West Cameron 3D Transition Zone Seismic Survey-A Practical Proof of Concept for Extended Resolution Imaging

Abstract

Abstract A recently developed alternative seismic imaging procedure which distinguishes between the imaging space (x, y, z) and the space of the Wavefield sampling (x, y, z, t) indicates that increased image resolution may be obtained from field data even when it is substantially decimated. Implications of the approach when applied to surveys having less spatial density of sampling significantly impact survey economics and timing. In the case of the Zydeco West Cameron 3D Transition Zone survey, $11 million in cost savings resulted representing a reduction of about 50% in expenditure. This survey which extended over some 230 square miles in the Louisiana Transition Zone was an important proof of concept for the technique which was fully explained in U.S. Patent No. 5,693,885, "Sampling and Reconstruction of Propagating Wavefields" (issued December 2 of 1997). Previous tests had consisted of systematically removing data from actual and model data sets with subsequent tests of attainable imaging resolution. While the multidimensional sampling theory that is needed to completely define limits for this approach is not yet in final form, all tests including this one demonstrate the viability of the concept. The survey described is of course an active exploration area having demonstrated hydrocarbon potential, but also proved ideally suited for the test purposes. This paper considers both the theory as it is applied and results of the test survey. Any project using real data is difficult to fully assess of course, particularly where alternatives have not been performed. That is, the conventional survey costing $24 million was not available for comparison purposes. Nevertheless, the resulting data quality was quite suitable for developing high quality prospects using pre-stack imaging, AVO methods as well as velocity inversion techniques. Comparisons with nearby existing survey data are quite favorable and data quality for the deeper strata compares exceptionally well with other surveys conducted in the same environment. Hence the test results are deemed to be a success especially in view of cost, time savings, and data quality. Text Validating a new seismic acquisition/imaging approach poses a host of difficult questions owing to the great variability of conditions which may characterize any survey. The surface terrain and conditions play an important role as will the subsurface geometry, geology, stratigraphy, lithology, etc. While model studies, both mathematical and physical, may suggest that a method will work, implementing it in the field may prove another matter as the special circumstances of the particular case come into play. Transition zones are not considered to be "easy" environments for developing seismic coverage, especially if one wishes to smoothly accomplish the imaging as one moves from a marine to a land regime. In addition, the marshy terrain is sometimes associated with statics problems which can be a challenge for the data processing but in this survey presented no problems. Further, a test case should embody some element of structural complexity to challenge the imaging task, but not to a pathological degree. The West Cameron area had significant structure including faulting both at intermediate and greater depths. Stratigraphy was highly variable involving both thick and thin stratigraphic units. Noise and statics problems while present were not extraordinary in their character.