New Tools for Geohazards- Combining Reservoir Evaluation Tools and Seismic Geomorphology to Improve the Interpretation of the Shallow Section

Abstract

Abstract Interpreters of deepwater 3-D seismic data have adopted or developed seismic geomorphologic techniques to make geohazard assessments. The overall quality and reliability of geohazards assessments based on map-view interpretations of 3-D seismic data have made them much more useful for making important well planning decisions for deepwater exploration/development wells than from interpretations based only on 2-D and/or traditional analogue geohazards data. Although far superior to traditional geohazards analysis, the assessments do not provide quantitative information about important and difficult deepwater geohazard problems such as shallow water flow (SWF) and buried gas hydrate that are not, or not directly, imaged by amplitudes. Because 3-D seismic data is being used for geohazard assessment, other techniques that use the same data set - such as seismic inversion - might be successfully applied to SWF and gas hydrate prediction and assessment. These seismic inversion techniques have been developed and applied to reservoir-level studies previously, and have only recently been used for shallow geohazard studies. There are data limitations to applying seismic inversion to the shallow section, namely the type and quality of the logs that constrain the widespread adaptation of seismic inversion techniques, although empirical models may be able to substitute for these in some cases. This paper describes the current seismic geomorphologic approach taken in geohazards when conventional 3-D or high resolution 3-D seismic data are available and briefly reviews work done by others using seismic inversion. In particular, there is an expanded discussion of methods developed by Lu and McMechan (2002) and Lu and others (2003) to characterize SWF and gas hydrates using inversion methods and a comparison to the seismic geomorphologic approach to addressing these geohazards. Introduction Geohazards practice has evolved with the tools made available to it. Until about nine years ago, the toolkits used by the geohazards practitioner and the exploration and/or development geoscientist had been separate, with little overlap. This separation changed with the need to assess geohazards in deepwater. The "shallow water" geohazards toolkit was not suitable to address geohazards in deepwater without literally taking the shallow water toolkit into the depths and/or dramatically increasing the minimum line spacing. Good 3-D seismic data used by explorationists, either used "as is" or reprocessed for the shallow section, became the basic dataset used in deepwater geohazardspractice. Since then, geohazards practitioners have become skilled in the map-view based seismic interpretation skills known as seismic geomorphology. Process-based good geologic models built from these techniques have been shown to be effective for making good well planning decisions with respect to shallow gas, shallow-water-flow and gas hydrates, but, as with all aspects of the industry, the geohazards industry must keep striving to improve the accuracy and reliability of predictions and assessments. There is a growing need for quantitative assessment of geohazards. Advanced techniques and tools developed by the energy exploration and development sector that use 3-D seismic data such as seismic inversion may be used or modified to help derive quantitative answers to difficult problems in geohazards.