Abstract Predicting deformation-driven permeability changes in the subsurface requires knowledge of the character and distribution of dilatant and compactant rock damage. Seismic reflection data can be used to gain insight into aspects of the deformation such as the geometry of seismically resolvable faults and bulk material property distributions. However, interpretations of material properties from seismic data are non-unique. This paper addresses the use of established seismic techniques to identify the signatures of fault-associated open fractures, modified and improved by a new linked geomechanics–seismic approach. The paper also addresses how each of stress state and open fractures affect seismic anisotropy. The geomechanics–seismic approach is demonstrated using a model of a North Sea hydrocarbon field in which a series of potential fracture arrays are assumed and the fracture apertures are modified to reflect the geomechanically generated stress states. Seismic anisotropy predictions based on these modified fracture distributions are then compared with a pre-existing seismic anisotropy interpretation to determine the best match. Using geomechanical simulation to support a seismic anisotropy-based method produces a higher-confidence result and can lead to better prediction of altered permeabilities in faulted regions. Because of the geomechanical focus of this Special Publication, the background for seismic identification of faults and inter-fault damage is also outlined, including a review of current seismic practice.

Abstract Improving the accuracy of subsurface imaging is commonly the main incentive for including the effects of anisotropy in seismic processing. However, the anisotropy itself holds valuable information about rock properties and, as such, can be viewed as a seismic attribute. Here we summarize results from an integrated project that explored the potential to use observations of seismic anisotropy to interpret lithological and fluid properties (the SAIL project). Our approach links detailed petrofabric analyses of reservoir rocks, laboratory based measurements of ultrasonic velocities in core samples, and reservoir-scale seismic observations. We present results for the Clair field, a Carboniferous–Devonian reservoir offshore Scotland, west of the Shetland Islands. The reservoir rocks are sandstones that are variable in composition and exhibit anisotropy on three length-scales: the crystal, grain and fracture scale. We have developed a methodology for assessing crystal-preferred-orientation (CPO) using a combination of electron back-scattered diffraction (EBSD), X-ray texture goniometry (XRTG) and image analysis. Modal proportions of individual minerals are measured using quantitative X-ray diffraction (QXRD). These measurements are used to calculate the intrinsic anisotropy due to CPO via Voigt-Reuss-Hill averaging of individual crystal elasticities and their orientations. The intrinsic anisotropy of the rock is controlled by the phyllosilicate content and to a lesser degree the orientation of quartz and feldspar; the latter can serve as a palaeoflow indicator. Our results show remarkable consistency in CPO throughout the reservoir and allow us to construct a mathematical model of reservoir anisotropy. A comparison of CPO-predicted velocities and those derived from laboratory measurements of ultrasonic signals allows the estimation of additional elastic compliance terms due to grain-boundary interactions. The results show that the CPO estimates are good proxies for the intrinsic anisotropy of the clean sandstones. The more micaceous rocks exhibit enhanced anisotropy due to interactions between the phyllosilicate grains. We then compare the lab-scale predictions with reservoir-scale measurements of seismic anisotropy, based on amplitude variation with offset and azimuth (AVOA) analysis and non-hyperbolic moveout. Our mathematical model provides a foundation for interpreting the reservoir-scale seismic data and improving the geological modelling of complex reservoirs. The observed increases in AVOA signal with depth can only be explained with an increase in fracturing beneath the major unit boundaries, rather than a change in intrinsic CPO properties. In general, the style and magnitude of anisotropy in the Clair field appears to be indicative of reservoir quality.

Abstract Geomechanical simulations are used to demonstrate the importance of the way that models are loaded. In this paper the development of permanent damage during faulting using frictional-slip models of a reverse fault is investigated. Although the use of different loads and constraints can produce the same faulted geometry (for the same rock type, and at the same burial depth), the models develop very different stress and strain states. Permanent strain magnitudes and distributions between models are quite dissimilar, including the distributions of permanent dilation and compaction. This work demonstrates that boundary loads and boundary constraints are significant factors in determining what stress and deformation states evolve in the simulation model. The examples also illustrate that final (deformed) geometry alone is a very poor basis from which to predict either stress state or open fracture distribution. Bulk finite strain does not allow a prediction of local principal stress directions, magnitudes, or signs, at least in the vicinity of fault damage zones.

Abstract The concepts of damage mechanics are proposed as the logical basis for integration of seismic anisotropy based identification and geomechanics based prediction of subsurface fracturing. In such a context a damage tensor can be used to describe the evolution of (anisotropic) degradation of the elastic properties of a rock undergoing deformation. The concepts of damage mechanics are common to both geomechanical and seismic descriptions of rock elasticity but have not previously been connected. In geomechanical simulations of the evolution of geological structures, damage development can be predicted, described and recorded through a single damage tensor representation for each point in the model (with the inclusion of damage by a number of different mechanisms). With appropriate analysis, equivalent information may be determined from anisotropy analysis of seismic reflection data as the seismic wave propagation is sensitive to the total damage. Thus the proposed approaches for parameterization and modelling/analysis, using the damage domain, provide a common quantification of the damage (as detected using seismic data and predicted using geomechanical modelling) that permits integration of the two disciplines. This paper presents the damage-domain theory and linkage between the domains plus practicalities, with respect to both the geomechanical and seismic analyses, with illustration using a simple geomechanical model example.

Abstract The vector nature of the Earth's magnetic field dictates that interpreters must take care to understand pitfalls related to the orientation of the field (i.e., magnetic inclination and declination), and the relationship of the magnetic field to a region's geology. The case history presented here demonstrates one such pitfall. Present models for the formation of the Grenada Basin vary from north-south extension to northeast-southwest extension to east-west extension. Gridded magnetic anomalies over the basin provide a picture of the Earth's field that contributes to this spectrum of possible extensional origins. The Grenada Basin is a back-arc basin located near the eastern edge of the Caribbean Plate. The basin is bounded on the east and west by the roughly north-south-trending active Lesser Antilles and remnant Aves Ridge Island Arcs, respectively. Although this physiography, as well as gravity data, supports formation by near east-west extension, magnetic anomalies over the basin exhibit predominantly east-west trends. The crust of the Grenada Basin and of other back-arc basins forms similarly to the crusts of ocean basins. If the observed magnetic anomalies over the basin are produced by sea-floor spreading, then the orientation of extension may be complex. Extension in most back-arc basins is roughly normal to their trenches and subduction zones, but some basins appear to exhibit oblique extension. A careful interpretation of magnetic profiles reveals low-amplitude magnetic anomaly trends, oriented subparallel to the island arc, over the southern part of the Grenada Basin, which supports a model for basin development by near east-west extension.

A compilation of existing magnetic data clearly demonstrates the presence of extensive, NE-SW trending, linear anomalies over the central Venezuelan Basin. These long wavelength, small amplitude anomalies are truncated in the east by a series of N-S linear anomalies over the Aves Ridge, and in the south by E-W trending anomalies over the Aruba Basin, Curacao Ridge, and Los Roques Basin. In the southeastern corner of the basin, there is a magnetic quiet zone similar to that observed in the North Atlantic and Pacific Oceans. Analysis of the NE-SW anomalies reveals an axis of symmetry which crosses the basin from north of the Guajira Peninsula to near the Muertos Trough at 68° W. Modelling indicates that the linear anomalies are the result of a phase of seafloor spreading between 153 and 127 m.y. at a half rate of 0.4 to 0.5 cm y −1 . The quiet zone is therefore believed to correspond to a period in the Middle Jurassic which may be characterized by frequent short reversals. The magnetic study together with other geologic and geophysical evidence suggests that the Venezuelan Basin formed in the Pacific region as a western extension of the N. Atlantic in Middle-Late Jurassic. Spreading appears to have ceased when, in the early Cretaceous, the South Atlantic began to open. As a result of these changes in plate motion, the Venezuelan Basin became trapped behind the juvenile Antilles arc-trench system. The Venezuelan Basin was then gradually inserted into the Caribbean region as this system migrated eastward with respect to North and South America.