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.
Figures & Tables
Model-Based Depth Imaging
In this chapter the benefits of depth imaging are reviewed. The distinction between depth conversion through depth imaging and map-based depth conversion is made. The four principle geophysical advantages are defined. Traditional barriers to depth imaging are described.
Figure 1.1 is a flow chart which makes the distinction between traditional map-based depth conversion and image-based depth conversion. Both procedures begin with CMP gathers and end with depth maps. Map-based depth conversion relies on time imaging to define the structural framework. Velocity variation is expressed in the form of layer interval velocity maps, and time-depth conversion is performed through grid operations. In contrast, image-based depth conversion utilizes depth imaging to define the structural framework. The depth conversion is part of the imaging process and velocity information is obtained in the imaging process itself.
The key difference between the techniques is first, in image-based depth conversion, the structural interpretation is made utilizing the superior imaging capability of depth imaging, and second, that depth imaging is, in itself, a strong velocity estimation tool.
Before reviewing the geophysical basis for the expectation that depth images will be superior it is worth examining some data examples that show, from a geologic standpoint, the additional structural information gained. Figures 1.2 to 1:12 are time and depth image comparisons from a variety of structural settings.
Salt examples - Figure 1.2 is from a salt sill in the deepwater Gulf of Mexico. The time image shows a complex arrangement of reflectors in the subsalt section. In the depth image the subsalt section is shown to be planar and relatively unbroken (there remains a single shadow zone below the nose of the sill) with a slight dip away from the salt sill. The base of salt reflections show alternations of steep and moderate dip perhaps related to changes in sedimentation rate versus speed of salt sill advancement. The development of this depth image is reviewed in the case history by Egozi in this volume.
Figure 1.3 depicts a salt diapir from the transition zone of the Gulf Coast. Exploration in this area often relies on an accurate estimate of the position of the salt sediment interface. Where the diapir flank becomes steep the time image shows little information. The depth image renders the interface continuously; where it is vertical and even where it is overhung.
Figure 1.4 depicts a salt-cored anticline in the Southern North Sea. The high amplitude reflection near the base of the time image is a salt weld along which the upper Permian salt has been evacuated.