Lars Henrik Nielsen

Structural development of the Fennoscandian Border Zone, offshore Denmark

Abstract The Hans-1, Saeby-1 and Terne-1 wells are located within the Danish part of the Fennoscandian Border Zone and provide significant new data pertaining to the evolution of this important tectonic belt. All three wells encountered Palaeozoic and Mesozoic rocks. The Cambrian to Lower Silurian deposits can be closely correlated to sections known from Bornholm and Scania, and tectonic activity within the Fennoscandian Border Zone cannot be verified during this time interval. The thick Upper Silurian shales were probably deposited in a rapidly subsiding foreland basin, marginal to the speculative Caledonian deformation front which lies south of the Ringkobing-Fyn High. A major hiatus comprises the Devonian and Early Carboniferous. Upper Carboniferous sediments were deposited before a phase of rifting. Upper Carboniferous intrusive and extrusive volcanic rocks suggest tectonic activities, heralding the rifting phase. Extensive block faulting occurred in the border zone during the Rotliegende and more than 650 m of reworked volcanic rocks were deposited in an alluvial fan environment. Syndepositional erosion created a major unconformity before the deposition of post-rift Zechstein sediments. During the Mesozoic the Fennoscandian Border Zone was divided into two zones: the Sorgenfrei-Tornquist Zone to the west and the Skagerrak-Kattegat Platform to the east. The Sorgenfrei-Tornquist Zone continued to experience tectonic activity in Triassic and Middle Jurassic times. The zone has experienced an uplift of 1700–2000 m during Late Cretaceous to Early Tertiary inversion tectonics and the Late Tertiary uplift of Fennoscandia.

Modelling of hydrocarbon generation in the Cenozoic Song Hong Basin, Vietnam: a highly prospective basin

Abstract The Cenozoic Song Hong Basin, situated on the northern part of the Vietnamese shelf, has been only sporadically explored for hydrocarbons. A review of the results of the exploration efforts so far shows that the distribution of potential source rocks and their time of hydrocarbon generation are the critical risks for finding commercial amounts of hydrocarbons. In the Song Hong Basin, including the Hanoi Trough, the rocks most likely to have source potential are: (1) oil-prone Eocene–Lower Oligocene lacustrine mudstones and coals, (2) oil- and gas-prone Middle Miocene coal beds, (3) gas-prone Upper Oligocene–Lower Miocene coals, and (4) gas- and oil-prone Miocene marine mudstones. To assess the time of hydrocarbon generation from these units, relative to the formation of traps, the generation history was modelled at 32 well and pseudo-well locations. The modelling demonstrates that the two first-mentioned source rock units are especially important. In the northern and northeastern part of the basin and along its western margin traps may have been charged by Eocene–Lower Oligocene source rocks. In the Hanoi Trough, the excellent Middle Miocene coal beds have probably generated hydrocarbons within the last few million years. Thus the huge and still underexplored Song Hong Basin provides attractive areas for further exploration.

Petroleum potential of Oligocene lacustrine mudstones and coals at Dong Ho, Vietnam — an outcrop analogue to terrestrial source rocks in the greater Song Hong Basin

The outcrop of Oligocene age at Dong Ho, northern Vietnam, may constitute an immature analogue to offshore terrestrial source rocks in the greater Song Hong Basin. The outcrop includes an interval with two source rocks: (1) highly oil-prone carbonaceous mudstones containing kerogen types IIA and IIA/I, and with TOC contents from 6.48 to 16.89 wt%, and HI values from 472 to 690; and (2) oilprone humic coals (kerogen type III) with HI values from 200 to 242. The mudstones were deposited in oxygen-deficient lakes, which on occasion were subject to marine influence, and the coals accumulated in freshwater peat-forming mires. The coals have broad activation energy (Ea) distributions, while the mudstones have Ea distributions characterised by a pronounced principal Ea. During artificial maturation about 16‐17% of the organic carbon in the coals and 45‐50% of the organic carbon in the mudstones participated in petroleum formation. The two source rocks primary generate oil and secondary generate gas, however, the mudstones realised the majority of their potential over a more narrow temperature range than the coals. The excellent generative potential of the terrestrial source rocks at Dong Ho is encouraging for offshore exploration for reservoirs charged by Cenozoic rift-lake successions. q 2001 Elsevier Science Ltd. All rights reserved.

No causal link between terrestrial ecosystem change and methane release during the end-Triassic mass extinction

Profound changes in both marine and terrestrial biota during the end-Triassic mass extinction event and associated successive carbon cycle perturbations across the Triassic-Jurassic boundary (T-J, 201.3 Ma) have primarily been attributed to volcanic emissions from the Central Atlantic Magmatic Province and/or injection of methane. Here we present a new extended organic carbon isotope record from a cored T-J boundary succession in the Danish Basin, dated by high-resolution palynostratigraphy and supplemented by a marine faunal record. Correlated with reference C-isotope and biotic records from the UK, it provides new evidence that the major biotic changes, both on land and in the oceans, commenced prior to the most prominent negative C-isotope excursion. If massive methane release was involved, it did not trigger the end-Triassic mass extinction. Instead, this negative C-isotope excursion is contemporaneous with the onset of floral recovery on land, whereas marine ecosystems remained perturbed. The decoupling between ecosystem recovery on land and in the sea is more likely explained by long-term flood basalt volcanism releasing both SO2 and CO2 with short- and long-term effects, respectively.

Palaeocene–early Eocene inversion of the Phuquoc–Kampot Som Basin: SE Asian deformation associated with the suturing of Luconia

Abstract: The little explored Cambodian and Vietnamese Phuquoc–Kampot Som Basin is a Late Jurassic to Early Cretaceous foreland basin developed in response to the build-up of a palaeo-Pacific magmatic arc. A combination of seismic data, well data and outcrop geology complemented by fission track and U/Pb analysis is used to unravel the basin history. This reveals a hitherto unknown earliest Palaeogene basin inversion associated with the Luconian suturing to SE Asia and the shutdown of palaeo-Pacific subduction underneath SE Asia. The Phuquoc–Kampot Som Basin and the Khorat Basin in Thailand constitute the erosional remnants of a larger basin that covered large parts of SE Asia in Late Mesozoic time, and subsequently became segregated during earliest Palaeogene inversion and erosion. Inversion was focused along the several hundred kilometres long Kampot and Khmer–Chanthaburi fold belts that confine the Phuquoc–Kampot Som Basin and merge with the Mae Ping and the Three Pagodas fault zones. These connections, together with local NW–SE-trending sinistral transpressional faults offshore, indicate a link between initial SE Asian left-lateral strike-slip faulting and the Luconian suturing. The separation between the once unbroken Khmer–Chanthaburi Fold Belt and the Phetchabun Fold Belt in Thailand suggests a 50–100 km Cenozoic left-lateral offset across the Mae Ping Fault Zone.

Major coastal impact induced by a 1000-year storm event

Extreme storms and storm surges may induce major changes along sandy barrier coastlines, potentially causing substantial environmental and economic damage. We show that the most destructive storm (the 1634 AD storm) documented for the northern Wadden Sea within the last thousand years both caused permanent barrier breaching and initiated accumulation of up to several metres of marine sand. An aggradational storm shoal and a prograding shoreface sand unit having thicknesses of up to 8 m and 5 m respectively were deposited as a result of the storm and during the subsequent 30 to 40 years long healing phase, on the eroded shoreface. Our results demonstrate that millennial-scale storms can induce large-scale and long-term changes on barrier coastlines and shorefaces, and that coastal changes assumed to take place over centuries or even millennia may occur in association with and be triggered by a single extreme storm event.

Divergent development of two neighbouring basins following the jurassic north sea doming event: the danish central graben and the Norwegian-Danish Basin

The two neighbouring basins, the Danish Central Graben and the Norwegian-Danish Basin were both affected by the regional uplift of the North Sea and adjacent areas in the early Middle Jurassic that caused the formation of a regionally extensive unconformity. The uplifted area was not a simple dome structure but of a more irregular shape with an east-west oriented branch that included the Ringkobing-Fyn High and much of the Norwegian-Danish Basin and the Fennoscandian Border Zone. Late Aalenian-Bajocian deposition was confined to fault-controlled depocentres in both the Danish Central Graben and the Norwegian-Danish Basin. An initial southward slope in the Danish Central Graben changed to a north- to eastward slope before the end of the Middle Jurassic, and the change possibly coincides with the formation of a conspicuous sequence boundary in the Bathonian. The depositional area began to expand in the late Middle Jurassic as a result of a regional sea-level rise. In the Danish Central Graben, accelerating half-graben subsidence during the Callovian-Early Kimmeridgian enhanced the sea-level rise. Several periods of rapid subsidence during the Callovian-Volgian (mainly in the Oxfordian-Early Kimmeridgian and latest Kimmeridgian-Middle Volgian) gave accommodation space to more than four kilometres of marine mud. A break in subsidence in the late Kimmeridgian, probably related to a change of fault directions, resulted in deposition of shallow marine sandstones on platforms and hanging-wall slopes. The Norwegian-Danish Basin was characterised by a small rate of subsidence and continuous expansion of the depositional area throughout the Late Jurassic. The slow subsidence and a large supply of sediment from the Fennoscandian Border Zone caused repeated progradational events from the northeast. Hydrocarbon discoveries are known only from the Danish Central Graben where Middle Jurassic and Upper Jurassic reservoirs have been charged from Upper and to a smaller degree Middle Jurassic source rocks. Within the Norwegian-Danish Basin, reservoir rocks are abundant in the Upper Triassic-lowermost Jurassic, the Middle Jurassic and Upper Jurassic successions. The presence of mature source rocks, however, is the main risk factor as they most likely only occur within Lower Jurassic mudstones deeply buried in rim-synclines and in local grabens.

Intense and widespread seismicity during the end-Triassic mass extinction due to emplacement of a large igneous province

Multiple levels of earthquake-induced soft-sediment deformations (seismites) are concentrated in the end-Triassic mass extinction interval across Europe. The repetitive nature of the seismites rules out an origin by an extraterrestrial impact. Instead, this intense seismic activity is linked to the formation of the Central Atlantic magmatic province (CAMP). By the earliest Jurassic the seismic activity had ceased, while extrusive volcanism still continued and biotic recovery was on its way. This suggests that magmatic intrusions into sedimentary strata during early stages of CAMP formation caused emission of gases (SO 2 , halocarbons, polycyclic aromatic hydrocarbons) that may have played a major part in the biotic crisis.

A Middle–Upper Miocene fluvial–lacustrine rift sequence in theSong Ba Rift, Vietnam: an analogue to oil-prone, small-scale continental rift basins

The small Neogene Krong Pa graben is situated within the continental Song Ba Rift, which is bounded by strike-slip faults that were reactivated as extensional faults in Middle Miocene time. The 500 m thick graben-fill shows an overall depositional development reflecting the structural evolution, which is very similar to much larger and longer-lived graben. The basal graben-fill consists of thin fluvial sandstones interbedded with well-oxygenated lacustrine siltstones in the basin centre, while very coarse-grained fluvial sandstones and conglomerates dominate at the basin margins. With increased subsidence rate and possibly a higher influx of water from the axial river systems the general water level in the graben rose and deep lakes formed. High organic preservation in the lakes prompted the formation of two excellent oil-prone lacustrine source-rock units. In the late phase of the graben development sedimentation rate outpaced the formation of accommodation space and fluvial activity increased again. During periods when the general sedimentation rate was in balance with the creation of accommodation space the environment changed frequently between lake deposition and intermittent vigorous fluvial activity. It is likely that the resulting interbedding of fluvial sandstones and lacustrine sediments reflects variations in precipitation. In periods of little precipitation the lakes diminished and lake bottoms became exposed. After heavy precipitation, transverse river systems transported sands from the rift shoulders across the exposed lake bottom and fluvial sands were deposited on lake bottom sediments. Subsequently, lake level rose due to increased water supply from the axial river and the sands were drowned and topped by transgressive lacustrine mudstones. These sandstones may function as carrier beds, whereas the braided fluvial sandstones and conglomerates along the graben margins may form reservoirs. The Krong Pa graben thus contains oil-prone lacustrine source rocks, effective conduits for generated hydrocarbons and reservoir sandstones side-sealed by the graben faults toward the footwall granites. In addition to the structural and climatic signals recorded by the graben-fill, sediment partitioning among the partly isolated basins along the rift axis seems to have been important.

Accretionary, forced regressive shoreface sands of the Holocene-Recent Skagen Odde spit complex, Denmark — a possible outcrop analogue to fault-attached shoreface sandstone reservoirs

Spit systems are often important components of Recent deltas and coastal plains, where waves redistribute fluvial-derived sediments and they show large variations in terms of size, volume and grain size. However, spit systems are likely to form along any wave-influenced coastline and may form where faulted terrains of poorly to moderately lithified rocks are exposed to wave erosion. In such areas large spit systems are likely to be formed down-current from fault escarpments, along fault crests and on fault ramps, but appear to have been overlooked in the geological record. The Holocene-Recent Skagen Odde spit complex has developed within the last 8000 years and is still growing. The most important factors that have governed its growth are well-known, including climate, sea-level changes, palaeo-relief, free fetch, tidal range, progradation and sediment transport rates. The spit system has developed down-current from a steep slope comparable to a fault escarpment. It shows a triangular form with sides of 30-40 km and it contains 5-10 km 3 of sand. The main part of the spit system consists of five sedimentary units that constitute a shallowing-upward shoreface succession: (lower) storm sand; bar-trough sand; beach sand; peat and (upper) modern aeolian sand. The succession was formed during a forced regression as uplift outpaced eustatic rise. As the average basin-floor gradient has been steeper than the shoreline trajectory, the accommodation increased during basinward progradation of the spit system, and the resulting succession represents an accretionary, forced regression. The spit-system model may serve as an analogue to ancient fault-related shoreface sandstones without associated fluvial or deltaic deposits.