BSR/gas hydrates

Comparison of P-wave (a) and converted PS-wave (b) images through the same geological section. The BSR is clearly visible on the P-wave data (a) but is not observable on the PS-wave data (b). (Courtesy PGS).

This example is related to characterization of gas hydrates, which were introduced in Chapter 1. Gas hydrates are ice-like crystalline solids in which a gas molecule, normally methane, is included within a cage of water molecules. Methane hydrate is stable in

near sea-floor sediments at water depths greater than 300 to 400 m. The limited knowledge on the nature of naturally forming gas hydrates comes from shallow coring of deepwater drilling. Seismic reflection methods seem to be the most promising approach for indirect detection of marine gas hydrates. As discussed in Chapter 1, the bottom simulating reflection (BSR), as observed on conventional P-wave data (e.g., towed-streamer data), is the most commonly used indicator of the presence of gas hydrate accumulations below the sea floor. P-wave data alone, however, seem to fail in detecting gas hydrates when the BSR is absent.

On the other, hand P-S data integrated with P-wave data enable a better interpretation of the nature, structure, distribution, and quantification of gas hydrates, regardless of the existence of a BSR. In order to test what information PS data can provide about gas hydrate sediments and their characterization, PGS acquired a 4C line profile over a location in the Norwegian Sea, where a BSR has been identified on conventional streamer P-wave data. Parts of the P-wave and PS-wave migrated stacks from this multicomponent line are shown here. By comparing the migrated stacks, we observe that events at the BSR area and below are quite different on the two data sections. The BSR is clearly visible on the P-wave data but is not observable on the PS-wave data.

PS reflections are not masked by the gas effects, as P-P reflections are, thus providing better stratigraphic and structural information. P-S reflections seem to follow the sediment layers. In this area, the fact that the BSR is not detected on the PS-wave section suggests that the gas hydrates in the sediments above BSR have not stiffened the sediment framework, indicating that the hydrate is not cementing the grain contacts. By contrast, if the hydrate had formed at grain contacts, the hydrate could have acted as a cementing agent. Then the sediment framework would become stiffer, resulting in increased P-wave and S-wave velocities above the BSR. PS-wave data in this particular situation could potentially show the BSR and could potentially give more detailed information about the stiffness of the sediment and gas hydrate concentration.

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.