The Use of Exploration 3D Seismic Data for Geohazards Assessment in the Gulf of Mexico

Abstract

Abstract 3D seismic data. acquired for oil and gas exploration purposes. is now being used to supplement or even replace conventional high resolution geohazard surveys in some areas of the Gulf of Mexico. The use of 3D data has improved the identification and understanding of some types of geohazard. and the use of seismic interpretation workstations has made the data more accessible for geohazard analysis. and resultedin a more thorough interpretation of the shallow geologic section. Technologies used for exploration purposes have been adopted for use in the identification and assessment of drilling hazards in the near seafloor sedimentary section. Savings of time and money have been realized. as well as identification of areas where additional data is required. Some of the limitations and restrictions in using exploration N) data have also been identified. Introduction In the Gulf of Mexico. 3D seismic data. acquired for exploration purposes. is being used to supplement or replace conventional high resolution geohazard surveys Until recently, conventional geohazard survey were used to analyze the near seafloor sediments for constraints or hazards to drilling. In deepwater areas. a conventional survey might include an echo sounder and sub bottom profiler to analyze the very shallow section. and a single trace analog recording and maybe digital multifold seismic recording to analyze the intermediate section. In shallow water. a side-scan sonar and magnetometer would be added A typierd line spacing would be 300 meters by 900 meters. The hazards 10 be studied might include surface or near-surface faults. zones of probable instability. zones of potential gas accumulations. or zones with characteristics of seafloor fluid expulsion. This type of assessment is typically required by drilling contractors and government permitting agencies. While in shallower water depths these conventional surveys are still necessary. they can be supplemented and enhanced by using exploration 3D seismic data. In certain deepwater areas it has been possible to use 3D data instead of a conventional gcohazard survey Application Of Technique The use of 3D data is probably most feasible in areas of relatively stable near seafloor sediments, with little faulting or shallow diapiric uplift near the seafloor. A portion of a seismic line from a conventional exploration 3D survey is shown in Fig. 1. This line was processed with a standard 3D exploration processing flow. and no high resolution processing enhancement. The water depth is over 4000'. As can be seen. there are numerous shallow faults as well as an irregular seafloor. probably caused by shallow diapiric uplift. This is an area that would require either higher resolution processing. or more likely. a conventional shallow tow or deep tow geohazard survey. before the area could be properly analyzed for potential well locations. A nearby line from the same dataset is shown in Fig. 2. As can be seen in the area covered by this line. the first 60' to 700' of sediment is relatively smooth and has very few seismically interesting or anomalous events. such as near surface faulting.