CASE HISTORY 10: Integrated Interpretation, 3-D Map Migration and VSP Modeling Project, Northern U.K. Southern Gas Basin*
J. M. Reilly, 1991. "Integrated Interpretation, 3-D Map Migration and VSP Modeling Project, Northern U.K. Southern Gas Basin", Seismic Modeling of Geologic Structures: Applications to Exploration Problems, Stuart W. Fagin
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Depth conversion in the northern portion of the U.K. Southern Gas Basin is complicated by the presence of (Permian) Zechstein salt swells and diapirs. In addition, the post-Zechstein (post-Permian) section displays large lateral velocity variations. The primary agents which control the velocity of this stratigraphic section are (1) depth of burial dependency, (2) lithologic variation within individual formations, and (3) the effects of subsequent tectonic inversion. An integrated approach which combines well velocity, seismic velocity, and seismic interpretation is required for accurate depth estimation.
In 1988 Mobil and partners drilled an exploratory well in the northern portion of the U.K. Southern Gas Basin. This well was located near the crest of a Zechstein salt diapir. Over 2000 m of Zechstein were encountered in the well. The Permian Rotliegendes objective was penetrated at a depth of over 3700 m.
The initial delineation of the objective structure was based on the results of three-dimensional (3-D) map migration of the seismic time interpretation. Spatially variant interval velocity functions were used to depth convert through five of the six mapped horizons. Both well- and model-based seismic interval velocity analysis information were used to construct these functions.
A moving source well seismic survey was conducted. The survey was run in two critical directions. In conjunction with presurvey modeling, it was possible to immediately confirm the structural configuration as mapped out to a distance of seven kilometers from the well. Post-survey 3-D map migration and modeling was employed to further refine the structural interpretation. Although the question of stratigraphic anisotropy was considered in the evaluation of the long offset modeling, no evidence was found in the field data to support a significant effect.
Finally, comparisons were made of: curved ray versus straight ray migration/modeling, midpoint-depth velocity versus (depth dependent) instantaneous velocity functions, and Hubral versus Fermat based map depth migration algorithms. Significant differences in the results were observed for structural dips exceeding 15 degrees and/or offsets exceeding 6 km. Map depth migration algorithms which employed both curved rays and spatially variant instantaneous velocity functions were found to best approximate the “true” geologic velocity field in the study area.
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Seismic Modeling of Geologic Structures: Applications to Exploration Problems
Seismic interpretation apparently is becoming primarily a geologic rather than a geophysical skill. This observation has been true from the moment seismic reflection data were displayed as a continuous record with the intention of creating an image of subsurface structure. The imaging advances that have occurred in the past two decades only reinforce the tendency. More effective migration algorithms making use of faster and less expensive computers, as well as high-fold and, in particular, 3-D data all serve to make the seismic picture better. As the image increasingly reveals more geology, the geologic skills become more crucial to the task of extracting the information made available. As seismic artifacts such as multiples, sideswipe, and raypath distortion effects are successively eliminated from the image, the geophysical sophistication of the interpreter becomes increasingly less important. At first glance it would seem that these tendencies can only intensify as these technological trends continue.
And yet the depiction of complex structures remains elusive. Migration programs have been developed that can manage the severe raypath bending attendant with complex structures. Moreover, the ever decreasing costs of computation make the application of these programs increasingly more feasible. Unfortunately, to benefit from these imaging approaches requires, a priori, an increasingly more precise definition of the velocity field which often is, in itself, an expression of geologic structure. Therefore, before we can create the image, we require an understanding of what the image is supposed to show. This circumstance implies that the preparation of the seismic image has become, and will likely remain, inextricably bound up with its interpretation.