For non-fission track visitors : why FTD in oil exploration ?
Written by Michael Krochmal   
Tuesday, 04 September 2007 07:03

The question is :

What is the utility of thermochronology in facilitating oil exploration ? Does it better define the production environment and so the likelihood of finding oil ? Do you still need to obtain the (core) sampling array, but are better able to define what is there ? Or is it something else ?

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(Mike's total layman's take on FTD - ie The Complete Idiot's Guide To ... ) :

You have hit the nail on the head : my understanding is that the core samples are still taken, but the analysis is of higher quality.

Oil tends to occur in places where suitable organic matter was present in prehistoric times, and where the eventually resultant rock containing this matter has experienced a thermal history that has turned the organic matter into oil. The two prerequisites for finding oil are that the rock is of a particular age, and that it has been compressed over vast time scales (thus increasing the temperature and enabling the necessary chemical processing to proceed). FTD is a technique which allows such assessments to be made, using suitable marker materials.
 
The most commonly used marker materials are apatite [Ca5(PO4)3(F,Cl,OH)], zircon (ZrSiO4) and sphene (also known as titanite) [CaTiO(SiO4)]. These materials are chosen because they incorporate suitable concentrations of naturally occurring uranium. The marker minerals are separated from the rest of the rock matrix by mechanical, magnetic and other means (eg the use of very heavy liquids). Each of the three minerals mentioned above has different characteristics, and each is used as a slightly different indicator.

The FTD technique for which our equipment is used, and which is known as the "External Detector Method" (EDM) of Fission Track Dating, is most often used with apatite. The technique allows the scientist to find the age of the rock sample by comparing the number of damage tracks (fission tracks) which have arisen spontaneously over the millenia in the mineral crystals (grains), due to the nuclear breakdown of the uranium, with the number of damage tracks created in an external detector, usually a thin sliver of muscovite mica, by exposure to a slow neutron beam. In Australia, this exposure is carried out at the OPAL reactor facility at ANSTO in Lucas Heights, NSW. The exposure causes secondary emission of radiation which damages the mica, whose activity depends on the age of the mineral (greater ages being associated with greater uranium depletion, and thus fewer tracks on the mica). Old samples have accumulated lots of ("spontaneous") grain tracks, but create few ("induced") mica tracks, and young samples are the opposite. For both the grain and mica mounts, the tracks to be analysed can be made visible under an optical microscope by chemical etching.

The thermal history can be established because an increase in temperature gives rise to a shortening of the tracks by a "self-healing" process in the crystal lattice, which is called "annealing". By taking a statistical sample of track lengths, the scientist can establish a time/temperature history.

The observation of the fission tracks is a challenge, as they are usually less than 16 microns long and only about one micron in diameter. The first component of the challenge is that the Raleigh limit for optical resolution in ordinary white light lies somewhere not too far below one micron. (The wavelength in mid-spectrum for visible light is about 550 nanometers, or .55 micron, and the limit under ideal conditions is about a half wavelength, but in practice it is usually much worse than this). The second component of the challenge is that the depth of focus at the high magnifications (X1000) needed to clearly see the tracks (especially the ends of the tracks, in order to accurately measure their lengths) means that the whole track is never in focus at the same time. This is why, to improve the analytical process, it is essential to first establish a high-quality 3D image of the tracks.

As you can imagine, the determination of the likelihood of finding oil in a given spot is of crucial importance in the oil exploration industry, where drilling can cost perhaps a million US dollars a day, and causing someone to drill in the wrong spot is not going to improve one's popularity !

 

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