Erik S. Rasmussen

Aspects of the structural evolution of the Lusitanian Basin in Portugal and the shelf and slope area offshore Portugal

Abstract The study provides a regional seismic interpretation and mapping of the Mesozoic and Cenozoic succession of the Lusitanian Basin and the shelf and slope area off Portugal. The seismic study is compared with previous studies of the Lusitanian Basin. From the Late Triassic to the Cretaceous the study area experienced four rift phases and intermittent periods of tectonic quiescence. The Triassic rifting was concentrated in the central part of the Lusitanian Basin and in the southernmost part of the study area, both as symmetrical grabens and half-grabens. The evolution of half-grabens was particularly prominent in the south. The Triassic fault-controlled subsidence ceased during the latest Late Triassic and was succeeded by regional subsidence during the early Early Jurassic (Hettangian) when deposition of evaporites took place. A second rift phase was initiated in the Early Jurassic, most likely during the Sinemurian–Pliensbachian. This resulted in minor salt movements along the most prominent faults. The second phase was concentrated to the area south of the Nazare Fault Zone and resulted here in the accumulation of a thick Sinemurian–Callovian succession. Following a major hiatus, probably as a result of the opening of the Central Atlantic, resumed deposition occurred during the Late Jurassic. Evidence for Late Jurassic fault-controlled subsidence is widespread over the whole basin. The pattern of Late Jurassic subsidence appears to change across the Nazare Fault Zone. North of the Nazare Fault, fault-controlled subsidence occurred mainly along NNW–SSE-trending faults and to the south of this fault zone a NNE–SSW fault pattern seems to dominate. The Oxfordian rift phase is testified in onlapping of the Oxfordian succession on salt pillows which formed in association with fault activity. The fourth and final rift phase was in the latest Late Jurassic or earliest Early Cretaceous. The Jurassic extensional tectonism resulted in triggering of salt movement and the development of salt structures along fault zones. However, only salt pillow development can be demonstrated. The extensional tectonics ceased during the Early Cretaceous. During most of the Cretaceous, regional subsidence occurred, resulting in the deposition of a uniform Lower and Upper Cretaceous succession. Marked inversion of former normal faults, particularly along NE–SW-trending faults, and development of salt diapirs occurred during the Middle Miocene, probably followed by tectonic pulses during the Late Miocene to present. The inversion was most prominent in the central and southern parts of the study area. In between these two areas affected by structural inversion, fault-controlled subsidence resulted in the formation of the Cenozoic Lower Tagus Basin. Northwest of the Nazare Fault Zone the effect of the compressional tectonic regime quickly dies out and extensional tectonic environment seems to have prevailed. The Miocene compressional stress was mainly oriented NW–SE shifting to more N–S in the southern part.

Episodic uplift and exhumation along North Atlantic passive margins: implications for hydrocarbon prospectivity

Abstract We present observations that demonstrate that the elevated passive margins around the North Atlantic were formed by episodic, post-rift uplift movements that are manifest in the high-lying peneplains that characterize the coastal mountains, in the unconformities in the adjacent sedimentary basins and in accelerated subsidence in the basin centres. Results from West Greenland show that subsidence of the rifted margin took place for c . 25 Myr after rifting and breakup in the Paleocene, as predicted by classical rift theory, but that this development was reversed by a series of uplift movements (starting at c . 35, 10 and 5 Ma) that remain unexplained. East Greenland and Scandinavia seem to have had a similar evolution of post-rift subsidence followed by uplift starting at c . 35 Ma. There was no notable fall in sea-level at this time, so the subsiding basins must have been inverted by tectonic forces. We speculate that the forces causing this phase were related to the plate boundary reorganization in the North Atlantic around Chron 13 time. One feature that these areas have in common is that uplift took place along the edges of cratons where the thickness of the crust and lithosphere changes substantially over a short distance. It may be that the lateral contrasts in the properties of the stretched and unstretched lithosphere make the margins of the cratons unstable long after rifting. These vertical movements have profound influence on hydrocarbon systems, not only in frontier areas such as West and East Greenland, where Mesozoic basins are deeply truncated and exposed onshore, but also for the understanding of near-shore hydrocarbon deposits in mature areas such as the North Sea Basin, where low-angular unconformities may represent episodes of deposition and removal of significant sedimentary sections.

Late Cenozoic depositional history of the Danish North Sea Basin: implications for the petroleum systems in the Kraka, Halfdan, Siri and Nini fields

The Central Graben area was filled with a thick pile of sediments during the Middle Miocene – Quaternary, corresponding to a period of 15Ma. As hydrocarbon expulsion from the most prolific source rock, the Upper Jurassic Bo Member, was initiated only 20 Ma BP and still occurs today, the Middle Miocene – Quaternary evolution is important. In the Middle Miocene, the Central Graben area was covered by a sea with water depth of 500–700 m. During the Late Miocene (Tortonian), the basin was successively filled by prograding slope and deltaic sediments from the northeast. The progradational infill resulted in local tilting of the substratum due to the loading effect of the deposits. In the latest Late Miocene (Messinian), the main input of sediments occurred from the south, as illustrated by a thick onlapping succession of upper Messinian sediments. Pliocene sedimentation was characterized by regular infill from the east within a shelf to shallow marine depositional environment. Following the Miocene and Pliocene, the North Sea Basin tilted due to strong uplift of the Fennoscandian shield and increased subsidence and sedimentation rates within the Central Graben area. This further complicated the maturation of the source rock, migration pathways and accumulation of hydrocarbons. The consequence of this complex burial history is exemplified by the Kraka and Halfdan fields. The Kraka Field has a large down-flank oil accumulation, which is the result of a porosity anomaly resulting from an early invasion of oil in this position before the late tilting of the North Sea Basin. The history of the non-structural accumulation of the Halfdan Field can be readily modelled; it constituted a simple four-way dip closure during the Late Miocene when peak oil migration occurred. The Quaternary tilting of the North Sea Basin due to uplift of the Fennoscandian Shield and strong subsidence of the Central Graben area resulted in a distinct gradient favouring long-distance migration of hydrocarbons. The occurrence of viable migration routes, especially within Paleocene sand layers, has resulted in long-distance migration of oil into the Siri submarine valley system. The most northern indication of hydrocarbons has been recognized as far as 75km from the source area. Long-distance migration of hydrocarbons is also indicated by direct hydrocarbon indications (DHIs) throughout the Cenozoic succession in the Danish North Sea. DHIs are particularly prominent above known hydrocarbon accumulations in the Central Graben. This indicates pronounced vertical migration, for instance along active faults, above these structures.

Early Miocene pollen and spores from western Jylland, Denmark - environmental and climatic implications

Abstract A palynological analysis of a Lower Miocene cored section from Sønder Vium in western Jylland, Denmark, provides new data regarding the vegetation and climate during the earliest Neogene. Most samples yielded well-preserved palynomorphs. Terrestrial pollen and spores dominate, with lesser proportions of dinoflagellates. A fluvial input into the marine setting is corroborated by the presence of freshwater algae, indicating an inner-neritic setting. A level containing comparatively abundant dinoflagellate cysts probably represents a transgressional event. The late Aquitanian age of the sequence as suggested by previous studies is supported by the composition of the palynoflora, e.g., by the presence of Ephedripites, Platycarya, and the relatively frequent occurrence of Engelhardtia. The pollen record is dominated by Taxodiaceae-Cupressaceae suggesting that swamp forests dominated the onshore region, which is consistent with previous results from central and northern Europe. Besides Taxodium, the swamp forest also contained angiosperm taxa such as Myricaceae, Nyssa, Betula, and Alnus. Elevated or better drained hinterland areas hosted a diverse mesophytic forest, with a ground cover of reeds, sedges and pteridophytes. Abundant pollen taxa derived from mesophytic forests indicates the presence of evergreen conifers, such as Pinus, Sequoia and Sciadopitys, and deciduous angiosperms, including Fagus and Quercus. A decrease in relative abundances of thermophilous elements such as Arecaceae (palms), Ilex, Mastixiaceae and Engelhardtia, in the middle part of the studied succession indicates a possible correlation to the late Aquitanian climatic deterioration. The composition of the palynological assemblages including widely distributed Taxodium swamps, suggests a warm, frost-free temperate climate during the Aquitanian in Denmark.

Sequence stratigraphy of Upper Jurassic reservoir sandstones in the northern part of the Danish Central Trough, North Sea

Abstract The Jurassic succession of the northern part of the Danish Central Trough is subdivided into six depositional sequences based on a multi-disciplinary study, including sedimentology, biostratigraphy, seismic interpretation, structural interpretation, log correlation, geochemistry and palynofacies analysis. Three of these sequences include significant sandstone deposits that represent potential reservoirs. The lowermost sequence 1 contains Upper Kimmeridgian backbarrier and shoreface sandstones of thebasal sandstone unit and the Heno Formation deposited within a transgressive and a highstand systems tract, respectively. Sequence 2 includes Upper Kimmeridgian middle shoreface sandstones of the Heno Formation deposited within a transgressive systems tract. Within sequence 6, Middle-Upper Volgian distal turbidite sandstones have been recognized. These are interpreted as having been deposited within a lowstand and transgressive systems tract and may constitute a reservoir elsewhere in the basin. The sequence stratigraphic boundaries of this study do not match those proposed by the Exxon group. In this rift setting, sequence development was probably controlled predominately by tectonism; eustasy was of subordinate importance.

The Ribe Formation in western Denmark- Holocene and Pleistocene groundwaters in a coastal Miocene sand aquifer

Abstract The Ribe Formation is a regionally extensive Miocene sand aquifer that is present in western Denmark at depths ranging from 100 to 300 m below ground surface. Groundwater chemistry and isotope data collected from more than 40 wells show that the Ribe Formation mainly contains high quality Cabi-carbonate type groundwater of Holocene age (100–10 000 a bp). Pleistocene age groundwaters, identified on the basis of stable isotopes, noble gases and corrected 14C values, are present below the island of Rømø, in discharge areas near the coast, and in hydraulically isolated inland areas. The groundwater age distribution in the Ribe Formation was successfully simulated with a numerical groundwater flow model and particle tracking only when the 14C content in groundwater was corrected for both geochemical reactions and diffusion. The results indicate that geochemical and physical processes significantly influence the 14C content of groundwater and that the correction factors required for the two processes are of the same magnitude. Flow modelling results indicate that Pleistocene groundwaters were emplaced at depth within the Ribe Fromation under low base-level conditions that prevailed throughout the late Pleistocene — near the coast these waters are essentially isolated from the present flow system, and Pleistocene freshwater may be present offshore. Seismic surveys show that conditions offshore are favourable for the presence of Pleistocene freshwater within the Ribe Formation and other aquifers.

Structural evolution of the Gert-Mjølner area

Abstract The Gert-Mjolner area is located in the northern part of the Danish Central Trough at the border between Denmark and Norway. A three-dimensional survey covering the Gert-Mjolner area and selected two-dimensional seismic lines which reveal the regional geology are interpreted here. The structural evolution of the Danish Central Trough from the Late Jurassic to the Early Cretaceous was characterized by a tensional/transtensional tectonic regime. In general, this resulted in the subsidence of major fault blocks dipping towards the north-east, forming a series of half-graben. Two of these present day half-graben are known as the Feda Graben and the Gertrud Graben. The two graben are separated by a NW-SE anastomosing fault zone named the Gert Fault. The structural development of the area was characterized by three main fault phases. During the Late Kimmeridgian a series of active N-S striking faults controlled the depositional conditions in the Gert-Mjolner area. The N-S trending faults became inactive in the latest part of the Kimmeridgian and were replaced by subsidence along NW-SE trending faults that were especially important in the formation of the Volgian Feda Graben. It is suggested that during the Late Volgian and Early Cretaceous, minor clockwise rotation of the Feda Graben relative to the Gertrud Graben created a local compressional tectonic regime along the so-called Gert Fault zone, most pronounced in the northern part of the investigated area. During the Cretaceous the rifting phase of the Central Graben ceased and subsidence was driven by thermal cooling. However, additional subsidence related to salt movements is observed in both the Feda and Gertrud Grabens. In the Late Cretaceous, former depocentres experienced inversion within the western part of the Feda Graben and the eastern part of the Gertrud Graben.

Organic-walled dinoflagellate cyst stratigraphy in an expanded Oligocene–Miocene boundary section in the eastern North Sea Basin (Frida-1 Well, Denmark) and correlation from basinal to marginal areas

The organic-walled dinoflagellate cyst (dinocyst) assemblages in an unusually thick (>800m) Oligocene–Miocene boundary succession from the eastern North Sea Basin (the Frida-1 Well) were studied. Six successive dinocyst assemblages are described: the Wetzeliella gochtii Assemblage (early Chattian), the Distatodinium biffii Assemblage (Chattian), the Deflandrea phosphoritica Assemblage (latest Chattian), the Homotryblium spp. Assemblage (early Aquitanian), the Caligodinium amiculum Assemblage (Aquitanian) and the Cordosphaeridium cantharellus Assemblage (latest Aquitanian to early Burdigalian). The dinocyst assemblages are compared with informal dinocyst zonations proposed for the southern North Sea Basin (Germany, Belgium and the Netherlands). A correlation of the expanded basinal succession in Frida-1 with the marginal marine succession found onshore Jylland, based on the dinocyst stratigraphy combined with well logs and seismic data, is proposed. This correlation confirms earlier proposed datings of the onshore deposits and the presence of several hiati. Furthermore, the correlation made it possible to subdivide the succession in Frida-1 into the sequences A–C, as defined onshore. The Mi-1 glaciation event and thus the Oligocene–Miocene boundary are proposed to correlate to the boundary between sequences A and B.

Structural evolution and sequence formation offshore South Gabon during the Tertiary

Abstract The present investigation of the Tertiary succession offshore South Gabon is based on seismic surveys of 1985–1989 vintage and wells covering the southern part of Gabonese waters. In general, the Tertiary basin off South Gabon evolved during two structural stages. During the Paleocene and Early-Middle Eocene, a trough-like basin subsided slowly towards the west. During this period there was a cyclic development of the succession which consists of up to four sequences mostly bounded by Type 2 sequence boundaries. The sequential changes were induced by eustatic sea-level changes. This pattern changed during the Neogene when the evolution was characterized by continental crustal flexuring in Early-Middle Miocene times. This resulted in strong subsidence of the western part of the basin and uplift of the eastern part. Consequently, the eastern part was exposed to strong erosion and a thick Middle Miocene succession was deposited west of the shelf (approximately the present-day shelf), especially on the slope. The clinoformal and lens-shaped Middle-Late Miocene succession which consists of four sequences bounded by Type 1 sequence boundaries has been controlled by eustatic changes in sea level. In addition to the overall subsidence pattern, salt tectonics has modified the external and internal architecture of the sequences on a local scale. Canyon formation in connection with slumping results in the establishment of well developed fan systems on the lower part of the slope and on the basin floor area.

Depositional Evolution and Sequence Stratigraphy of the Shelf and Slope Area Off South Gabon, West Africa

ABSTRACT The Tertiary succession off South Gabon is divided into stratigraphic sequences. The study includes interpretation of seismic data and data from seven wells within the study area, covering 26,000 km2. Four Paleogene sequences (P1-P4) are recognized within the Danian to earliest early Ypresian time interval. There was a major hiatus from the middle Eocene to the early Miocene in the study area. Six sequences (N1-N6) are identified in the Neogene, N1-N4 lower and middle Miocene and N5 and N6 probably upper Miocene to recent. The Danian and Thanetian sequences (P1-P3) are separated by Type 2 sequence boundaries, and the most widespread flooding of the area occurred at the Danian-Thanetian transition within sequence P2. The boundary between P3 and P4, close to the Thanetian-Ypr sian transition, is probably of Type 1. The Paleocene sequences consist of transgressive systems tracts and highstand systems tracts. The lower Miocene sequence (N1) was formed mainly by progradation of the shelf as a result of high sediment supply and had a tectonic origin. The middle Miocene sequence boundaries are characterized by deep erosion, e.g., canyon formation into the substratum, and thus are Type 1 sequence boundaries. The middle Miocene sequences (N2-N4) consist of lowstand systems tracts, transgressive systems tracts (early part), and late highstand systems tracts. The upper Miocene to Holocene interval (N5-N6) could not be subdivided because the deposits are relatively thin. The Tertiary mainly siliciclastic sediments are mainly fine-grained sediments, though sandstones are identified at sequence boundaries of Danian to Ypresian ages. Sandstones are also inferred to have been deposited during the early fill of canyons and basinward in the middle Miocene. The processes involved in the formation of these sequences is thought to be a combination of basin type (tectonics), sediment supply, and sea-level changes. Curves of relative sea level and eustatic sea level were constructed for the studied interval, and these curves are compared with earlier published sea-level curves for the Tertiary.