Stratigraphic interpretation involves delineating seismic sequences, which represent different depositional units, recognizing seismic facies characteristics, which suggest the depositional environment, and analyzing reflection character variations to locate both stratigraphic changes and hydrocarbon accumulations. Three-D work is especially important in recognizing stratigraphic features by their distinctive shapes. Hydrocarbon accumulations are sometimes indicated by amplitude, velocity, frequency, or waveshape changes. The variation of amplitude with angle (or with offset) is one of the newer hydrocarbon indicators (HCI).