ALBERT

Description: An integrated multidisciplinary workflow has been implemented for quantitative lithology and fluid predictions from prestack angle gathers and well-log data within the Realgrunnen Subgroup in the Goliat Field, southwestern Barents Sea. We have first performed a qualitative amplitude-variation-with-angle (AVA) attribute analysis to assess the spatial distribution of lithology and fluid anomalies from the seismic data. A simultaneous prestack elastic inversion was then carried out for quantitative estimates of the P-impedance and [Formula: see text] ratio. Probability density functions, a priori lithology, and fluid class proportions extracted from well-log training data are further applied to the inverted P-impedance and [Formula: see text] seismic volumes. The AVA qualitative analysis indicates a class IV response for the top of the reservoir, whereas anomalies from the AVA attribute maps agree largely with the clean sand probabilities predicted from the Bayesian facies classification. The largest misclassification in the lithology classification occurs between shaly sands and shales. A mixed lithology and fluid classification indicates a smaller degree of overlap and allows for the discrimination of hydrocarbon sands. Integration of a qualitative AVA analysis and a quantitative Bayesian probability approach helps in constraining the depositional facies variability within the Realgrunnen Subgroup. Finally, a possible influence of tectonic activity during the deposition of the Realgrunnen reservoir is inferred based on the facies distribution maps.

Description: An integrated innovative multidisciplinary approach has been used to estimate effective porosity (PHIE), shale volume ([Formula: see text]), and sand probability from prestack angle gathers and petrophysical well logs within the Lower Triassic Havert Formation in the Goliat field, Southwest Barents Sea. A rock-physics feasibility study revealed the optimum petrofacies discriminating ability of extended elastic impedance (EEI) tuned for PHIE and [Formula: see text]. We then combined model-based prestack inversion outputs from a simultaneous inversion and an EEI inversion into a multilinear attribute regression analysis to estimate absolute [Formula: see text] and PHIE seismic attributes. The quality of the [Formula: see text] and PHIE prediction is shown to increase by integrating the EEI inversion in the workflow. Probability distribution functions and a priori petrofacies proportions extracted from the well data are then applied to the [Formula: see text] and PHIE volumes to obtain clean and shaly sand probabilities. A tectonic-controlled point-source depositional model for the Havert Formation sands is then inferred from the extracted sand bodies and the seismic geomorphological character of the different attributes.

Description: Seismic sequences are stratigraphic units of relatively conformable seismic reflections. These units are intervals of similar sedimentation conditions, governed by sediment supply and relative sea level, and they are key elements in understanding the evolution of sedimentary basins. Conventional seismic sequence analyses typically rely on human interpretation; consequently, they are time-consuming. We have developed a new data-driven method to identify first-order stratigraphic units based on the assumption that the seismic units honor a layer-cake earth model, with layers that can be discriminated by the differences in seismic reflection properties, such as amplitude, continuity, and density. To identify stratigraphic units in a seismic volume, we compute feature vectors that describe the distribution of amplitudes, texture, and two-way traveltime for small seismic subvolumes. Here, the seismic texture is described with a novel texture descriptor that quantifies a simplified 3D local binary pattern around each pixel in the seismic volume. The feature vectors are preprocessed and clustered using a hierarchical density-based cluster algorithm in which each cluster is assumed to represent one stratigraphic unit. Field examples from the Barents Sea and the North Sea demonstrate that the proposed data-driven method can identify major 3D stratigraphic units without the need for manual interpretation, labeling, or prior geologic knowledge.

Description: Faults may be barriers or conduits for fluid flow in sedimentary basins. The properties of faults, however, depend on stress conditions and rock properties at the time of deformation and subsequent diagenesis of the fault zone. Several recent publications have suggested that petroleum reservoirs in the North Sea and at Haltenbanken, offshore mid-Norway, have experienced leakage along faults caused by imposed stresses, related to glacial loading during the Quaternary. The Jurassic reservoirs in these areas are, however, bounded by faults produced during the Upper Jurassic rifting, when the sediments were still soft and, for the most part, uncemented. These faults do not represent zones of weakness. Because of strain hardening and later diagenesis in sandstones and cementation in mudstones, the fault zones are commonly stronger than the adjacent rocks. They are therefore not likely to be reactivated tectonically. Furthermore, there is little evidence of glacial deformation in the Quaternary sediments overlying these oil fields. It has been proposed that very large horizontal stresses, inferred to be related to periods of glacial loading, caused shear failure at pore pressures below fracture pressure and subsequent leakage along these shear zones. We argue that this is not a likely mechanism during progressive burial in sedimentary basins. Very high horizontal effective stresses, up to 60 MPa, at about 3 km (1.8 mi) depth, at Haltenbanken would have caused more mechanical compaction and grain crushing than that observed in situ. External stress, i.e., plate-tectonic stress from spreading ridges (ridge push), will be transmitted primarily through the basement and not through the much more compressible overlying sedimentary rocks. During progressive basin subsidence, chemical compaction, i.e., caused by quartz cementation, causes rock shrinkage, which will relax differential stresses. This makes brittle deformation (shear failure), resulting in open fractures less likely to occur at stresses below the fracture pressure. In subsiding sedimentary basins with progressive compaction, horizontal stress will normally not exceed the vertical stress except when there is significant shortening of the underlying basement. Knut Bjørlykke received his Ph.D. from the University of Oslo, became a professor of petroleum geology at the University of Bergen (1976–1984), and is presently a professor at the University of Oslo. He has worked on different aspects of sedimentology, sedimentary geochemistry, and petroleum geology. Recently, he has attempted to apply the principles of diagenesis to rock mechanics.Kaare Høeg received his Doctor of Science from the Massachusetts Institute of Technology (United States) in 1965 and was professor of civil engineering at Stanford University (1968–1974). He was managing director of the Norwegian Geotechnical Institute (1974–1991) and is presently a professor at the University of Oslo. He is an elected member of the Norwegian Academy of Science and Letters and the U.S. National Academy of Engineering. Jan Inge Faleide is a professor at the Department of Geosciences, University of Oslo, where he is the leader of the Passive Margin Research Group and coordinator of the new M.Sc. study in petroleum geology and geophysics. He is mainly involved in research projects covering the Norwegian continental shelf and margin and adjacent areas in close collaboration with international partners from academia and industry. Jens Jahren received his M.Sc. degree (1988) and his Ph.D. (1991) from the University of Oslo. He has been an associate professor in geology in the same university since 1994. His research focuses on rock properties with applications to petroleum reservoirs.

Description: Vertical and lateral changes in physical properties in Cenozoic mudstones from the northern North Sea Basin reflect differences in the primary mineralogical composition and burial history, which provides information about sedimentary facies and provenance. Integration of well-log data with mineralogical information shows the effect of varying clay mineralogy on compaction curves in mudstones. The main controlling factor for the compaction of Eocene to early Miocene mudstones within the North Sea is the smectite content, which is derived from volcanic sources located northwest of the North Sea. Mudstones with high smectite content are characterized by low P-wave velocities and bulk densities compared to mudstones with other clay mineral assemblages at the same burial depths. Smectitic clays are important during mechanical compaction because they are less compressible than other types of clay minerals. A comparison between well-log data and experimental work also shows that smectite may be a controlling factor for overpressure generation in the smectite-rich Eocene and Oligocene sediments. At greater burial depths and temperatures (〉70–80°C), the dissolution of smectite and precipitation of illite and quartz significantly increases velocities and densities. Miocene and younger mudstones from the northern North Sea have generally low smectite contents and as a result have higher velocities and densities than Eocene and Oligocene mudstones. Lateral differences in the compaction trends between the north and south for these sediments also exist, which may be related to two different source areas in the Pliocene. The log-derived petrophysical data from the northern North Sea Basin show that mudstone lithologies have very different compaction trends depending on the primary composition. Simplified compaction curves may therefore affect the outcomes from basin modeling. The amplitude-versus-offset response of hydrocarbon sands and the seismic signature on seismic sections are also dependent on the petrophysical properties of mudstones and will vary depending on the mineralogical composition. Øyvind Marcussen received his M.S. degree in 2003 from the University of Oslo. He is presently a Ph.D. student in petroleum geology at the same university. Brit I. Thyberg received her M.S. degree in geology in 1993 from the University of Oslo. She is presently a researcher in petroleum geology at the University of Oslo. Christer Peltonen received his B.S. in geology from California Lutheran University (1996) and his M.S. and Ph.D. in petroleum geology from the University of Oslo, Norway (2007). He is currently working as a development geologist for Venoco Inc. located in Santa Barbara County, California. Jens Jahren received his M.S. degree (1988) and his Ph.D. (1991) from the University of Oslo. He has been an associate professor first in mineralogy and petrology (from 1994) and then in petroleum geology (from 2003) at the same university. His research focuses on mechanical and chemical compaction processes in sediments. Knut Bjørlykke is a professor at the Department of Geosciences, University of Oslo. He has worked in the field of sedimentology and clastic diagenesis. In recent years, he has led a research group on sediment compaction and rock physics at the University of Oslo. Jan Inge Faleide is a professor at the Department of Geosciences, University of Oslo. He has been project leader and principal investigator for several interdisciplinary and international research projects focusing on the formation and evolution of sedimentary basins and continental margins. Most of his studies have been located offshore Norway and conducted in close collaboration with the petroleum industry.

Description: Fault systems in extensional basins commonly display geometries that vary with depth, reflecting depth- and lithology-dependent mechanical strength. Using an experimental approach, we investigate this relationship by deploying physical analog models with stratified sequences consisting of brittle–ductile (sand–silicone polymer) sequences subject to single and polyphase deformation. The experiments were used as analogs for a sandstone sequence interlayered by beds of evaporates or overpressured or unconsolidated mudstone in nature (the latter being representative of decollement horizons).Experiments (series 1 [S1]) using homogeneous and stratified quartz and feldspar sand produced asymmetric, composite single grabens with diverse fault frequencies and fault styles for the graben margin faults.For the mechanically stratified experiments with one decollement level (series 2), contrasting graben configurations were produced, in that the lowermost sequence was characterized by graben geometries of similar type to that of the S1 experiments, whereas the sequence above the decollement was characterized by large fault blocks, delineated by steepened or oversteepened faults.The experiments with two decollements (series 3) were displayed similarly but included graben geometries that widened upward, with each level being characterized by independent fault systems.The results can be used to explain strata-bound fault patterns and depth-dependent extension as seen in several places along the Norwegian continental margin and elsewhere.

Description: Extracting key horizons from seismic images is an important element of the seismic interpretation workflow. Although numerous computer-assisted horizon extraction methods exist, they are typically sensitive to structural and stratigraphic discontinuities. As a result, these computer-assisted methods have difficulties in extracting noncoherent dislocated horizons. We have developed a new data-driven method to correlate, track, and extract horizons from seismic volumes with complex geologic structures. Our method correlates seismic horizons across discontinuities and does not require user input in the form of seed points or prior identification of faults. Furthermore, the method is robust toward amplitude changes along a seismic horizon and does not jump from peak to trough or vice versa. We use a large sliding window and match full-length seismic traces using nonlocal dynamic time warping to extract grids of correlated points for our target horizons. Through computed accuracy measurements, we discard nonaccurate correlations before interpolating complete seismic horizons. Because our method does not require manually picked seed points or prior structural restoration, it does not rely on interpretive experience or geologic knowledge. The proposed method is applied on different real and complex seismic images, with two case examples from the southwestern Barents Sea, and one on the open source Netherlands F3 seismic data.