1. An Introduction to AVO Analysis

Abstract

Since the 1930s, exploration for hydrocarbons has been dominated by the seismic reflection technique. The high resolution of the data, the continuous sampling obtainable, and the low cost compared with drilling make the method an indispensable part of modern oil and gas exploration. In many cases, seismic reflection profiles are used to locate structures that have the potential to trap hydrocarbons. Anticlines, fault traps, and stratigraphic pinch outs are all easily mapped using seismic reflection data. The risk lies in the possibility that the trap will contain no hydrocarbons. Exploration would be more effective if the hydrocarbons could be detected directly on the seismic sections. In the 1960s, geophysicists discovered that the presence of gas often resulted in high-amplitude reflections known as “bright spots.” Bright spot exploration has been extremely successful, especially in the offshore Gulf of Mexico and other young clastic basins. The bright spot method was found to have limitations in that factors other than gas can cause bright reflections. Dry holes drilled into bright spots found igneous intrusions, carbonate or hard streaks, lignites, or wet sands. A test more definitive than bright spots on a stacked section was sought for the direct detection of gas on seismic records. This book describes the use of one such test, the analysis of amplitude variation with offset (AVO).