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GCFlat Spots Are Not Always Flat
By
Alistair Brown1
Search and Discovery Article #40119 (2004)
*Adapted from the Geophysical Corner column in AAPG Explorer, October, 2003, with the same title and prepared by the author. Appreciation is expressed to the author, to R. Randy Ray, Chairman of the AAPG Geophysical Integration Committee, and to Larry Nation, AAPG Communications Director, for their support of this online version.
1Consultant, Allen, Texas ([email protected])
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.
uGeneral statementuFigure captionsuExamplesuConcluding remarks
uGeneral statementuFigure captionsuExamplesuConcluding remarks
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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 reservoirs. 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
Concluding Remarks
A flat
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.
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