Ole Valdemar Vejbæk

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.

Cretaceous-Early Tertiary inversion tectonism in the Danish Central Trough

Abstract The inversion of the Danish Central Trough was heralded by a mild precursor during the Late Hauterivian, before rifting had ended. The main inversion took place during the Late Cretaceous. It can be subdivided into several sub-phases, of which those occurring during the Turonian-Santonian, mid-Maastrichtian and post-Danian-Early Tertiary are the most pronounced. Minor inversion tectonism occurred more or less continuously between these major events. The Turonian-Santonian event had the greatest impact in the northern part of the Danish Central Trough, whereas the effects of later events are most pronounced in the south. Throughout the Late Cretaceous-Danian a gradual change from more brittle deformation, with faulting, to more plastic deformation characterised by folding and flexuring, can be seen.

The Horn Graben, and its relationship to the Oslo Graben and the Danish Basin

Abstract The tectonic development of the Horn Graben, the Danish Basin and the Oslo Graben are compared with special emphasis on the timing of their initiation. The comparison is based on seismic interpretation and is supported by analysis of tectonic subsidence. Devonian and Carboniferous sedimentary strata and Rotliegende volcanics constitutes the pre-rift succession in the Horn Graben. In the central part of the graben it rests unconformably on Precambrian (possibly Dalslandian) basement, to the south on Caledonian metamorphic basement, whereas Lower Palaeozoics occur to the north. The distribution of these pre-Devonian assemblages suggests initial early Devonian uplift of the Ringkobing-Fyn High as the Lower Palaeozoic sediments are believed to have covered the high originally. Further uplift appears to have taken place in the Early Permian. A subdivision of the Horn Graben into a northern and a southern segment are recognized. The rifting was apparently heralded only in the southern segment in the Rotliegende, and the main rifting phase did not start before the Triassic. Subsidence modelling shows that the thermal anomaly created by rifting of the Horn Graben was less significant than in the Danish Basin and resulted in much less post-rift thermal subsidence. The reduced thermal anomaly may account for an earlier consolidation of the crust, thereby rendering the area resistant to late Cretaceous-early Tertiary inversion tectonism. The Oslo Graben was tectonically active during late Carboniferous to early Permian times, and essentially became inactive during Triassic times when the main tectonism of the Horn Graben occurred. Subsidence modelling shows that the Danish Basin, south of the Fjerritslev Trough, was tectonically active mainly during the Zechstein, whereas the rapid Triassic subsidence is modelled to be caused by phase transformations, as indicated by the minor amount of basement attached faulting. Based on the differing subsidence patterns, it is concluded that the three basins did not develop simultaneously, and that they are not genetically related.

Influence of porosity and pore fluid on acoustic properties of chalk: AVO response from oil, South Arne Field, North Sea

Amplitude versus offset (AVO) inversion provides direct evidence for the presence of light oil in high-porous chalk in the South Arne Field, North Sea. The elastic properties of the chalk were estimated at three scales by analysing core data, log-readings and AVO-inversion results. The velocity–porosity relation of the core data matches a modified upper Hashin–Shtrikman model for Ekofisk Field chalk and the model is extended to 45% porosity. A small clay content reduces porosity without affecting chalk stiffness and this content can be estimated from the water saturation, which is controlled by silicate content and particle sorting in the zone of irreducible water saturation. The model is, thus, scaled according to clay content estimated by the water saturation. Based on comparison with the model and measurements on core samples, it is found that the sonic log data represent chalk characterized by forced displacement of the oil by mud filtrate and, thus, a much higher water saturation than estimated from, for example, a shallow resistivity log. Forward modelling of the acoustic properties of the virgin zone results in a characteristic pattern of Poisson ratio versus depth. This pattern agrees with inverted seismic data, whereas it is not captured by conventional fluid substitution.

Disequilibrium compaction as the cause for Cretaceous–Paleogene overpressures in the Danish North Sea

Cretaceous–Paleogene overpressure distribution in the Danish Central Graben shows a remarkable coincidence with the thickness of the rapidly deposited middle Miocene to Holocene succession. Slow deposition of smectite-dominated clays in a deep-marine environment occurred from the late Paleocene until the middle Miocene, and the resultant mudstone succession constitutes the main barrier that delays pressure dissipation. Between the late Miocene and the Holocene, the Upper Cretaceous–Paleogene succession became overpressured, probably because of accelerated depositional rates. Quantification of this disequilibrium compaction mechanism relies mainly on a determination of permeability and effective compressibility of the Paleogene shales. This article shows that realistic permeabilities can be assumed, provided that compressibilities describing the plastic process of compaction are used in the pressure equation instead of the elastic compressibilites that, for instance, can be derived from log data. One-dimensional (1-D) modeling is applied in two cases: a well from the Dan chalk field, where accelerated deposition since the Tortonian (11.2 Ma) produced a present-day overpressure of 7.97 MPa (1156 psi); and a well from the South Arne chalk field, where accelerated deposition since the early Serravallian (14.6 Ma) produced a present-day overpressure of 13.9 MPa (2016 psi). This is based on an identical set of parameters and compares with the observed 7.7 and 14.8 MPa (1117 and 2147 psi ) overpressure at the two locations. The modeled development of the pressure profiles shows that an effective stress minimum occurred in the upper part of the Paleogene succession. This is consistent with the observed ubiquitous intraformational faulting at that level. About 80% of the added Neogene load is estimated to have been converted to overpressure.

The history of hydrocarbon filling of Danish chalk fields

It is well established that dynamic conditions expressed as tilted fluid contacts characterize most hydrocarbon accumulations in North Sea Chalk reservoirs. Chalk is a low-permeability, high-porosity rock and properties grade smoothly from reservoir over baffle to seal. The natural dynamic conditions prevail because pressure dissipation takes place through the rockmatrix, as fracture-supported flow often is minimal. The dynamic conditions are imposed by processes occurring on a geological time-scale and result mainly in lateral pressure differences in the water zone and even in lateral pressure differences in the oil zone. Re-equilibration of fluid contacts also occurs on a geological time-scale. These processes are of paramount importance for trap definition and impose severe restrictions on migration distances. Reservoir simulation techniques are applied, in combination with back-stripping, to the simulation of geological time-scale secondary migration and trapping. Flow simulation of the filling dynamics of a chalk reservoir shows a complex filling geometry due to the high capillary entry pressures in the low-permeability chalks. Such internal barriers will re-direct hydrocarbons and residual oil can be left on the migration route. The process of hydrocarbon charging is slow and equilibration of hydrocarbons with respect to pressure gradients, therefore, also occurs very slowly. The Kraka Field and the Dan and Halfdan fields are subjected to studies of primary oil charging and re-migration in this paper and the dynamic oil on Dan Field west flank is successfully mimicked. Results show that a time span in the order of 2 Ma is required for the hydrocarbons to reach the summit in an approximately equilibrium state from a flanking position. However, even dynamic equilibrium may not be fully obtained due to re-perturbation by tectonic movements and changing water zone pressure gradients. Results show that saturation profiles in drilled wells may appear in drainage equilibrium while under partial re-imbibition, which impairs saturation modelling.

Seismic Texture Classification: A Computer-aided Approach to Stratigraphic Analysis

Statistical recognition of seismic reflection patterns is a useful aid to seismic data interpretation. It classifies windows of carefully processed seismic data into a number of groups, each characterized by a distinct reflection pattern. The recognition is based on a set of reference patterns extracted from representative or geologically well understood zones. The seismic data is modelled as a Markov random field. The probability distribution of patterns in a given data window is used to characterize its seismic texture. The recognition of seismic texture can be reduced to a simple mathematical operation which makes the method very efficient. In addition, the method has a relatively low demand in computer storage during computation. The method of recognizing seismic texture is illustrated on a set of offshore reflection data. Applied to these data the method distinguishes clearly between different layers and efficiently separates zones of different internal stratification, even when they are hard to see for the naked eye.

Downflank hydrocarbon potential identified using seismic inversion and geostatistics: Upper Maastrichtian reservoir unit, Dan Field, Danish Central Graben

Porosities considerably higher than anticipated from porosity/depth trend models are encountered in a Maastrichtian reservoir unit on the western flank of the Dan Field. Because there is a good correlation between seismic impedance and well log porosity, inverted seismic data are used to infer that highly porous zones are widely distributed. The distribution of these high porosity zones is predicted using geostatistical methods based on the inverted seismic data. These predictions contradict the general assumption that porosity deteriorates with depth in the study area. Annealing cosimulation is applied using inversion-derived seismic impedances as soft data and well log porosities as hard data. The sensitivity of the porosity characterization to hydrocarbon in-place estimates is investigated through the calculation of water saturations using height above free water level and simulated porosity as input parameters. Multiple realizations show that calculated hydrocarbon in-place estimates are more sensitive to the location of the free water level than to uncertainties related to the geostatistical reservoir characterization.

Modelling seismic response from North Sea chalk reservoirs resulting from changes in burial depth and fluid saturation

Changes in seismic response caused by variation in degree of compaction and fluid content in North Sea chalk reservoirs away from a wellbore are investigated by forward modelling. The investigated seismic response encompasses reflectivity, AVO and acoustic impedance. Synthetic seismic data are calculated on the basis of well data from the South-Arne and Dan fields, Danish North Sea and compared to field records. Seismic response predictions are based on three main tools: (1) saturation modelling, (2) compaction/decompaction modelling and (3) rock physics. Hydrocarbon saturation in North Sea chalk is strongly affected by capillary forces and transition zones in the order of 50 m are common. Advanced saturation height modelling is applied, which has proved robust for the prediction of saturation profiles in Danish chalk. Compaction modelling relies on exponential decay of porosity with depth, where abnormal fluid pressures are accounted for. A new set of compaction parameters is presented based on a normal velocity–depth trend and a velocity–porosity transform for North Sea chalk. The parameters appear to allow fairly precise predictions of abnormal fluid pressures from observed average porosity. Based on this, the relative contribution to porosity preservation by abnormal fluid pressure and early hydrocarbon invasion may be estimated. Rock physics theory is applied to obtain all necessary parameters for the complete set of elastic parameters for seismic modelling. Modelling results of importance in the search for subtle traps include: (1) correlation of reflectivity with porosity; (2) primary sensitivity of acoustic impedance is to porosity variation rather than hydrocarbon saturation; (3) the Poisson ratio is very sensitive to hydrocarbon saturation at high porosity, depending on fluid density contrasts. In addition, compaction modelling shows a clear effect of porosity preservation by hydrocarbons in the South-Arne Field, whereas this effect is negligible in the Dan Field. In both fields seismic signatures in field records originating from fluid changes are identified.

Reservoir characterization of the Roar Gas Field, Danish North Sea

The Roar Field consists of a gentle inversion anticline in which porosity, permeability and hydrocarbon saturation distributions in the Danian–Upper Cretaceous Chalk reservoir have been mapped using seismic data. The seismic data are inverted to support porosity prediction in a stochastic modelling approach. The uniform chalk matrix causes seismic impedance to correlate mainly with porosity and fluid content. Replacement of oil by water has negligible effect on seismic impedance but correction for fluid content in the gas cap is needed to increase correlation between seismic impedance and porosity. Porosity in excess of 40% is characteristic, probably reflecting early invasion of hydrocarbons. A slight tilting since Miocene times is suggested by higher porosity on the southwest flank. Permeability is generally below 10 mD in the reservoir and around 1 mD below. A slight south-dipping free water level is consistent with a regional water zone pressure gradient, if local high permeability is considered. A gas cloud that produces a velocity anomaly not apparent in well velocity data hampers depth conversion. The velocity anomaly is revealed from horizontal wells and has led to the identification of a new culmination at the north end 15 m shallower than the culmination at the south end; this has increased the estimated initial hydrocarbons in place from around 510 to 720×109 SCF gas. Analysis of uncertainty on hydrocarbon in-place calculations resulting from the characterization is limited to studying the effects of the gas cloud anomaly and the value added by the two horizontal wells on porosity field determination.