Diagram of a gas hydrate

Diagram of a gas hydrate, showing how water molecules form a lattice around 'guest' gas molecules. The chemical name for this cage-like solid structure is 'clathrate'. Diagram courtesy of Bjorn Kvamme.

Mathews, in The Log Analyst, described hydrates as follows:

"Hydrates from when one species (known as the host) forms a crystal lattice containing voids or cavities that physically entrap other guest molecules. In natural gas hydrates, the host is water and the guest can be any common constituent of natural gas such as CH₄, C₂H₆, C₃H₈, C₄H₁₀, CO₂ or H₂S. Natural gas hydrates can have two possible structures, depending on the size of the guest molecules. Both structures consist of a network of water molecules hydrogen-bonded to each other in a manner similar to water ice. (Mathews (1986))"

The figure shows a diagram of gas hydrates; the water molecules form a lattice containing 'guest' (gas) molecules - this is also called a clathrate substance in chemistry. The conditions under which gas hydrates are stable are tightly controlled by temperature and pressure, but are also affected by the chemistry (i.e., including the salinity) of the pore water and the composition of the 'guest' gases. Chemicals (such as salts and methanol) that reduce the pressure/temperature envelope of hydrate stability are called hydrate inhibitors.

Other Images in Chapter 1

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.