Dip and azimuth displays for 3D seismic interpretation

Abstract

In the last decade, the sophistication of three-dimensional (3D) seismic data has been applied increasingly to the more precise definition of geological structures containing oil and gas accumulations. In favourable circumstances, it is even possible to define the limits of the hydrocarbons themselves and, in rare cases, show the retreat of hydrocarbon-water contacts in response to sustained oil and gas production. In the early days, 3D seismic data were interpreted in a manner comparable to the interpretation of a grid of two-dimensional (2D) profiles. Equispaced in-lines and cross-lines were abstracted from the total data volume and, with the additional benefit of arbitrary lines and time-slices, interpreters were able to construct time contour maps of selected horizons with increased confidence. However, a significant percentage of the total volume of acquired data was by-passed in this process, and it was not until the advent of interactive trace interpretation systems (the so called work stations) that the interpretation of the full data volume became feasible. Feasibility turned into reality with the development of new and powerful automatic tracking (horizon picking) programs. These enable the interpreter to incorporate data from every common midpoint (CM) in the data volume. In some more advanced programs, selected ‘seed’ lines, which are initially interpreted by hand, are fed to the automatic tracking programs which, in turn, extrapolate the horizon pick until every CMP in the survey has been covered. For any given reflection horizon it is thus possible to generate files of two-way time, amplitude, instantaneous phase, etc. in a framework of x,y coordinates. These files permit the calculation of additional horizon attributes, especially dip and azimuth (Shell Internationale Petroleum Maatschappij BV 1988. Horizon processing techniques for recognition of structural geology on 3D seismic. Research Disclosure 29473). This paper is concerned with the determination and display of the latter two attributes. It will be shown that, with carefully chosen colour display parameters, subtle laterally continuous features that would have been overlooked during traditional interpretation may be interpreted with relative ease and confidence as having geological significance. The techniques employed have much in common with those used in photogeology.