Common Mapping Errors in Edge Water
Net Pay Isochore Maps
This article is the last in a ten (10) part series on Quick Look
Techniques (QLT's). During this series we briefly reviewed pitfalls
involving 1) equal spaced contours, 2) contour compatibility, 3)
an odd number of contours along a fault, 4) throw versus vertical
separation, 5) the additive property of faults, 6) restored tops,
7) normal fault dip as a sand indicator, 8) screw faults, 9) implied
fault strike and 10) isochore reservoir mapping. These QLT's and
many others are discussed thoroughly in a text by Tearpock, Bischke
and Brewton (1994) entitled "QUICK LOOK TECHNIQUES AND PITFALLS OF
PROSPECT EVALUATION."
A common error made in the preparation of net pay isochore maps
relates to the incorrect mapping of the water wedge zone in an edge
water reservoir. This error is usually caused by incorrectly connecting
the net pay isochore contours within the full sand thickness zone
to those within the water wedge. An incorrectly constructed net pay
isochore map can result in an unrealistic over or under estimation
of reserves.
How can you check for this type of contouring error? Consider the
two net oil isochore maps shown in Figures 1A & 1B. Which of
the two maps is correctly contoured?
Fig. 1a
Fig. 1b In both maps, the interpreter identified the Inner Limit of Water
(ILW). The ILW is defined as the intersection of the hydrocarbon
water contact with the base of porosity. In Figure 1A, notice how
the isochore contours in the full thickness area make an abrupt turn
at the ILW. The ILW controls the updip limit of water within the
reservoir. The full thickness sand contours cannot continue unaffected
past the ILW into the water wedge because the sand in the wedge contains
both hydrocarbons and water. Therefore, the contours must make an
abrupt turn at the ILW toward the thicker sand, and should often
be subparallel to the structural contours between the ILW and the
water contact. In Figure 1B, several net sand contours, which represent
full sand thickness, extend past the ILW into the water wedge zone.
Why is this incorrect and what is the effect of this mistake?
A cross-section of a net pay isochore map shows that the reserves
are divided into two zones: the full thickness zone and the water
wedge (Figure 2). These two zones are separated by the inner limit
of water (oil/water contact on the base of porosity). The entire
net pay isochore map is bounded by the zero contour line, which represents
the limits of the hydrocarbons in the reservoir. Within the full
sand thickness zone, updip to the ILW, net pay is equal to net sand.
Within the water wedge zone, the sand contains both hydrocarbons
and water. Therefore, the net pay (net sand) contours in the full
thickness area cannot continue unaffected into the wedge, but must
turn abruptly at the ILW toward the thicker sand. The major elements
used to contour the water wedge zone are the structural attitude
of the formation and the sand shale distribution within the formation.
Fig. 2 In order to correctly construct an edge water net
pay isochore map, the interpreter must have the following information.
A structure map on the top and base of porosity with the hydrocarbon
contacts marked, a total net sand isochore map and net pay values
for all wells within the reservoir. For correct net pay isochore
mapping, the Wharton method must be used. Using this method and the
above information, you can construct an accurate net pay isochore
map. A detailed description of the method can be found in "APPLIED
SUBSURFACE GEOLOGICAL MAPPING" by Tearpock and Bischke (1991). The planimetered volume for Figure 1B is 13,937 AcFt in contrast
to 12,401 AcFt for the correctly contoured map in Figure 1A. This
results in a 1536 AcFt overestimation of reservoir volume. Considering
a reasonable recovery factor, the overestimated reserves are approximately
three quarters a million barrels of oil. How would you explain this
volume discrepancy to an investor or supervisor after buying this
property?
When evaluating a net pay isochore map for an edge water reservoir,
always look for an abrupt turn of the net pay contours at the ILW.
If the ILW is not marked on the map, simply overlay the base of sand
map, trace the ILW onto the net pay isochore map and then review
the map. By using this simple QLT, you can quickly verify the accuracy
of the map and estimate of reserves, thus avoiding the mistake of
paying for more reserves then actually exist within the reservoir.
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