Sandstone bar

Crossbedding in a Devonian fluvial sandstone bar, from the Catskill delta, New York, USA.

Like all substances, rock formations are made of atoms, and they contain gaps or empty spaces (see Figure 2-3). This feature is especially true for sedimentary rocks, which comprise most petroleum reservoirs (see Appendix A). Except in chapters 1 and 2, our discussion in this book has so far totally disregarded the atom scale (microscopic scale) by assuming that most rock formations can be described as isotropic piecewise-continuous regions, separated by interfaces in which the medium parameters are discontinuous. In other words, we have so far described the subsurface as an isotropic heterogeneous medium in which elastic properties can vary from one point to another

(i.e., a heterogeneous-medium assumption), but for any given point of the medium, these elastic properties cannot vary with direction (i.e., the isotropic-medium assumption). The word point here represents a particle (representitive of a volume) at a macroscopic scale whose size is of the order of a quarter of a seismic wavelength, about 6 m or more.

Evidences of heterogeneities much smaller than the particle are abundant. The crossbedding in this figure and the photomicrograph of limestone in the next figure are just a few of many evidences of heterogeneities at scales much smaller than those of particles. Therefore, a model of the earth which ignores small-scale heterogeneities is bound to be inadequate for describing some rock formations. However, the laws of continuous mechanics that we are using today to study seismic wave propagation and to analyze seismic data are valid only at the particle scale. So the dilemma that petroleum seismologists face is how to process and interpret seismic data at the particle scale while taking into account some of the behaviors of rock formations at a scale much smaller than that of particles. One way of addressing this dilemma is to consider that rock formations can behave as an anisotropic medium at the particle scale; i.e., elastic properties at a given point of the medium can vary with direction (i.e., the anisotropic-medium assumption) in addition to the fact that these properties can vary from one point to another (i.e., the heterogeneous-medium assumption).

Other Images in Chapter 12

An example of the sailing path of a marine vessel in a towed-streamer survey

Images - Chapter 8 An example of the sailing path of a marine vessel in a towed-streamer survey. Note that the time for turning from one sailing line to another is about nine hours for vessels carrying streamers that are 10 km long. The dotted line indicates the turning legs of the sailing path.

This figure illustrates a typical sailing path of a 3D survey; the vessel travels back and forth, shooting and collecting data along many parallel lines, resulting in seismic data generated along lines 25 to 50 meters apart. Note that it takes about nine hours to turn from one sailing line to another for a vessel carrying 10-km-long streamers. Today, data are recorded even when the vessel is turning.

A shot diagram

Images - Chapter 7 A display of source and receiver distribution of a 2D seismic line in the so-called shot diagram. The rows corresponding to common-shot gathers and columns to common-receiver gathers. The diagonal is the zero-offset section, and all the other lines parallel to the diagonal are common-offset gathers (also known as common-offset sections). The lines perpendicular to the diagonal are the CMP gathers.

Another illustration of towed streamer acquisition

Images - Chapter 7 Another illustration of towed streamer acquisition

Interference noise

Images - Chapter 7 An illustration of interference noise in seismic data before and after. This figure shows the stack of seismic interference noise contaminated shots from another line in the Gulf of Mexico. Interference noise is clearly visible. Attenuation of seismic interference noise can be achieved by the use f-x prediction filters. Courtesy of Western Geco.

Other examples of structural traps

Images - Chapter 1 Structural traps. (A) Tilted fault blocks in an extensional regime. The seals are overlying mudstones and cross-fault juxtaposition against mudstones. (B) Rollover anticline on thrust. Petroleum accumulations may occur on both the hanging wall and the footwall. The hanging wall accumulation is dependent on a subthrust fault seal, whereas at least part of the hanging wall trap is likely to be a simple, four-way, dip-closed structure. (C) Lateral seal of a trap against a salt diapir and compactional drape trap over the diapir crest. (D) Diapiric mudstone associated trap with lateral seal against mud wall. Diapiric mud associated traps share many common features with that of salt. In this diagram, the diapiric mud wall developed at the core of a compressional fold. (E) Compactional drape over a basement block commonly creates enormous low-relief traps. (F) Gravity-generated trapping commonly occurs in deltaic sequences. Sediment loading causes gravity-driven failure and produces convex-down (listric) faults. The hanging wall of the fault rotates, creating space for sediment accumulation adjacent to the fault planes. The marker beds (grey) illustrate the form of the structure that has many favourable sites for petroleum accumulation. Adapted from Gluyas JG and Swarbrick RE (2003) Petroleum Geoscience. Oxford: Blackwell Science.