Rock-physics templates (RPT), in combination with seismic AVO inversion data, can be used to screen for hydrocarbon prospects during exploration. With the improved quality and increased use of elastic seismic inversion, there has recently been a paradigm change in prospect mapping in the oil industry, and quantitative interpretation has become a widely used jargon. Rock-physics models are essential in that they help in converting elastic parameters from inversion data to reservoir parameters. In the screening phase of inversion data, rock-physics models also can reveal hydrocarbon-associated anomalies. Two new rock-physics attributes help in detecting hydrocarbons from seismic—the curved pseudo-elastic impedance and the trend angle. The first of these is similar to the extended elastic impedance or the fluid factor in that it represents a deviation from a wet-background trend in a rock-physics template. However, it honors the nonlinear nature of a compaction trend. The trend angle is a measure of slope angle between two adjacent data points in the AI-versus-VP/VS crossplot, and this attribute will highlight fluid trends in the data. Those two attributes can be used complementarily to detect and highlight hydrocarbon accumulations, as demonstrated on data from the Norwegian shelf.
AVO inversion of prestack seismic data, constrained by geologic knowledge and rock-physics modeling, is an essential technology that can greatly reduce interpretation risk during exploration. However, in spite of its great potentials, this technology suffers from a wide range of pitfalls and many uncertainties. In this study, we demonstrate the use of simultaneous AVO inversion in an area of the Norwegian Sea where several proven discoveries with strong AVO anomalies are present, all of which are structural traps. The inversion successfully delineated these discoveries, and a pure blind test of a prospect on a structural high correctly predicted the presence of hydrocarbons. The AVO inversion also gave support for a stratigraphic trap in a graben setting between the prolific structural highs. However, this prospect turned out to be a failure, as the observed AVO anomaly was false. Postdrill analysis showed that a thin, very hard, calcareous event right above the target had created a refraction that interfered with the target horizon below, creating a false AVO class II to III anomaly. The target interval was predominantly a very soft, thick, and immature organic-rich shale, which normally should show a class IV AVO anomaly. Moreover, the low-frequency model used in the inversion was derived from wells on structural highs combined with interval velocities and turned out to not be representative in the graben areas. The key learnings from this study are not to use angle ranges exceeding about 40° during AVO inversion in this area and, in general, to be careful extrapolating the low-frequency model away from well control, even when interval velocities are used, especially in areas with complex geology. More integration between geology and geophysics is necessary during all steps of the AVO inversion to avoid such pitfalls.
In some areas of the North Sea there are irregular highamplitude events that some call "gull-wing" reflections because of their characteristic appearance on seismic sections. The Oseberg area, the site of a recently acquired ocean-bottom cable (OBC) 3D survey, is severely affected by these anomalies, which are scattered over the area at a depth of about 1.5 km. Some gull-wing anomalies have been drilled and are seen in log data to have a thickness of up to about 50 m and a P-velocity of around 5500 m/s. The high velocity of these irregular features causes significant distortion in depth imaging of Oseberg seismic data, causing both vertical and lateral displacement of deeper events. Different approaches were explored to resolve the high-velocity anomalies during depth imaging of the OBC 3D data. One method that has proven successful is to use results of prestack AVO inversion to insert anomalies in the velocity model. Another method is high-resolution common-image-point (CIP) tomography using offset-vector-tile (OVT) input, which is also able to resolve a smooth representation of the gull-wing anomalies. Both of these methods reduce the distortion caused by the gull-wing anomalies and give improved depth-imaged results.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.