This paper describes a method of mapping the top reservoir using acoustic reflection surveys. A high frequency seismic survey is acquired around the horizontal wellbore with a drillpipe conveyed wireline tool. When the reservoir-to-shale acoustic contrast is enough to make a discernable reflection, this image can be used for both geosteering validation in the surveyed well and to improve the accuracy of geometric well plans of adjacent new wells. This makes the geosteering of nearby wells or sidetracks easier by providing a tighter limit on the top reservoir uncertainty. This is particularly useful when the placement of the wellbore near the reservoir top is critical, but is difficult to establish from conventional logging while drilling measurements when the markers within the reservoir cannot be used to infer the well location with respect to the reservoir cap.

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