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GCPrevious HitFlatNext Hit Spots Are Not Always Previous HitFlatNext Hit

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])
 

General Statement 

Everyone has heard of a Previous HitflatNext Hit spot, so, interested as we are in hydrocarbons, we tend to look for something Previous HitflatNext Hit! Also, Previous HitflatNext Hit spots have some mystical appeal for explorationists.

A Previous HitflatNext Hit spot is, of course, the seismic expression of a fluid contact, and normally the fluid contact will be Previous HitflatNext Hit. 

Within the complexities of the seismic response are many reasons why a fluid contact reflection may not be Previous HitflatNext Hit; so “Previous HitflatNext Hit 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 statement

uFigure captions

uExamples

uConcluding remarks

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uGeneral statement

uFigure captions

uExamples

uConcluding remarks

 

 

 

 

 

 

 

 

 

 

Figure and Table Captions

Figure 1. 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. (Courtesy of BP [Amoco Egypt]).

Figure 2. A prominent fluid contact reflection from the Gulf of Mexico. It is, in fact, a single contact affected by a tuning phenomenon. The strong reflections dipping down above the contact are internal reflections caused by "illumination of internal layering." (Courtesy of ChevronTexaco [Chevron USA]).

Figure 3. A Previous HitflatNext Hit spot associated with a bright spot offshore Indonesia. The Previous HitflatNext Hit spot is fairly Previous HitflatNext Hit, but actually dipping slightly to the left -- that is, against structure. (Courtesy of Premier Oil Natuna Sea).

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Examples 

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-Previous HitflatNext Hit, 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 Previous HitflatNext Hit spot associated with a bright spot in offshore Indonesia. The Previous HitflatNext Hit spot is fairly Previous HitflatNext Hit, 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.

 

Concluding Remarks 

A Previous HitflatNext Hit spot should never be sought or studied in isolation from the other reservoir reflections. The key is to see the Previous HitflatNext Hit 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.

Previous HitFlatNext Hit spots are wonderful indicators of hydrocarbon, but please consider all the characteristics of the reservoir reflections together. Remember, Previous HitflatNext Hit spots don't have to be Previous HitflatTop, but they do have to be discrete and fit with top and base reflections.

 

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