Gas chimneys are visible in seismic data as columnar disturbances, where the continuity of reflectors is missing, and reflection amplitudes are weaker than in the surrounding areas. In this chapter, gas chimneys interpreted from threedimensional seismic data, some of which have been confirmed by wells, have been sorted into two kinds. Type 1 chimneys are associated with faults. These chimneys commonly have a circular and limited horizontal cross section with a diameter in the order of 100 m (330 ft). The presence of gas chimneys along faults indicates that the faults are open or have been open for a time, in which case fluids can migrate through the faults. Type 2 chimneys are not associated with faults, and their lateral extent can be in the order of several hundred meters. Because open faults are not capillary barriers for hydrocarbons, as opposed to shales, type 1 chimneys can indicate hydrocarbon-migration pathways where relatively high flux rates can occur. Because type 2 chimneys are not associated with faults, capillary resistance in the shales will prevent upward movement of free gas (and oil), and the chimneys are regarded to represent gas having a very slow or no upward movement (low to zero flux rate). However, fractures beyond seismic resolution may exist, which may account for gas migration through the shales. Another explanation for the presence of gas type 2 chimneys is that gas-saturated water may release gas during upward movement caused by a drop in the pressure.
Examples from the North Sea, Gulf of Mexico, Nigeria, and the Caspian Sea show a consistency in the appearance and distribution of types 1 and 2 chimneys above hydrocarbon-charged reservoirs, as well as above dry reservoirs. Type 1 chimneys have also been observed below hydrocarbon-charged reservoirs, in which case, they indicate migration pathways into the reservoir.