1. n. [Geophysics]

A type of seismic amplitude anomaly, seismic event, or characteristic of seismic data that can occur in a hydrocarbon-bearing reservoir. Although "bright spots," as hydrocarbon indicators are loosely called, can originate in numerous ways, they are not all indicative of the presence of hydrocarbons. Criteria to distinguish true hydrocarbon indicators (sometimes called HCIs) from other types of amplitude anomalies include:

• amplitude variation with offset
• bright or dim spot(s) in amplitude as a result of variations in lithology and pore fluids, sometimes occurring in groups of stacked reservoirs
• change or reversal in polarity because of velocity changes, also called phasing
• conformity with local structures
• diffractions that emanate from fluid contacts
• flat spot that represents a fluid (gas-oil or gas-water) contact, which can also show the downdip limit of the reservoir in some cases
• gas chimneys above leaking reservoirs
• shadow zones below the accumulation
• velocity push-down because of lower velocities of hydrocarbons than rocks
• difference in response between reflected pressure and shear energy.

Hydrocarbon indicators are most common in relatively young, unconsolidated siliciclastic sediments with large impedance contrasts across lithologic boundaries, such as those in the Gulf of Mexico and offshore western Africa. An ongoing issue in exploration for hydrocarbon indicators is the difficulty in distinguishing between gas accumulations and water with a low degree of gas saturation ("fizz water").

Alternate Form: bright spot

See related terms: amplitude anomaly,  amplitude variation with offset,  attribute,  diffraction,  dim spot,  gas chimney,  phase,  push-down,  velocity