Figure 5-1 shows a bright spot presented by Tegland (1973). This was one of the early examples studied and was observable because amplitude had been preserved in seismic processing. In earlier years, when records were normally made with automatic gain control, there was little opportunity for studying amplitudes. The bright spot of Figure 5-1 is actually a very good one for its era because it also shows a flat spot, presumably a fluid contact reflection. The flat spot terminates laterally at the same points as does the bright spot; we would consider this a simple form of bright spot validation, increasing the interpreter’s confidence that the anomaly indicates the presence of hydrocarbons.
With the improvements in seismic processing over two decades, we can now consider polarity and phase as well as amplitude and spatial extent. Frequency, velocity, amplitude/offset and shear wave information can also help in the positive identification of hydrocarbon indicators. These are all subjects of this chapter and the direct observation of hydrocarbon fluids is now very widespread.
Most direct hydrocarbon indication relates to gas rather than oil reservoirs as the effect on acoustic properties of gas in the pore space is significantly greater than oil.Figure 5-2(derived from Gardner, Gardner, and Gregory, 1974) summarizes the different effects of gas and oil and shows that the effect of either diminishes with depth.