Memetakan oil water contact atau gas oil contact atau juga gas water contact mempunyai cara yg kadang kala perlu mikir "thithik2".
Sekedar mengingatkan bagi yg sering melakukan, juga bagi yg belum pernah semoga menambah ilmu ======================================== Quick Look Techniques Common Mapping Errors in Edge Water 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? 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. 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. 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." === permission for discussion to http://www.scacompanies.com/ -- my blog : http://putrohari.tripod.com/Putrohari/
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