Alba channel

(a) Conventional streamer P-wave image of the Alba channel. Note the weak top sand event in the mid of the section at around 2 s traveltime. (b) The converted wave PS image shows dramatically improved imaging of the sand channel due to the high PS reflectivity between shale and sand. (c) The dipole sonic log. through the Alba reservoir sand shows a large contrast in shear-wave velocity (left) and a small contrast in P-wave velocity (right) with the surrounding shales. The green curves represent velocities in sand. The red and blue curves represent velocities in shales above and below the sand channel, respectively.

The Alba field in the central UK North Sea consists of channel sands sealed by shales.

The channel sand is roughly 9 km long, 1.5 km wide, and up to 100 m thick. On conventional P-wave seismic data shown in (a) one observes a weak, inconsistent reflector at the top of the reservoir and intrareservoir shales, but a strong oil-water contact response. The reason that the top of the Alba reservoir is almost seismically invisible is that there is little or no contrast in P-wave reflectivity between the reservoir sands and the shale cap rock. However, as displayed in (c, wireline sonic logs show an increase in shear-wave impedance at the shale-sand interface that is significant; hence it was expected that P-S converted reflection data would properly image the Alba reservoir.

In early 1998, Chevron acquired a 67 km² 3D-4C survey at Alba.The P-S data shown in (b) provided a clear and high-quality image of the reservoir body. By comparing the OBS data with streamer data, reservoir fluid changes after four years of production and water injection have been directly imaged. Chevron's study has improved the reservoir characteristics of Alba, allowing better placing of new development wells, and it furthermore documented the usefulness of P-S converted data to image low P-wave impedance contrast reservoirs displaying large shear-wave impedance contrast.

Even though reservoirs with low P-wave reflectivity due to the small contrast in P-wave impedance are well imaged by P-S waves, due to the large contrast in S-wave impedance, large-angle stacks of conventional P-wave towed-streamer data can potentially provide a satisfactory image. P-wave AVO predicts that shale-sand interfaces with small P-wave impedance contrast, but high S-wave impedance contrast should have significant P-wave reflectivity for large angles of incidence. Therefore, before deciding on 4C-OBS acquisition for imaging of low P-wave impedance contrast reservoirs, the petroleum seismologist should evaluate whether towed-streamer AVO sections can solve the problem at hand. In some cases, however, the reservoir may be an obstructed area limiting the access for towed-streamer operations. In this case, 4C-OBS is definitely the best solution.

Other Images in Chapter 7

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.