A hydrocarbon indicator (HCI) or direct hydrocarbon indicator (DHI), is an anomalous

seismic attribute In reflection seismology, a seismic attribute is a quantity extracted or derived from seismic data that can be analysed in order to enhance information that might be more subtle in a traditional seismic image, leading to a better geological or geo ...

value or pattern that could be explained by the presence of

hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or e ...

in an oil or gas reservoir. DHIs are particularly useful in

hydrocarbon exploration Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth using petroleum geology. Exploration methods Vis ...

for reducing the geological risk of exploration wells. Broadly, geophysicists recognize several types of DHI: *

Bright spot In reflection seismology, a bright spot is a local high amplitude seismic attribute anomaly that can indicate the presence of hydrocarbons and is therefore known as a direct hydrocarbon indicator. It is used by geophysicists in hydrocarbon expl ...

s: localized amplitudes of greater magnitude than background amplitude values. Equipment prior to the 1970s had the bright spots obscured due to the automatic gain control. *

Flat spot A flat spot, or wheel flat, also called spalling or shelling, is a fault in railroad wheel shape. A flat spot occurs when a rail vehicle's wheelset is dragged along the rail after the wheel/axle has stopped rotating. Flat spots are usually cau ...

s: nearly horizontal reflectors that cross existing stratigraphy, possibly indicating a hydrocarbon fluid level within an oil or gas reservoir. * Dim spots: low amplitude anomalies. * Polarity reversals can occur where the capping rock has a slightly lower seismic velocity than the reservoir and the reflection has its sign reversed. Some geoscientists regard amplitude versus offset anomalies as a type of direct hydrocarbon indicator. For example, the amplitude of a reflection might increase with the angle of incidence, a possible indicator of natural gas.

References

{{petroleum-stub Geophysical imaging Petroleum geology