Stress field

Stress field near the end of a fault. (a) Homogeneous stress

field before displacement occurs on the fault. (b) Stress field after displacement on the fault, in homogeneous stress field.

For a given medium, the stress field is simply the values of stress components at every point in the medium. It can be characterized as homogeneous (or uniform) if all of the stress components are the same at every point. This is the case for the tank of water (described in Section 1) before the stone is dropped. However, the stress field can be inhomogeneous even in the absence of external forces, as the example in this figure shows.

Most petroleum seismology models of the subsurface assume, sometimes implicitly, that the stress field is homogeneous in the absence of external forces (i.e., before triggering the external force responsible for generating seismic waves). This assumption is obviously not true. First, there are always body forces present in rock formations which introduce gradients in stresses from point to point. Second, all rocks are made up of grains, layers, or other elements which are mineralogically distinct from their adjacent parts and therefore have somewhat different responses to stress. Some of these parts resist deformation more than others and thereby carry higher stresses; other parts resist deformation less and carry lower stresses. The parts with lower stresses may decay, given sufficient time or temperature, but they will always be present to some degree. Third, as reservoirs are depleted, they may be subjected to subsidence or to damage due to stress fatigue. Therefore, we must consider the state of the stress field as an important parameter in our theory of wave propagation, especially if the medium is considered nonlinearly elastic. We can seek to recover the stress field from measurements, or we can enter it into our theory as an initial condition if it becomes available from independent measurements.

Other Images in Chapter 2

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.