A Case History of 3-0 Seismic Application in the Gulf of Mexico

Abstract

ABSTRACT Atypical faulted Tertiary structure in the Gulf of Mexico was surveyed to obtain dense 3-D coverage for the purpose of ultimately guiding field development efforts targeted on proven productive horizons. Comparisons can be drawn between the interpretive results of this program and more conventional grid shooting. The benefits gained from data density and 3-D migration are explored in this geologic setting. The interplay of the data density with the velocity evaluation and direct hydrocarbon indicators is reviewed along with various aspects of display versatility. INTRODUCTION The escalated use of oil and gas products by each individual in recent years has generated concern that the known oil and gas reserves are not sufficient to provide current and near future energy needs of this nation. Consequently, the oil and gas industry is expected to find more reserves and produce them at "tomorrow's cost" and receive "yesterday's prices" for its efforts. With these facts in mind, the oil and gas industry is faced with three challenges. First, where to search; second, how to locate; and third, how to develop these new reserves efficiently. Since new frontiers are not as plentiful as they have been in the past, it is necessary that a portion of these future reserves be located in problem areas evaluated by "yesterday's technology". These areas include oil and gas fields currently producing, fields considered depleted, areas with limited surface access, and areas with complicated subsurface geology which prevents the making of an accurate subsurface map. CASE HISTORY DISCUSSION The area selected for this discussion could be described by a number of the above problem areas. It is located Offshore Louisiana (figure 1) in about 15 feet of water and has wells producing from multiple zones within a series of faulted blocks. Subsurface maps made from the available 2-D seismic data and well information have defined the major fault blocks from which the present production exists. Amore detailed seismic evaluation of each block could possibly pinpoint smaller traps that were overlooked by previous efforts. The remainder of this discussion will review the results from a three-dimensional (3-D) seismic survey which was used to improve the subsurface picture and in turn provide valuable information for optimum field development. This survey technique is now considered a production method in the industry and has proved to be a valuable asset to field development. In areas where no access problems exist, the conventional "3-D swath" technique may be used; whereas, areas having surface restrictions may require an adaptation of the 3-D method, such as the Seisquare1 or Seisloopl methods (figures 2 and 3). The initial step in any 3-D survey is to evaluate the range of dip, velocity, frequency and depth of the zone of interest expected in the area. These attributes control the spatial sampling interval required to obtain optimum results.