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The fault shadow problem as an interpretation pitfall

Tecpetrol SA, Reservoir Dept., Buenos Aires, Argentina

Corresponding author: TECETR@tecpetrol.com

Editor's note: This article was originally submitted to the Pitfalls in Seismic Interpretation department which is designed to update Tucker and Yorston's classic book of the same name originally published in 1973. However, because of its length and number of illustrations, the article was deemed more appropriate for Interpreters Corner. For more information about the new series or an example of the type of material desired, see page 1320 of the November 1999 issue of TLE or contact Steve Henry (geolearn@aol.com).

The first 20% of the full text of this article appears below.

Seismic interpretation pitfalls are false structures on seismic created by regional or local velocity changes, acquisition patterns, or processing artifacts. These false structures could lead an unsuspecting interpreter to an erroneous interpretation and a noncommercial well or dry hole. This is a brief analysis of one of the velocity-induced pitfalls, the fault shadow pitfall, and how to recognize it.

Numerous dry holes have been drilled into small rollover structures seen on seismic in the footwall of a fault, either the upthrown block of a normal fault or the downthrown block of a reverse fault. Some structures in the shadow of the fault plane may have been real and had closure, but other structures may have been false and created by lateral velocity contrasts across the fault—effects of the fault shadow.

In order to visualize the fault shadow pitfall, I created simplified, two-dimensional geologic models and constructed synthetic seismic lines from them.

The first model consists of a series of flat layers with a normal fault. Velocity increases with depth, and a high-velocity layer is inserted in the middle (Figure 1). Figure 2, the depth model converted to time, shows apparent structures in the footwall of the fault; however, . . . [Full Text of this Article]