reflection seismology Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismi ...

, a bright spot is a local high amplitude

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 ...

anomaly that can indicate 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 ex ...

and is therefore known as a

direct hydrocarbon indicator A hydrocarbon indicator (HCI) or direct hydrocarbon indicator (DHI), is an anomalous seismic attribute value or pattern that could be explained by the presence of hydrocarbons in an oil or gas reservoir. DHIs are particularly useful in hydrocarb ...

. It is used by

geophysicists Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' so ...

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 ...

History

Bright spots were not commonly identified until the early 1970s because of the extensive and industry-wide use of

automatic gain control Automatic gain control (AGC) is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, the purpose of which is to maintain a suitable signal amplitude at its output, despite variation of the signal amplitude at the inpu ...

, which obscured the amplitude effects of hydrocarbon accumulations.

Theory

A bright spot primarily results from the increase in

acoustic impedance Acoustic impedance and specific acoustic impedance are measures of the opposition that a system presents to the acoustic flow resulting from an acoustic pressure applied to the system. The SI unit of acoustic impedance is the pascal-second per cub ...

contrast when a hydrocarbon (with a lower acoustic impedance) replaces the brine-saturated zone (with a higher acoustic impedance) that underlies a

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

(with a higher acoustic impedance still), increasing the

reflection coefficient In physics and electrical engineering the reflection coefficient is a parameter that describes how much of a wave is reflected by an impedance discontinuity in the transmission medium. It is equal to the ratio of the amplitude of the reflected wa ...

. The effect decreases with depth because compaction for sands and shales occurs at different rates and the acoustic impedance relationship stated above will not hold after a certain depth/age. Below this depth, there will be a crossover of shale and sand acoustic impedances and a

dim spot In reflection seismology, a dim spot is a local low-amplitude seismic attribute anomaly that can indicate the presence of hydrocarbons and is therefore known as a direct hydrocarbon indicator. It primarily results from the decrease in acoustic im ...

is more useful to hydrocarbon exploration.

Caution

The relationship between hydrocarbons and direct hydrocarbon indicators, such as bright spots, is not straightforward and not all bright spots are caused by the presence of hydrocarbons and therefore they should not be treated as conclusive evidence of hydrocarbon accumulations.

References

{{Reflist *http://www.glossary.oilfield.slb.com/Display.cfm?Term=bright%20spot Schlumberger: Oilfield Glossary *Brown, Alistar. R., (2010), “Dim Spots in Seismic Images as Hydrocarbon Indicators”, AAPG Search and Discovery Article #40514. nline: http://www.searchanddiscovery.com/documents/2010/40514brown/ *Hilterman, Fred J., (2001), “Seismic Amplitude Interpretation”, SEG. Seismology

History

Theory

Caution

See also

References