ALBERT

Description: 〈span〉〈div〉ABSTRACT〈/div〉Using 3D broadband seismic data, we have investigated low-frequency seismic amplitude anomalies associated with and below various geologic formations in the Johan Sverdrup oil field situated in the central North Sea. Low-frequency anomalies are observed below the Intra-Draupne and the Heather and Hugin reservoirs, at the Svarte and Tor Fms, and below shallow channels. The Intra-Draupne Fm is the main oil reservoir of interest in the field, and it is relatively homogeneous, with observed low Q values of approximately 25–30. The Heather and Hugin reservoirs, which underlie the Intra-Draupne, are heterogeneous and oil bearing. We performed three-layer elastic modeling on a simple reservoir model based on the properties of the Intra-Draupne Fm, and the results suggest that as the thickness of the middle oil-bearing layer increases the dominant frequency decreases due to the tuning effect. The Svarte and Tor Formations are shallower in the section and are fractured. Low-frequency anomalies associated with these formations seem to indicate a clear correlation with zones of increased fracture density interpreted from high-resolution most-negative-curvature attribute maps. Low-frequency anomalies are also observed below shallow gas channels consisting of stratified sandy and shaly intervals with vertical variations in porosity and permeability. In addition, stacking tests using coarse and fine velocity analysis indicate no noticeable difference in the characteristics of the low-frequency anomalies, in general, at all levels. We conclude, therefore, that the observed low-frequency anomalies are unrelated to stacking issues.〈/span〉

Description: Our aim was to identify some of the characteristics of low-frequency anomalies. Specifically, we have looked, in 3D broadband data from the North Sea, for any offset dependence in these anomalies and any frequency-related change in normal moveout (NMO) velocity that could influence stacking power over different frequencies. After high-resolution spectral decomposition, two types of low-frequency anomaly have been identified associated with hydrocarbon-bearing reservoirs: (1) at the reservoir top and (2) below the reservoir, with a time delay of approximately 100–200 ms. Both types of anomalies indicate offset dependence. On the near-offset stacks, they are relatively strong, but they tend to be absent on the far-offset stacks. In addition, horizon velocity analysis, which was performed along the horizons picked at the tops of reservoir and nonreservoir intervals, has revealed frequency-dependent NMO velocity. For nonreservoir events, we found no significant difference between the NMO velocities for the low-frequency and high-frequency filtered common-midpoint gathers. However, along the anomalously low-frequency events observed at the tops of, and below, oil-bearing reservoirs, lower velocity is observed for low-frequency and higher velocity for high-frequency filtered gathers. If these properties turn out to be universally typical, increased understanding and inclusion of them could lead to improved workflows and help increase the reliability of low-frequency analysis as a hydrocarbon indicator.

Description: The 1300 km 3 tholeiitic lava flow field of the 14.98 Ma Roza Member of the Columbia River Basalt Province has the best-preserved vent system of any known continental flood basalt. Detailed geological mapping and logging of the exposed pyroclastic rocks along the 〉180-km-long vent system enable the reconstruction of pyroclastic edifices (partial cones) built around vents. The pyroclastic cones differ from those constructed during typical basaltic effusive eruptions and are characterized by low to moderate slope angles (〈20°), have minimum heights up to ~160 m, and are composed of dominantly coarse-grained, moderately to densely agglutinated and welded spatter and scoria that extend up to 750 m away from the vent. Thick, well-sorted fall deposits composed of moderately to highly vesicular scoria lapilli extend away from some vents and exhibit some characteristics comparable to the proximal deposits of violent Strombolian or subplinian eruptions. The recorded volcanic activity does not fit with common eruption styles of basaltic magmas, and the evidence indicates that the Roza eruption was punctuated by eruptive activity of unusually high intensity that was characterized by vigorous lava fountains. The extensive agglutinated deposits accumulated around the vents as a result of fallout from high (〉1 km) fountains enhanced by fallout from the lower parts of convective plumes that rose above the fountains.

Description: We assess bias in the record of kimberlite volcanism by using newly acquired size data on more than 900 kimberlite bodies from 12 kimberlite fields eroded to depths of between 0 m and 〉1200 m, and by a comparison with intraplate monogenetic basaltic volcanic fields. Eroded kimberlite fields are composed of pipes (or diatremes) and dikes, and within any one kimberlite field, regardless of erosion level, kimberlite bodies vary in area at Earth’s surface over 2–3 orders of magnitude. Typically 60%–70% of the bodies are 〈10% the area of the largest pipe in the field. The maximum size of a kimberlite pipe found in a field shows a relationship with estimated erosion levels, suggesting that the erosion level of a region could be used to predict the maximum potential size of a pipe where it intersects the surface. The data indicate that the selective removal of surface volcanic structures and deposits by erosion has distorted the geological record of kimberlite volcanism. Selective mining of preferentially large, diamondiferous kimberlite pipes and underreporting of small kimberlite pipes and dikes add further bias. A comparison of kimberlite volcanic fields with intraplate monogenetic basaltic volcanic fields indicates that both types of volcanism overlap in terms of field size, volcano number and size, and typical erupted volumes. Eroded monogenetic basaltic fields consist of dikes that fed effusive and weakly explosive surface eruptions, and diatremes (pipes) generated during phreatomagmatic eruptions, and they are structurally similar to eroded kimberlite fields. Reassessment of published data suggests that kimberlite magmas can erupt in a variety of ways and that most published data, taken from the largest kimberlite pipes, may not be representative of kimberlite volcanism as a whole. This refuels long-standing debates as to whether kimberlite pipes (diatremes) primarily result from phreatomagmatic eruptions (as in basaltic volcanism) or from volatile-driven magmatic eruptions.