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    Seismic Hydrocarbon Indicators

    Bright spot: Bright spots are the high amplitude seismic anomaly that indicates presence of hydrocarbon.Bright spots primarily results due to increase in acoustic impedance contrast such as when a low impedance gas zone is overlain by high acoustic impedance shale thus increasing the reflection coefficient.Bright spots can also happen due to change in lithology.

    Figure:- Acoustic impedance relation which leads to bright spot.

    Dim spot:- Dim spot are low amplitude seismic anomaly that indicates presence of hydrocarbon,Dim spot result due to  reduction in acoustic impedance contrast  when hydrocarbon  zone  is overlain by low impedance  shale .Similar to bright spot dim spot can also happen due to lithology.

    Acoustic impedance of sands and shales increase with age and depth but phenomenan is not uniform.Acoustic impedance of Younger shales are higher than younger sands,but this reverses at higher depth with acoustic impedance of older shales higher lower than older sands.

    Figure:- Acoustic impedance relation which leads to dim spot.

    Flat spot: Flat spot represents a fluid contact ,They appear on seismic section as horizontal reflector. They are very prominent on seismic section as they are flat compared to surrounding dipping reflector.They can show downdip limit of reservoir in some cases.

    Above mentioned hydrocarbon indicators are mostly found in relatively young,unconsolidated siliciclastic sediments,as these sediments may have large acoustic impedance contrast,prominent examples are Offshore western Africa and Gulf of Mexico.One big problem with these hydrocarbon indicators is they cant differentiate between high gas saturation and low gas saturation (fizz water).AVO effect can also be tracked across the reservoir to identify a fluid contact.

    Some operators use AVO as standard procedure  to reduce risk associated with identification of drilling location and other operator find it to be to cumbersome and time consuming.

    Figure:-Flat spot in seismic section