A and B crossplotting of AVO

A and B crossplotting of AVO.

Based primarily on the correlations between the changes in Poisson's ratio and the presence of hydrocarbons in a reservoir, petrophysists have established four classes of AVA responses. Class I occurs when the upper layer has lower acoustic impedance than the lower layer. The normal-incidence P-wave reflection coefficient is strongly positive and shows a strong amplitude decrease with angle and possibly a phase change at large angles. Class II has very little difference in acoustic impedance between the upper and lower layers, and in many cases the velocities and densities will change in opposite directions. The normal-incidence P-wave reflection coefficient is either slightly positive or slightly negative. In the first case, a phase change occurs at small or moderate-size angles. Class III occurs when the upper layer has a higher acoustic impedance than the lower layer. For Class III anomalies, the normal-incidence P-wave reflection coefficient is strongly negative. The reflection coefficient becomes more negative with angle. Classical bright spots show class III behavior. For shale over gas sand reflections, a simple, often-used rule of thumb is that the reflection coefficient gets more negative with increasing angle. Classes I-III originally were introduced to detect and describe gas sands. However, some gas sands turn out to exhibit AVA behavior contrary to the much-used rule of thumb. While the normal-incidence P-wave reflection coefficient is strongly negative, the reflection coefficient decreases with angle. This is the characteristic of Class IV sands. Class IV sands occur in many basins throughout the world, including the Gulf of Mexico.

The cross-plot of the intercept and the gradient in this figure illustrates these four AVO classes. For a shale unit over sand, Class I sands occur in quadrant 4 of the A-B plane: A is positive while B is negative. Class II sands have about the same P-wave impedance as the overlying unit. Such sands may occur in quadrants 2, 3, and 4 of the A-B (intercept-gradient) plane. Class III sands, which frequently are bright, have lower P-wave impedance than the overlying shale. They occur in quadrant 3. Class IV sands occur in quadrant 2.

In addition to A_PP and B_PP, numerous combinations of A_PP and B_PP are used in the AVO analysis, such as A_PP + B_PP, A_PP - B_PP, A_PP x B_PP. The correlation of these combinations to petrophysical parameters or lithology is still a wide-open question.

Other Images in Chapter 3

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.