Exploring the subsalt

A geological model of a small region of the Gulf of Mexico contructed by the SMAART JV group, which included representatives of four major oil companies (BP, BHP, ChevronTexaco, ExxonMobil). "S" indicates salt sheets.

To gain a better insight into this problem, we have displayed in this figure an example of a geological model constructed by the SMAART JV group, which included representives of four major oil companies (BP, BHP, ChevronTexaco, and ExxonMobil).

The physical properties of salt---a density of 2.1 g/cc and a velocity of 4400 m/s or more---are in sharp contrast with the surrounding sediments, which are generally denser and have lower velocities. The strong contrasts in velocity and density at the sediment-salt interface acts like an irregularly shaped lens. In the past, petroleum seismologists have treated this contrast like a mirror, producing images that portrayed salt features as bottomless diapirs extending to the deepest level of seismic data. Once seen as impenetrable barriers to petroleum seismology probing, many salt structures are now proving to be thin blankets shielding rich reserves. Detecting these reserves requires more energy penetration than in traditional seismic acquisition. It also requires attenuating multiple reflections (i.e., unwanted reflections which arrive at almost the same time as the desired subsalt reflections but with higher energy than that of the desired reflections). Moreover, drilling through the salt is particularly difficult because the properties of salt---pseudoplastic flow under subsurface temperatures and pressures, and low permeability---make drilling through salt bodies like drilling through fluids. These difficulties increase the need for accurate imaging of the subsalt traps to reduce drilling risks. (From Bishop et al., 2001; Miley at al., 2001; and Stoughton et al., 2001.)

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.