Juxtaposition at a given location can be checked by means of a cross-section, but a large number of cross-sections would be required to evaluate each fault trap. The best method to evaluate the potential for a juxtaposition trap is to map the stratigraphy at the fault surface in both the hanging wall and the footwall (Figure 1)6;7. These maps are often referred to as "Allan" fault plane maps' or "Allan Sections". In order for "Allan Sections" to be meaningful, structure maps and fault surface maps must be properly integrated as described in Tearpock and Bischke.

Construction of "Allan Sections" by hand is very time consuming. Fortunately, FAPS (Fault Analysis Projection System) software written by Badley Earth Sciences, Ltd. has automated this process. A geoscientist can extract fault and horizon interpretations from 3-D interpretation packages such as Landmark, GeoQuest and Charisma, or digitize them from maps, and import them into FAPS. Stratigraphic information can be added to these interpretations and FAPS will construct "Allan Sections" quickly and painlessly (Figure 2).

In a recent study of Vermilion Block 331 Field, Broussard and Lock9 concluded that juxtaposition explained 70% of the hydrocarbon/water contacts. In the other cases, hydrocarbons are trapped in sands which are juxtaposed against other sands across faults. In these cases the petrophysical properties of the fault zone itself are such that the fault zone acts as a seal. Such "sealing faults" are not uncommon in the Gulf Coast and other similar settings such as the Niger Delta 12-14 and the Columbus Basin of Trinidad.

Impermeable fault zones can result when clay-rich rocks are entrained in the fault zone or when coarser-grained rocks are ground into finer particles. Fault zone permeability can also be reduced by diagenesis, but only in rare cases will diagenesis be pervasive enough to form an effective seal. The entrainment of clay-rich rocks into fault zones, commonly referred to as clay smear, is probably the most common mechanism for creating sealing faults in young clastic basins (Figure 3).

Clay smear is a function of the thickness of individual shale beds and the distance of each shale bed from the point of interest along the fault zone. Multiple shale beds have an additive effect, so that the Clay Smear Potential at any point along a fault surface is the sum of the clay smear of all of the individual shall beds. The Clay Smear Potential has to be computed for all points along a fault where reservoir rocks are in juxtaposition. Some large oil companies have proprietary computer programs which perform these calculations"-". Recently FAPS has added this capability as well, allowing the Clay Smear Potential to be calculated wherever sand-on-sand juxtapositions occur across a fault (Figure 4).

An alternative approach to estimating the sealing potential of a fault in areas of complicated stratigraphy is to calculate the "Shale Gouge Ratio'?. The Shale Gouge Ratio is the fraction of shale in the slipped interval. FAPS also allows the calculation of Shale Gouge Ratio along a fault surface.

For best results in determining the likelihood that a fault will seal, the sealing potential of various Clay Smear Potentials or Shale Gouge Ratios needs to be locally calibrated. Recent studies from three different geologic regimes have indicated that as little as 20% shale in a faulted section can result in sealing faults16 .

"Allan Sections" should be created for all prospects where a fault forms one of the boundaries of the trap. In cases where a sand is isolated in a thick shale section, it may be sufficient to check that the vertical separation exceeds the true vertical thickness of the sand at all points along the fault. In cases where several sands are potential objectives in an alternating sand-shale sequence, detailed "Allan Sections" need to be created from the maps, either by hand or using computer programs such as FAPS. Where "Allan Sections" indicate the potential for sand-on-sand juxtaposition, the sealing potential of faults should be estimated using either Clay Smear Potential or Shale Gouge Ratio. Estimating either of these factors is easiest using computer programs such as FAPS.

James C. Brenneke

REFERENCES

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