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Abstract

The fault shadow problem is what explorationists call the zone of unreliable seismic imaging in the footwalls of faults. Although it can occur in all types of faults, the term is usually applied to extensional faults. These problems are common-place, and yet much current exploration still relies on images in which they remain. In this article I present an example of the fault shadow problem from the Wilcox trend (Eocene-Paleocene) of south Texas. The stratigraphic-velocity alternations in this area cause a distinctive suite of problems in conventional time imaging. Explorationists can cite numerous examples of wells drilled on time highs, but which are significandy off structure in depth. Moreover, imaging often degrades precipitously at levels below the Middle Wilcox even when wells indicate an uncomplicated structure. I'll begin by demonstrating, through schematic and synthetic examples, the nature of these problems and their elimination through prestack depth imaging. A real data example will then show identical effects.

Depth imaging is most commonly applied to imaging of complex structures, mainly subsalt and thrust belt. A principle theme of this article is that depth imaging can impact exploration in every structural play, including subtle ones like fault shadow areas, because contemporary structural play exploration tends to operate along the frontier where time imaging is no longer effective.

The fault shadow problems in the Wilcox trend are related to the stratigraphic-interval velocity relationships of the section. These are illustrated in Figure 1, check shot survey data from a well along a line which is presented later. From a velocity standpoint the section can be characterized by four units. Ranging from shallowest to deepest these are: the Weches Shale at 7,000-9,000 ft/s, the Queen City Sandstone at 12.000 ft/s, the Reklaw Shale at 9000 ft/s, and the Wilcox Sandstone at 10,500 ft/s. So the stratigraphic sequence can be summarized as: low-velocity shale, high-velocity sandstone, low velocity shale. high-velocity sandstone.

Synthetic seismic data were used to identify the fault shadow problems, understand their origin, and examine the ability of imaging strategies to remove them. Figure 2, the model from which the synthetic data were collected, includes four units with the same alternations shown in Figure 1. Unit thicknesses and velocities are typical for this area. Each unit is precisely horizontal and broken by a single extensional fault. In addition, there is a flat basement horizon which is unbroken by the fault.

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