distribution was assigned to each input parameter, and the sensitivity to these parameters was investigated by subsequently varying each parameter within the assigned ranges. The inaccuracy in the modelling results were mainly due to imprecise knowledge of hydrocarbon generation kinetics, depth conversion and present day virgin rock temperature (kerogen transformation modelling), and source rock richness/ thickness/areal extent (volumetric charge calculations). Monte Carlo/latin hypercube sampling based on the probability distribution of each parameter was performed, giving a 40% probability for a dry structure, a mean value of 10.7·10 6Sm 3 recoverable oil, and a considerable high side potential (4% probability for more than 50·10 6Sm 3.

Stetiges wachstum

Normal faulting and the deep subsurface flow of salt are key processes controlling the structural development of many salt-bearing sedimentary basins. However, our detailed understanding of the spatial and temporal relationship between normal faulting and salt movement is poor due to a lack of natural examples constraining their geometric and kinematic relationship in three-dimensions. To improve our understanding of these processes, we here use 3D seismic reflection and borehole data from the Egersund Basin, offshore Norway, to determine the structure and growth of a normal fault array formed during the birth, growth and decay of an array of salt structures. We show that the fault array and salt structures developed in response to: (i) Late Triassic-to-Middle Jurassic extension, which involved thick-skinned, sub-salt and thin-skinned supra-salt faulting with the latter driving reactive diapirism; (ii) Early Cretaceous extensional collapse of the walls; and (iii) Jurassic-to-Neogene, active and passive diapirism, which was at least partly coeval with and occurred along-strike from areas of reactive diapirism and wall collapse. Our study supports physical model predictions, showcasing a three-dimensional example of how protracted, multiphase salt diapirism can influence the structure and growth of normal fault arrays.