From Wiki:Flat spotsFlat spots represent a hydrocarbon contact...

From Wiki:

Flat spots
Flat spots represent a hydrocarbon contact seismic response where it is apparently flat. Such contact may be between gas and oil, oil and water, or gas and water. The hydrocarbon reservoir much be thicker than the vertical resolution in order to represent a flat spot. Flat spots are often difficult to find; the edge or base of channels, low angle faults, or processing artifacts can often be misconceived as flat spots. Flat spots may also be caused by low saturated gas in a reservoir.



My understanding is a 'flat spot' (i.e. horizontal indicator) may indicate the surface of a contact between oil/water or gas/water, and can be distinguished from other dipping structures in the rock by the difference in angle (see image above).

I dont think these necessarily indicate the size of the reservoir, just that there is potentially a different substance there (oil/gas not rock).

Must re-iterate I am certainly no petroleum engineer!

Cheers,