Marine electromagnetic surveying for hydrocarbon detection

Principle of marine electromagnetic logging; a resistivity-based hydrocarbon detection method. This method exploits the large resistivity contrast known to exist from standard borehole logging (left) between oil or gas reservoirs and surrounding water-filled sediments (note that hydrocarbons are highly resistant to the passage of electric currents). It consists of a line of receivers placed on the seafloor in and around a target area, while a powerful source transmits a low-frequency, electromagnetic signal down through the underlying rock formations. In the presence of hydrocarbons, signals are reflected back to the surface where they are recorded by the receivers. This information is then processed to obtain bulk resistivity images of the underlying rocks. However, the resulting images are far less detailed than the seismic counterparts. In the absence of hydrocarbons, no response is received. The technique is currently designed to work in offshore deep-water areas where the depths of potential reservoirs below the seafloor (which should be no greater than 2000 m) have been pre-determined from seismics.

The seismic method has long held sway in detailed exploration and, more recently, reservoir description and monitoring; and nobody expects this to be otherwise. However, research and development related to hydrocarbon E\&P are also going on in non-seismic areas. One of the areas with current high focus is marine electromagnetic (EM) sounding or seabed logging, which is essentially a marine electromagnetic acquisition technique using a mobile horizontal electric dipole source and an array of seabed electric field receivers (\cite{Eide02} and \cite{Elli02}), as illustrated in this figure.

This technique has originated from long-established principles of resistivity well logging. In general, the matrices in sedimented rocks are insulators, and their contained fluids determine the resistivity. Normally the formation fluids are compareble with seawater, which is conductive due to salts dissociated in an aqueous solution. However, hydrocarbons are an important exception when it comes to fluid conductivity. If a porous rock contains hydrocarbons, the overall resistivity will be very high; if the same formation contains salty water, its resistivity will be low. In other words, a large resistivity contrast is known to exist from standard petrophysical well logging between oil or gas reservoirs and surrounding water-filled sediments.

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.