ALBERT

Description: The 3D basin modelling of the Southwestern Barents Sea was planned with the aim of addressing the masses of petroleum generated, migrated, accumulated and lost during the basin evolution. The first model was constructed for the Hammerfest Basin considering three source rocks, which correspond to the Upper Jurassic Hekkingen Formation and the Triassic Snadd and Kobbe formations. The highest maturities for the three source rocks were reached in the western and northwestern margin of the basin. The model reproduced satisfactorily the hydrocarbon phases and distribution of the main fields and discoveries. Two events of petroleum re-distribution occurred in the basin: the first corresponds to the oil re-distribution (during the Oligocene–Miocene); the second corresponds to the gas leakage (during the Pliocene–Pleistocene) in connection to the glacial–interglacial cycles. At least 0.247 Gt of thermogenic gas leaked from the main reservoir and reached the sediment interface. The analysis of the volumetric proportions of oil and gas contributions to each field and discovery, suggest that the gas contribution stems mainly from Triassic source rocks, while the oil phases contain variable proportions from both the Jurassic Hekkingen Formation and the Triassic source rocks. Available fluid geochemical data from the main fields in the Hammerfest Basin allowed testing these results. The interpretation of gas isotopes and maturity related biomarker ratios confirms the maturity trends derived from basin modelling; and light hydrocarbons indicate the influence of secondary processes. However, age related biomarker ratios did not provide a clear separation when evaluating a contribution from Jurassic versus Triassic source rocks. The 3D basin modelling was extended to include the Loppa High as well as some other important frontier exploration areas; taking into account the same source rocks. Calibrated model predictions indicate that the three source rocks are overmature in the western margin and also have high maturities in the deepest parts of the Maud Basin to the east. However, in the Bjarmeland platform, only the Triassic source rocks have entered the oil window. Recent generation has been observed in the eastern part around the Bjarmeland Platform and generative potential is still available at present–day. The timing of generation in the western part is different in comparison to the east, with the Kobbe Formation starting to generate during the Late Triassic–Early Jurassic, the Snadd Formation during Late Jurassic–Early Cretaceous and the Hekkingen Formation during Middle Cretaceous. The three source rocks do not have any generative potential left; therefore, it is necessary to rely on younger source rocks. Additional results indicate that the main drainage directions do not change drastically during the evolution of the area, not even during the glacial–interglacial cycles. The model output shows changes in the sizes of the relative oil versus gas quantities in the modelled accumulations during the glacial cycles.

Description: In this study Monte Carlo solutions to the radiative transfer equations are used to model translational and rotational motion seismogram envelopes in random elastic media with deterministic background structure assuming multiple anisotropic scattering. The results of the Monte Carlo radiative transfer theory simulations are verified by comparisons with 3D full wave field finite difference simulations. The observation and modeling of the three additional components of rotational ground motions can provide independent information about seismic wave propagation in the Earth’s structure. Rotational motions around the vertical axis observed in the P-wave coda are of particular interest as they can only be excited by horizontally polarized shear waves and therefore indicate the conversion from P- to SH-energy by multiple scattering at 3D-heterogeneities. Scattering and attenuation parameters in south-east Germany beneath the Gräfenberg array and in the Vogtland region are estimated by comparisons of synthesized multi-component seismogram envelopes to seismic data from local and regional swarm earthquakes and to teleseismic events. In a first step, frequency dependent scattering and attenuation parameters from a local data set are estimated for the Vogtland region using nearby swarm earthquakes. The results from the elastic simulations are compared to outcomes from acoustic radiative transfer simulations. Both methods yield similar results and suggest that intrinsic attenuation dominates scattering attenuation. From the elastic simulations it is observable, that forward scattering is required to explain the data. However, the amount of forward scattering strength remains unresolvable. In a second step scattering and attenuation parameters beneath the Gräfenberg array are estimated using a nonlinear genetic inversion of seismogram envelopes from regional events at high frequencies (4–8 Hz). The preferred model of crustal heterogeneity consists of a random medium described by an exponential auto correlation function with a transport mean free path length of ∼ 420 km. The quality factor for elastic S-wave attenuation Q iS is around 700. In a final step simulations of teleseismic P-wave arrivals, using this estimated set of scattering and attenuation parameters, are compared to observed seismogram envelopes from deep events. Simulations of teleseismic events with the parameters found from the regional inversion show good agreement with the measured seismogram envelopes. This includes ringlaser observations of vertical rotations in the teleseismic P-wave coda that naturally result from the proposed model of wave scattering. The model also predicts, that the elastic energy recorded in the teleseismic P-coda is not equipartitioned, unlike the coda of regional events, but contains an excess of shear energy. The combined results from the three different data sets suggest that scattering generating the seismic coda mainly occurs in the crustal part of the lithosphere beneath the receivers. The observations do not require scattering of high frequency waves in the mantle, but weak scattering in the lithospheric mantle cannot be ruled out.

Description: In March 2010, the project CoCoCo (incipient COntinent-COntinent COllision) recorded a 650 km long amphibian N-S wide-angle seismic profile, extending from the Eratosthenes Seamount (ESM) across Cyprus and southern Turkey to the Anatolian plateau. The aim of the project is to reveal the impact of the transition from subduction to continent-continent collision of the African plate with the Cyprus-Anatolian plate. A visual quality check, frequency analysis and filtering were applied to the seismic data and reveal a good data quality. Subsequent first break picking, finite-differences ray tracing and inversion of the offshore wide-angle data leads to a first-arrival tomographic model. This model reveals (1) P-wave velocities lower than 6.5 km/s in the crust, (2) a variable crustal thickness of about 28 - 37 km and (3) an upper crustal reflection at 5 km depth beneath the ESM. Two land shots on Turkey, also recorded on Cyprus, airgun shots south of Cyprus and geological and previous seismic investigations provide the information to derive a layered velocity model beneath the Anatolian plateau and for the ophiolite complex on Cyprus. The analysis of the reflections provides evidence for a north-dipping plate subducting beneath Cyprus. The main features of this layered velocity model are (1) an upper and lower crust with large lateral changes of the velocity structure and thickness, (2) a Moho depth of about 38 - 45 km beneath the Anatolian plateau, (3) a shallow north-dipping subducting plate below Cyprus with an increasing dip and (4) a typical ophiolite sequence on Cyprus with a total thickness of about 12 km. The offshore-onshore seismic data complete and improve the information about the velocity structure beneath Cyprus and the deeper part of the offshore tomographic model. Thus, the wide-angle seismic data provide detailed insights into the 2-D geometry and velocity structures of the uplifted and overriding Cyprus-Anatolian plate. Subsequent gravity modelling confirms and extends the crustal P-wave velocity model. The deeper part of the subducting plate is constrained by the gravity data and has a dip angle of ~ 28°. Finally, an integrated analysis of the geophysical and geological information allows a comprehensive interpretation of the crustal structure related to the collision process.

Description: The Barents Sea is a frontier for hydrocarbon exploration where activity has been renewed after recent oil discoveries. However, previously this province has been dominated by gas finds, with the largest discoveries being Snøhvit, Albatross and Askeladd gas fields, located in the Hammerfest Basin. Cenozoic erosion and high latitude Quaternary glaciations are thought to have driven the hydrocarbons out of the traps and contribute thus to the lack of significant oil discoveries. Hydrocarbon leakage is a widespread phenomenon and has significant impact on climate, marine ecosystem, geotechnical installations and petroleum exploration. In this study, we aim to elucidate the impact of Cenozoic erosion and Pliocene-Pleistocene glaciations on the dynamics of hydrocarbon leakage from the thermogenic reservoirs. We use high resolution and vintage 3D seismic reflection datasets to analyse hydrocarbon plumbing system above the Snøhvit and Albatross gas fields to investigate the geo-morphological manifestation and the dynamics of leakage from the reservoir. We then use 3D Petroleum Systems Modelling (PSM) to simulate the basin history in terms of generation, migration and leakage of hydrocarbons through time in response to erosion, glacial loading and deglaciations. Based on this integrated approach, we then are able to compare numerical modelling results with seismically observed leakage indicators. Numerous EW trending reactivated faults are present in the study area which link the Jurassic hydrocarbon reservoirs of the Snøhvit and Albatross field with the shallow Paleocene strata. Reactivation of polygonal fault networks has formed an interconnected network of Paleocene faults, which served as migration avenues for thermogenic fluids in the vicinity of deep reactivated tectonic faults. Numerous pockmarks and mega pockmarks on the seabed and buried pockmarks on the base Quaternary Upper Regional Unconformity (URU) provide evidence of migration pathways as they are connected to seismic blow out pipes, Paleocene fault networks and deep reactivated tectonic faults. A gas cloud anomaly has been interpreted as a Bottom Simulating Reflector (BSR), whose depth coincides with the estimated base of the hydrate stability field for a thermogenically-derived gas hydrate with around 90 mol % methane. At least two fluid venting episodes have been inferred based on seabed and URU pockmark distributions, following the Last Glacial Maximum ~17-16 ka and prior to the Late Weichselian, older than ~0.7 Ma. Results of the 3D PSM modelling show that hydrocarbon leakage from the Jurassic reservoirs takes place through faults during each deglaciation, with most of accumulated mass lost (60-80 %) during the first instance of fault dilation. Subsequent leakage during deglaciations results in a sequential loss of remaining accumulated mass in the Snøhvit reservoir. The first modeled leakage event (0.8-0.78 Ma) coincides with a major fluid escape event at the time of a major regional unconformity (URU older than ~0.7Ma), and is in agreement with shallow subsurface hydrocarbon leakage indicators such as pockmarks, shallow gas clouds and blow out pipes observed in the seismic data analysis.