Hammerfest Basin Research Articles

Geochemically driven exploration models in uplifted areas: Examples from the Norwegian Barents Sea

The Norwegian Barents Sea with multiple source rock intervals represents a prime example of an overfilled petroleum system. However, several episodes of uplift and erosion from the Paleocene until the Pliocene-Pleistocene have caused the depletion of hydrocarbon accumulations in the region. These uplift events were not only potentially catastrophic but have also caused the redistribution of the remaining oil and gas over laterally large distances in the Barents Sea region. This redistribution directed petroleum to distal parts of the various hydrocarbon-generating basins, thus charging traps, which otherwise would not have been reached. Therefore, we expect that discoveries will be made in distal basin settings, particularly in traps with partly leaking cap rocks, which can bleed off gas and thereby retain oil. Many oil accumulations in the region represent various mixtures of oils from several different stratigraphic source intervals. This suggests that Triassic and Paleozoic oils may be trapped below the presently drilled targets, which are mostly Jurassic in the Hammerfest Basin and older to the north and east. Deeper exploration targets also stand a higher chance of containing oil because the amount of gas being released from oil during uplift, erosion, and subsequent pressure release is lower. Uplift and erosion are followed by a reduction in temperature. This is why hydrocarbon generation is believed to have ended in uplifted areas. Some discoveries in this data set suggest, however, a significant fresh gas charge.

Severity and timing of Cenozoic exhumation in the southwestern Barents Sea

The juxtaposition of the North Atlantic rift system with the borderlands of the Eurasian Arctic shelf has resulted in an unusual basin evolution in the southwestern Barents Sea. Exploration has concentrated on the Mesozoic sandstone plays of the Hammerfest Basin. These have yielded abundant gas and very little oil. Such findings are typical of peripheral North Atlantic margin basins that have undergone Cenozoic exhumation. This is generally thought to have occurred during the Palaeocene, Oligocene–Miocene or Pleistocene. Hammerfest Basin erosion is widely estimated to be in the range of 500–1500 m. However, the relative severity of individual episodes and impact on fluid dynamics within the basin are poorly constrained. We apply basin modelling tools to establish the sensitivity of the petroleum system to three cited scenarios for Cenozoic exhumation. Vitrinite reflectance and temperature data constrain burial and thermal histories. Calibration is not possible using heat flow variations alone; erosion timing and magnitude are required to match available data. The calibrated model indicates a significant thermal disequilibrium in the Hammerfest Basin at the present day resulting from Late Cenozoic exhumation. Forward modelling of Pleistocene ice sheets allows for an estimation of pressure and temperature fluctuations in response to glacial–interglacial cycles.

Lower Cretaceous: the next target for oil exploration in the Barents Sea?

During the early days of exploration in the southwestern Barents Sea, Lower Cretaceous wedges and mounds were identified on seismic data along the margins of the Hammerfest Basin above rotated Jurassic fault blocks. They were ranked as secondary targets after the giant Jurassic closures in the central part of the basin. Drilling of the latter proved the Snøhvit area gas fields and Snøhvit’s thin oil leg. However, the majority of the wells drilled in the Hammerfest Basin have oil shows in Lower Cretaceous strata. Even the claystones in the most basinal part have oil shows, interpreted by the industry to be remnants of oil leaked from Jurassic reservoirs to the surface. Lower Cretaceous sandstones have also been drilled along the Troms–Finnmark Platform, Loppa High and Senja Ridge, where almost all wells have oil shows. Three wells drilled along the southern margin of the Loppa High have even proved small amounts of oil ranging from 31° API to 38° API and the westernmost well, 7120/1-2, is ranked as an oil discovery. Thick (123–157m) massive sandstones of Valanginian to Hauterivian age in wells 7120/1-2, 7122/2-1 and 7120/10-2 represent the reservoir analogues for a possible Lower Cretaceous oil giant. The Gamma gas discovery 7019/1-1 at the eastern margin of the Tromsø Basin could be an indication of such a giant. Lower Cretaceous sandstones were also the primary target for 7120/10-2. The reservoir sandstones were deposited as submarine fans and the drilled massive sandstones have good reservoir parameters.

Assessment of undiscovered resources on the Norwegian Continental Shelf

The Norwegian Petroleum Directorate (NPD) makes annual assessments of the total petroleum resources on the Norwegian Continental Shelf (NCS). Major updates of the undiscovered resources have been done every second year, most recently in 2003. The resource assessment provides an overview of the resource potential that can be materialized in the future. This is used by the government to plan future licensing rounds and to make forecasts on petroleum activity and it provides oil companies with valuable information about exploration potential. The NPD defines resources as the total recoverable petroleum. The estimation is done using a probabilistic method, using the mean value as the official number. The undiscovered resources are estimated as a total figure for both the mapped (prospects) and the unmapped (potential prospects) for each play. The input parameters are standard volumetric parameters, with the addition of distribution of prospect size, number of prospects likely to occur in a play and the average chance of success. Discoveries and prospects provide crucial information to the play analysis. The North Sea, although fairly mature, still holds a considerable potential of undiscovered resources and is assumed to be the most oil-rich province. Pre-Jurassic plays are poorly explored in the Norwegian sector, as are stratigraphic plays. A large part of the remaining resources is assumed to be in medium to small discoveries. The Norwegian Sea holds the largest potential of undiscovered resources, with potential large structural traps in Cretaceous and Tertiary deep-water plays in the Vøring and Møre basins. The Barents Sea has potential for large discoveries of both oil and gas. Recent identification of pre-Jurassic source rocks increases the chance of discoveries in pre-Jurassic plays. The uncertainty in the estimates is large due to few wells drilled outside the Hammerfest Basin.

Statoil awards sn↓hvit contract to Dragados

The Late Cretaceous foraminiferal assemblages from five wells drilled in the southwestern Barents Sea were analysed using morphogroup analysis in order to understand the palaeoenvironment and palaeobathymetry of the study area. Morphogroup analysis of assemblages in the Upper Cretaceous Kveite Formation suggests stable, possibly middle to lower bathyal, mesotrophic and relatively well-oxygenated environments in the Tromsø Basin during the Late Cretaceous. The depositional setting of Upper Cretaceous Kviting Formation in the tectonically more stable Hammerfest Basin was probably shallower, but still represents a deep-water environment similar to that of the Kveite Formation. A shallowing regional bathymetry in the southwestern Barents Sea is indicated by the general decreasing trend in the relative abundance of tubular forms and the species diversity of assemblages, which reflects the regional Late Cretaceous–Paleocene uplift prior to the break-up of the Greenland–Norwegian Sea. Changes in the relative abundance of morphogroups and in the main constituent in the cumulative length of tubular taxa from Rhizammina to Rhabdammina near the top of Kveite Formation agree with the possible existence of bottom water currents.

Compositional variations and provenance of Triassic sandstones from the Barents Shelf

ABSTRACT Triassic sandstones from a large area ranging from the Pechora Sea and Novaya Zemlya in the southeast to the western Barents Sea and Svalbard have been analyzed by petrographic, mineralogical, mineral-chemical, and Sm-Nd bulk-rock isotopic methods. The largest compositional variation is seen in the Lower Triassic deposits. Epidote-bearing, lithic-rich sandstones are characteristic of the eastern Barents Sea, Novaya Zemlya, and the Timan-Pechora Basin, and indicate a provenance from the Polar Urals. Quartzo-feldspathic sandstones in the Hammerfest Basin may have been derived from deeper crustal plutonic and metamorphic rocks of the Caledonides and Baltic margin, whereas the source of quartz-dominated sandstones on Spitsbergen was a paleoland in the northwest, in the vicinity of paleo-Greenland. Calculated Sm-Nd provenance ages for Lower Triassic deposits are in the range of 1500-1880 Ma for sandstones from Spitsbergen, the Hammerfest Basin, and the Finnmark Platform. Sandstone provenance ages for samples analyzed from the eastern Barents Sea are significantly younger, with a range of 600-1280 Ma and an average of 780 Ma. Sandstones in central shelf areas, i.e., the Nordkapp Basin-Bjarmeland Platform, are rich in both lithics and feldspar and have intermediate provenance ages (960-1630 Ma). The largest influence of older components occurs in samples from the Nordkapp Basin. This may be related to sediment supply both from the Caledonized Baltic margin and Uralides, and from local uplifted areas composed of Paleozoic sedimentary rocks. Middle and Upper Triassic sandstones display compositional variations at different times in different areas due to local changes in provenance. However, in the Upper Triassic there appears to be a convergence in sandstone compositions reflecting both more extensive mixing of components from the eastern and western provenance areas as well as changes to more evolved compositions of the erosional products derived from the paleo-Urals. There is also a question of increasing influence from possible paleolands in the northeast. The most important compositional change was the distinct increase in sandstone maturity that took place on a regional scale in the Arctic in the latest Triassic-Early Jurassic. This may in part have been caused by extensive reworking connected with a regional transgression and sea-level raise.

A, B AVO cross plotting and its application in Greenland and the Barents Sea

Recent amplitude versus offset (AVO) interpretations in the Norwegian Barents Sea and offshore West Greenland have identified subtle and complex anomalies which are best interpreted using a layer-specific, amplitude and gradient cross plot analysis scheme. The creation of AVO intercept (A) and gradient (B) seismic sections allows for a variety of stacks including A, B, A times B, and A plus B. Also, A versus B cross plots are easily generated to help define shale, wet sand and hydrocarbon trend lines that better differentiate the anomalies and provide information concerning the geological framework that created the AVO response. Determination of the background trend on an A versus B cross plot is problematic in AVO analysis since variations in lithology and depth are interdependent, with both factors greatly influencing this trend. Also, the background trend is empirically defined by the AVO response of shale-on-shale boundaries or shale-on-silt reflectors, and this may not be indicative of the trend line for a shale-on-wet sand response. However, once the empirical background trend or wet trend is established, A and B data can be rescaled such that the shale line takes on a slope of −1. Potential hydrocarbon anomalies will then plot as data points with maximum scatter away from the normalized shale line. An A plus B section based upon a normalized shale line shows the potential hydrocarbon anomalies at maximum amplitude and the wet sands at minimum amplitude. AVO analysis of A and B is generally performed together with forward modelling that generates theoretical responses for intercept amplitude (A) and gradient (B) parameters. The forward modelling helps validate the processing steps, give clues as to the non-uniqueness of various A and B responses, and integrates the geology into the study. In the Fylla area, offshore West Greenland, an AVO analysis focused on large flat spots identified on the seismic data, as part of a licence application. A and B analysis was used to delineate trends in the data, to isolate three different classes of AVO anomalies which suggest the presence of hydrocarbons, and to provide insight as to the rock properties of the interbedded sandstones and shales. An AVO analysis was also performed to help evaluate the prospectivity of an area in the Norwegian Hammerfest Basin. The purpose of the study was to establish the expected AVO response for the prospective interval from nearby gas discoveries and determine if similar anomalies exist within the prospect area. The AVO work consisted of forward modelling and A plus B analysis. Subtle AVO effects associated with gas in the Middle Jurassic section were identified.

Origin of low δ 18O, pre-compactional ferroan carbonates in the marine Stø Formation (Middle Jurassic), offshore NW Norway

Siderite (δ 18O PDB= −9.3to−5.7‰) and Fe-dolomite/ankerite (δ 18O PDB= −13.7to−7.7‰) occur as texturally early diagenetic, pre-compactional cements in sandstones and mudstones of the nearshore to inner shelf Stø Formation (Middle Jurassic), Troms I area, Barents Sea. However, these carbonates have anomalously low δ 18O, probably due to alteration of volcaniclastic materials to clay matrix, as well as to clay ooids and peloids through the interaction with marine porewaters. The lowermost δ 18O values could, however, be related to replacement during deep burial and/or to upwelling of hot fluids related to the Jurassic rifting in the Hammerfest Basin. The alteration of volcaniclastic materials also has most likely been an important source of Fe and Mg for the formation of the carbonates.

Three-phase fluid migration with solubilities in a two-dimensional basin simulation model

Three-phase fluid migration with solubilities is examined in a two-dimensional basin simulation model. Darcy's law is applied with the driving forces of excess pressure from disequilibrium compaction, buoyancy and capillary pressure controlling the flow of different fluid phases. Relative permabilities are related to the saturation fractions taken by each fluid phase in the pore space. The test results show that solubilities have a significant impact on both primary and secondary hydrocarbon migration and accumulation. The spatial and temporal patterns of hydrocarbon migration and accumulation with solubility are various, depending on the specific geological, geothermal and geochemical conditions. Because the dynamic features of hydrocarbon migration are tied to phase saturation through relative permeability variations, and to phase solubility through temperature and pressure dependence, this study indicates that the hydrocarbon accumulation history must be considered in exploration assessments of basins. Application of the phase-dependent migration model to a geological section descriptive of the Hammerfest Basin, Barents Sea shows that Tertiary uplift and erosion, which lead to the leakage of hydrocarbons, are probably the dominant causes of the majority of exploration sub-commericial holes in the Hammerfest Basin.

Petroleum geology of the proven hydrocarbon basins, offshore Norway

The Norwegian continental shelf stretches for over 3000 km from 56°N to 82°N and covers an area of more than 1.4 million km2 of which Svalbard and its shelf cover about 300000 km2 (Fig. I). To the north and west the shelf area is limited by the 1500m isobath and the water depth is generally less than 500 m. There are three main petroleum provinces: the North Sea, the Haltenbank area offshore mid Norway and the Hammerfest Basin in the southwest Barents Sea (Fig. 2). In this article we aim to give a brief overview of these areas, describing the hydrocarbon play types developed. Readers interested in further details can consult the selected references quoted in the bibliography (e.g. Glennie 1990).

Variation in stable carbon isotope ratios of individual hydrocarbons as a function of artificial maturity

This paper reports isotopic variations in n-alkanes, isoprenoids and terpenoids from different source rock types as a function of maturity by analysis of these compounds in hydrous pyrolysis products and oil. At high pyrolysis temperatures n-alkanes, isoprenoids and terpenoids in the pyrolysis products become depleted in 12C. The most marked depletion occurs in the terpenoids, particularly in the C 29C 31 hopanes. These compounds are isopically very light in the immature unheated samples of the lacustrine Green River shale (Tertiary) and a Guatemalan carbonate source rock. There is a marked depletion in 12C at 330°C and higher pyrolysis temperatures for these two samples. Moderately mature samples of the Toarcian Paris Basin shale and a Jurassic coal from the Hammerfest Basin showed less or no 12C depletion. Fairly uniform values were recorded for the hopanes in pyrolysates of all samples heated at temperatures of 350°C and above. The isotopic composition of n-alkanes and isoprenoids are fairly similar to the hopanes in samples pyrolysed at the higher temperatures. It appears, therefore, that with increasing maturity n-alkanes, isoprenoids and terpenoids become fairly similar in their carbon isotopic composition. The isotopic compositions within the different compound groups in the different source rocks are sufficiently different for this technique to be used in source rock/oil correlation.

Early Cenozoic sedimentation in the Hammerfest Basin

Using seismic stratigraphy combined with information from 24 wells, a detailed history of the early Cenozoic sedimentation of the Hammerfest Basin in the southwestern Barents Sea has been revealed. During the early Paleocene the Hammerfest Basin was subject to non-deposition or minor erosion, and sedimentation in the basin was initiated after a transgression in the mid-Paleocene. Coeval erosion on the basin margin ceased as the transgression gradually spread in the late Paleocene, and a south-southwesterly sediment input from the Loppa High to the Hammerfest Basin was initiated during the late Paleocene. The late Paleocene/early Eocene environment was dominated by SSW-directed progradation from the Loppa High and westward progradation in the central part of the basin. The early Tertiary sedimentation of the Hammerfest Basin is speculated as mirroring reactivation of older fault regimes, and transpression and transtension along the western margin during the latest phase of rifting and earliest phase of spreading in the Norwegian-Greenland Sea are indicated. Mid-Eocene to Pliocene sediments have not been found in the Hammerfest Basin. It seems, however, that sedimentation continued in the basin in post-early Eocene time. During the Oligocene and Miocene accumulation and erosion alternated in the Hammerfest Basin. The erosion increased during the late Miocene and Pliocene and culminated with the Pleistocene glaciations.

Shallow Bedrock Geology of the Olga Basin Area, Northwestern Barents Sea (1)

The east-west-trending Olga Basin has possibly the largest post-Jurassic sedimentary rock thickness in the northwestern Barents Sea. The thickness of these mainly shallow-marine deposits exceeds 750 m in the study area, and increases eastward. The Olga Basin is characterized by relatively undisturbed layers, which except for local areas, rarely dip more than 3 degrees. The Storbanken area is heavily penetrated by normal faults, which probably are related to the Late Cretaceous uplift in the region. Differential uplift along the flanks of the Olga Basin during the Oxfordian-Hauterivian led to deposition of onlapping units in the central parts, whereas erosion and limited deposition prevailed on the topographic highs. The overlying Barremian sequence is more uniform and is t least 350 m thick. In the Hammerfest Basin in the southern Barents Sea, corresponding Barremian deposits are found. The post-Jurassic strata contain sandstones of three different kinds: subarkose-sublitharenite, litharenite, and lithic arkose. The Barremian subarkoses-sublitharenites have relatively high porosities and may be potential reservoir rocks. The organic geochemistry analyses show somewhat higher hydrogen index values than those found in the corresponding sedimentary strata on Spitsbergen; however, the strata are slightly less mature than the corresponding strata on Spitsbergen.

Tectonic Development and Hydrocarbon Potential Offshore Troms, Northern Norway: ABSTRACT

The study area, offshore Troms, is located from 16 to 25°E long. and 70 to 72°N lat. The geologic border between continental and oceanic crust is defined by a dramatic increase in water depth in the western part of the area. The area with water depth less than 500 m (1,640 ft) covers approximately 6,000 km2 (2,300 mi2). In this area, several highs and basins are defined. The Troms and Hammerfest basins are situated between the Loppa high on the north and the Troms-Finnmark basement high on the south. The southwestern continuation of the Troms basin is a strongly faulted depression, the Harstad basin. Thinned continental crust under the Troms basin is covered with more than a 15 km (9.3 mi) thick sedimentary sequence of Paleozoic and younger rocks. Early Permian evaporites have formed a few large diapiric structures. The main rifting phase in the Troms basin occurred during Late Jurassic-Early Cretaceous. Several thousand meters of Cretaceous claystones were subsequently deposited. During early Tertiary, a second rifting phase resulted in sea floor spreading west of the studied area. During the cooling stage, the southwestern Barents Sea acquired a westward dip. Tertiary sediments onlap the Upper Cretaceous unconformity from the west. Quaternary sediments lie directly on Cretaceous sediments in the eastern part of the area. The north-south-oriented Ringvassoy-Loppa fault complex separates the Troms from the Hammerfest basin. In the Hammerfest basin, the Permian is developed as a shallow shelf carbonate facies. Tertiary and Mesozoic sedimentation was dominated by clastics. The Harstad basin is dominantly filled with Jurassic and younger sediments, and Paleozoic sediments are thought to underlie. Minor amounts of diapirism are present. Several compressional structures, formed by mid-Cretaceous strike-slip forces, are associated with the Troms-Finnmark fault complex. The Loppa high is a triangular-shaped structure covered by relatively thin Jurassic to Quaternary sediments. The southeastern part is an inverted Triassic basin. Shallow shelf carbonates define a good seismic marker in the upper Paleozoic. Seismic control of the area offshore Troms, at present, is generally a 2 by 4 and 2 by 2 km grid. More detailed surveys cover the licensed areas, totaling 2,200 km2 (850 mi2). The first exploratory well was spudded in spring 1980, and 15 wells, dominantly wildcats, have now been drilled. Five gas discoveries have been made in the western Hammerfest basin, two of them with recoverable reserves in the range of 50-100 × 109 Sm3 (1.76-3.52 tcf) gas. All discoveries have been made in Early to Middle Jurassic sandstones. Rocks of Triassic and Permian age will be increasingly important toward End_Page 1206------------------------------ the east and north. In the Troms basin, these sub-Cretaceous rocks are deeply buried, so younger reservoir zones have been drilled. Jurassic and younger rocks will be the main targets in the virgin Harstad basin. The gas has been generated from Lower Jurassic coals and shales. These, and Upper Jurassic shales with oil source potential, will probably also be the source for most hydrocarbons to be discovered in the future. Some deep Paleozoic structures will be dependent on older source rocks. The hydrocarbon potential of the relatively well-known Troms and Hammerfest basins is considered low and moderate, respectively. More promising today are Paleozoic structures on the Loppa high and structures in the Harstad basin. End_of_Article - Last_Page 1207------------

The cretaceous opening of the Northeast Atlantic

A continuous rift system from the Rockall Trough through the Faeroe-Shetland Channel, and the Møre and Vøring basins up to the Tromsø and Bear Island basins is inferred to have developed during the Cretaceous. Oceanic crust was generated in its southern part but its width presumably decreases toward the northeast and probably ends in the Møre Basin. This rift/spreading system requires a clockwise rotation of Greenland and the Rockall Plateau during the Cretaceous around a rotation pole at its northern end at about 74°N 21°E. North of the pole of rotation crustal shortening must have occurred. This compression is interpreted to have taken place on West Spitsbergen. The orogeny there is reinterpreted as 1. (1) a Cretaceous folding phase, 2. (2) a phase of overthrusting at the Paleocene-Eocene boundary, and 3. (3) an extensional faulting phase during the Oligocene. This plate-tectonic model can resolve a series of problems in the Northeast Atlantic region: the connection of the Caledonian fronts of Scotland and Greenland can be easily established. The northern ends of the West Shetland Basin and the North Sea graben; the unusual depth of the Møre and Vøring basins; the marked obliqueness of structural trends between the Northeast Greenland shelf and the Norwegian shelf; the fact that the western Hammerfest Basin was intersected by a younger north-south trending graben which formed the deep Tromsø Basin—all these enigmatic observations can be explained by the Cretaceous rift system.