Everyone has heard of a flat spot, so,
interested as we are in hydrocarbons, we tend to look for something
flat! Also, flat spots have some mystical appeal for explorationists.
A flat spot is, of course, the seismic expression
of a fluid contact, and normally the fluid contact will be flat.
Within the complexities of the seismic response are
many reasons why a fluid contact reflection may not be flat, so
flat spot is actually a rather poor name. Velocity of the hydrocarbon
in the reservoir, velocity effects in the overburden and tuning
effects all can introduce significant dip.
Unconformability rather than flatness is the important
property of a fluid contact reflection in seismic data.
Figure 1 shows some gas
reflections from the Nile Delta in Egypt. The high amplitude red
reflection is from the top of the gas in an anticlinal trap. The
high amplitude blue reflection is from the base of the gas -- in
other words, from the fluid contact.
This fluid contact reflection is significantly un-flat,
bowing down in the center where there is maximum thickness of low
velocity gas sand.
Figure 2 shows a prominent
fluid contact reflection from the Gulf of Mexico. It might be interpreted
by some as multiple contacts, suggesting that there are multiple
This is, in fact, a single contact affected by a
tuning phenomenon. The strong reflections dipping down to the east
above the contact are internal reflections that are caused by "illumination
of internal layering."
In a thick clastic reservoir this is a fairly common
effect of gas. The strong internal reflections then interfere or
tune with the fluid contact reflection causing these apparent steps
in the contact.
Figure 3 shows a flat
spot associated with a bright spot offshore Indonesia. The flat
spot is fairly flat, but actually dipping slightly to the left --
that is, against structure. This again is the effect of the gas
within the reservoir. There is a clear top and base gas reflection,
as there should be.
A flat spot should never be sought or studied in isolation
from the other reservoir reflections. The key is to see the flat
spot at the downdip limit of brightness and fitting in between the
top and base reflections. The whole package of reflections must
always be studied together.
The character and color of the reflections is also
of the utmost importance and this depends on the phase and polarity
of the data. But this is the subject of another article.
Flat spots are wonderful indicators of hydrocarbon,
but please consider all the characteristics of the reservoir reflections
together. Remember, flat spots don't have to be flat, but
they do have to be discrete and fit with top and base reflections.