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Mass balance method for quantifying average erosion in the source area

Mass balance method for quantifying average erosion in the source area

Presentation at the conference "Vinterkonferansen", 7. - 9.01.19, Bergen, arranged by NGF (Norsk Geologisk forening). https://www.geologi.no/konferanser/vinterkonferanser/item/947-vk19.

The circum North Atlantic-Arctic continental margin and adjacent land areas have experienced several episodes of uplift and erosion during the Cenozoic. A series of efforts quantifying this erosion for the Barents Sea shelf, where the Arctic shelf is at its widest and deepest have been done since the early 90’s using different methods. As the seismic and well database have expanded considerably, our understanding of the Cenozoic evolution of this climatic sensitive and hydrocarbon prospective area has improved. This review includes a comparison of results from different methods (e.g. the mass balance technique,shale compaction, apatite fission track, sandstone diagenesis, and seismic velocities). The Cenozoic erosion is divided into a pre-glacial and a glacial erosion. The pre-glacial erosion is related to the early Cenozoic tectonics and riftflank uplift due to the onset of rifting, shear, and compression followed by sea-floor spreading between Norway and Greenland, whereas the glacial erosion occurred during the late Cenozoic Northern Hemisphere Glaciations when grounded ice sheets repeatedly covered the Barents Sea shelf. The different methods generally show the same order of magnitude of erosion for the major source areas in the Barents Sea, i.e. northern Norway, the Loppa High, the Stappen High, Svalbard, and from the northern Barents Sea margin. Furthermore, we compare sediment load and size of drainage area from various settings and different periods. For similar size of drainage area, sediment load for glacial period is generally higher than for the pre-glacial one. Our review shows that the ratio between the Cenozoic pre-glacial and glacial sedimentation along this part of the Arctic margin is ~40%, ~50%, and ~70% for the southwestern, northwestern, and the northern Barents Sea, respectively. Thus, there is a N-S trend of increasing pre-glacial erosion of the Barents Sea shelf, whereas an W-E trend of increasing erosion is inferred for the glacial period. Future directions of research in refining the erosion estimates and better understanding the mechanisms of uplift and erosion will be addressed.

Accepted manuscript version. Published version available at https://doi.org/10.1016/j.jog.2018.03.004. Accepted manuscript version, licensed CC BY-NC-ND 4.0. The evolution of the northwestern Barents Sea continental margin, part of a NW-SE trending mega shear zone, has been reconstructed in order to quantify the sedimentation and erosion affecting this area during and after its formation in the Paleogene–Neogene. This development was closely related to the sea-floor opening of the Norwegian-Greenland Sea. Our study incorporated 2D seismic data, well data, and information from shallow cores. During the Paleocene–Eocene, the northwesternmost Barents Sea margin was subjected to compression-transpression that led to the development of the West Spitsbergen Fold-Thrust Belt (WSFTB) and largely affected the northern part of the study area. To the south, the Vestbakken volcanic province developed in a pull-apart setting. A transition zone separates these two areas marked by a basin morphology becoming more pronounced to the south suggesting increasing subsidence and extension. Subsequently, during the Oligocene, extension and sea-floor spreading were initiated along the whole margin, resulting in the opening of the Fram Strait between Spitsbergen and NE Greenland in the Miocene. During the Paleocene, the Stappen High and a part of the NE Greenland shelf sourced sediments into the newly developing basins. The southwestern part of the WSFTB, the Stappen High, and part of the NE Greenland margin are interpreted as the main sediment source areas in the Eocene. During the Oligocene and Neogene, a larger part of the northwestern Barents Sea shelf is interpreted to have acted as source area including the Edgeøya platform. As a result of this development, the wider Barents Sea shelf itself is inferred to have been a lowland prior to the northern hemisphere glaciations. We found that the average sedimentation rate for the Paleogene–Neogene at the northwestern Barents Sea margin is about 0.034 m/k.y. This number is in agreement with the sedimentation rate reported from present-day fluvial systems and modern rates coastal erosion. By using a mass-balance approach, we have also estimated the average net erosion and erosion rate for the Paleogene–Neogene period to be ∼2440 m and 0.038 m/k.y, respectively. This erosion rate is two times higher compared to the southwestern Barents Sea margin, probably reflecting erosion of a more tectonically active northwestern margin. Thus, for the western Barents Sea margin, a general increasing trend of pre-glacial erosion northwards can be inferred. This study also suggests that more than half of the Cenozoic erosion affecting the studied part of the northwestern Barents Sea were of pre-glacial origin.

The papers 1, 2 and 3 are not available in Munin.Paper 1: Lasabuda, A., Laberg, J. S., Knutsen, S.-M., Høgseth, G. Early to middle Cenozoic paleoenvironment and sediment yield of the southwestern Barents Sea continental margin: Insights from a regional mass-balance approach. (Manuscript). Accepted manuscript version available in Marine and Petroleum Geology, June 2018 Paper 2: Lasabuda, A., Laberg, J. S., Knutsen, S.-M, Safronova, P. A. Cenozoic tectonostratigraphy and pre-glacial erosion: A mass-balance study of the northwestern Barents Sea margin, Norwegian Arctic. (Manuscript). Accepted manuscript version available in Journal of Geodynamics, Special Arctic Issue 2018. Paper 3: Lasabuda, A., Geissler, W. H., Laberg, J. S., Knutsen, S.-M, Rydningen, T. A., Berglar, K. Late Cenozoic glacial sediment input to the Arctic Ocean – quantifying the contribution from the Barents Sea. (Manuscript). Final version with altered title, published in Geochemistry, Geophysics, Geosystems, 19(12), 4876-4903, is available at https://doi.org/10.1029/2018GC007882 The Barents Sea shelf has been experienced extensive uplift and erosion in the Cenozoic. However, the pre-glacial erosion has been so far less constrained in comparison to the glacial erosion. This paper aims to better understand the early–middle Cenozoic erosion in the western Barents Sea and the late Cenozoic erosion in the northern Barents Sea by using the mass-balance approach. The study utilizes seismic and well data to map the Cenozoic sediments and to estimate their volumes. The paleoenvironmental reconstruction is used to delineate the size of the likely source areas. From this, the average erosion and erosion rates are estimated. It is important to quantify the erosion as an effort to lower the risk in petroleum exploration in the Barents Sea. This study is also essential to understand the sedimentation pattern and their relation to the tectonic development. The paper suggests an N-S trending of uplift and erosion in the early–middle Cenozoic and E-W trending of uplift and erosion in the late Cenozoic. For the first time, the glacial erosion is estimated in the northeastern Svalbard/northern Barents Sea continental margin, which reflects the sediment input to the Arctic Ocean. The average erosion rates show one order of magnitude lower than the glacial erosion rates. The estimated numbers from this thesis are compared to the other high-latitude margins and in agreement with present-day systems.

High resolution 3D seismic dataset has been used to examine gravity-driven deposits from the Plesitocene to present-day in the structurally complex area of the western Nile deepwater. The underlying Messinian salt and its associated faults and the existence of fluid-migrated features generated local seabed topography that affected sedimentation and seismic facies distribution. Seismic interpretation combined with planform analysis of attribute maps were utilized to define five seismic facies which are distinguished by amplitude strength, reflections, and geometry. The observation of the temporal and spatial relationship of five seismic units using isochron map analysis offers an insight into seismic stratigraphy development which can be described in three main phases. The first phase is characterized by the deposition of two main mass transport deposits (seismic unit 1 and 2). The second phase is defined by the development of channelized-fan deposit (seismic unit 3). The final phase is characterized by the deposition of near-seabed MTD and present-day channel-levee system (seismic unit 4) and the development of major mud diapirs (seismic unit 5). Spectrally decomposed and colour-blended seismic volume analysis combined with seismic facies interpretation in seismic cross-section have been useful to morphologically investigate and quantify three submarine channels and their associated elements (e.g. thalweg and erosional depth longitudinal profile, levee width and thickness). The results revealed an architectural element variation and the channel pathways with respect to local seabed topography. Increases in sinuosity are related to decreases in channel axis gradient and followed by evidence of channel aggradation Levee thickness development shows an overall decrease in thickness and width downslope with local variations due to structural bathymetry changes (e.g thickness increase towards the hanging wall of the main fault). Overall a change downslope from degradational to aggradational following equilibrium profile is observed. This is evidenced by a deeply incised channel erosional depth in the upperslope and increasing in sinuosity in the lower slope. Sinuous form is observed when the channel experiences a shallow gradient and availability of accommodation space followed by the development of lateral accretion packages (LAPs). Local structural growth (e.g. minibasin and anticline) has a big influence on the channel path geometry. Submarine channel deposit is often a prominent reservoir target in many deepwater explorations around the world. A near-seafloor study with a higher seismic resolution may give an analogue to image turbiditic reservoir architecture. Therefore, this study improves the understanding in the interaction of sedimentation and structural variations which can be applied to hydrocarbon exploration to predict control and distribution of deepwater turbiditic reservoirs. Master i Geovitenskap MAMN-GEOV GEOV399

A meteor impact event on the Bjameland Platform in the Norwegian Barents Sea about 142 ± 2.6 million years ago formed the Mjølnir Impact Crater. The structure has diameter of 40 km impacting the Mesozoic stratigraphy, and has previously been investigated with emphasis on the dynamics of the impact, the structure formation, sedimentation and post-impact tectonics. Impact structures have proven to be petroliferous elsewhere in the world. By applying new seismic 2D- and P-cable data and integrating prior studies, this thesis focuses on how the structure of the Mjølnir Impact Crater might represent a petroleum system in the Barents Sea. The structural setting of the Mjølnir impact Crater shows several potential structural traps, including horst and graben structures within the crater, and listric rim faults at the periphery. A simplified 1D maturation model of potential source rocks in the area, indicate that the Hekkingen and Steinkobbe formations are sufficiently matured and have generated oil, while the Lower Triassic formations have generated gas. These source rocks may potentially charge the Mjølnir Impact Crater by petroleum migration vertically along deep-seated faults and laterally in the southward dipping stratigraphy, from the Nordkapp Basin area. The southern section of the Mjølnir Impact Crater is considered have the highest hydrocarbon potential, due to a thicker overburden and favorable migration pathways. Five seismic anomalies of high amplitude have been identified in the southern Mjølnir Impact Crater. These are interpreted to represent hydrocarbon accumulations, structurally trapped in horsts and by listric rim faults, and overlain by Upper Jurassic shales with sealing properties. Volumetric calculations for one of the amplitude anomalies suggest that 24,4 MMSm3 of oil may be accumulated at the southern boundary of the crater. In conclusion, presence of source rocks, migration pathways, reservoir and traps supported by observations of seismic amplitude anomalies suggests that the Mjølnir Impact Crater represents a potential hydrocarbon system.

During initial exploration efforts in the Barents Sea several gas prone and underfilled prospects were drilled which significantly dropped the interest in the area. The Hammerfest Basin however turned out to hold significant hydrocarbon volumes. Although still mostly underfilled, the discoveries here today make up the Snøhvit oil and gas field. The Hammerfest Basin is an overfilled petroleum basin, meaning that the several source intervals have produced enough hydrocarbons over time to fill all the traps. This, and the hydrocarbon shows below the hydrocarbon-water contacts inside the traps indicate that the reason behind underfilled structures is leakage. This means that the main exploration risk in the area is trap integrity. Given the large amount of evidence for fault-assisted remigration, this thesis focuses on explaining the hydrocarbon column heights observed today in these structures with the help of fault valve theory. The theory suggests that a subset of faults oriented at the right angles to the stress field can control the pore pressures in a pressure compartment in a dynamic equilibrium. When pressure increases with hydrocarbon charge, or gas dissolution the effective stress drops which can reactivate the faults leading to the leakage of hydrocarbons from the trap. In this study, the slip tendencies of faults around the Albatross, Snøhvit and Askeladd structures were calculated and plotted on the interpreted fault planes. This gave the critical pressure perturbations of the faults, i.e. the pressure increase needed to cause slip on them. Faults and fault segments with low critical pressure perturbations are regarded as areas of high risk of leakage. After identifying high risk segments, their intersection with the top reservoir was found and compared with the level of hydrocarbon-water contacts. On Albatross Sør and Snøhvit Nord high risk faults and fault intersections intersect the top reservoir on the level of the gas-water contact. On Albatross and Snøhvit, both of which have leaked considerably, high risk faults were identified near the culmination of the trap. Askeladd, Askeladd Vest and Askeladd Gamma which are not as severely underfilled, the fault planes show higher critical pressure perturbations. Delta Vest, an empty structure also has higher pressure perturbations, however it has a fault intersection right at the culmination. Askeladd Beta, the second dry structure showed higher risk on a fault directly at the top of the structure.