A 3 D Seismic Interpretation Of Ship Shoal 253 Field Offshore Louisiana PDF Download

In 1977 G.S.I. collected a three dimensional seismic survey over a gas producing salt dome offshore Louisiana. The purpose of this study was to test the 3D seismic system as a development tool to aid bringing new fields into production. The producing structure in this area contains one of the larger gas fields in the United States with reserves of 1.5 trillion cubic feet. The field was discovered in 1950 and has a total of forty-nine wells, thirty-seven producers and twelve dry holes. In this study the well log analysis and the seismic interpretation were carried out completely independent of each other. Direct hydrocarbon indicators, in this case flat spots (hydrocarbon-water contact reflections), were used to seismically determine the extent of the gas accumulation. Thick gas sands in young offshore clastic basins, like the Gulf of Mexico, are the easiest reservoirs to identify using direct hydrocarbon indicators such as bright spots or flat spots. The high density surface coverage associated with a 3D survey enhances flat spot detection. The structure of the reservoir units as well as the hydrocarbon extent in two of the major reservoirs were studied both seismically and by using the wells drilled in the field. These two independent study methods yielded very similar interpretations. With the larger survey area and the higher density surface coverage, the 3D seismic survey yielded a more refined structural interpretation than the well log analysis. In areas of good well control both the interpretations are the same; where the well control diminishes there are some discrepancies. Two additional faults which affect the hydrocarbon accumulation in the D sand were identified using the 3D survey. A major growth fault which controlled the deposition of the reservoir units was mapped more accurately using the 3D survey. The seismic estimates of the hydrocarbon extent are found to be a little conservative compared with the proven producing areas. The reason for this conservative estimate is that the seismic data is low frequency 10-20 Hertz. With data of this bandwidth a significant gas thickness (approximately 100 feet) is required before a gas-water contact reflection can be reliably mapped. Thinner gas pays near the outer producing boundaries were not detected using the 3D survey. In this study the 3D seismic system was used to accurately map the structure and hydrocarbon extent in two of the major reservoirs. Wells producing from these reservoirs confirm findings. If this survey was done in the early 1950’s at least three dry holes could have been prevented during the development of this field. The 3D seismic system is an excellent development tool and in most cases its use will reduce the number of dry holes drilled while defining the limits of new discoveries.