6.2 Magnetics for Hydrocarbon Exploration


How can a tiny mineral called magnetite help to unravel hydrocarbon seepage and subsurface petrology? It’s not magic – it’s magnetics… in integrated workflows!


Magnetism, as you recall from physics class, is a powerful force that causes certain items to be attracted to refrigerators. Dave Barry


Magnetite has the ability to respond to a magnetic field by genera ting its own field, known as the induced magnetic field. Magnetite is the most important member in the family of magnetic minerals, which are found in varying amounts in crystal line rocks and to a lesser extent in sediments. In oceanic crustal rocks, it is common for magnetic minerals to per manently hold substantial magnetic fields. Tese remanent fields date back to the moment when the rocks in the hot and just solidi fied magmas of mid-oceanic ridges cooled through the Curie point (the temperature at which magnetic materials undergo a sharp change in their magnetic properties) and captured and stored the acting magnetic field characteristics. Tis can be used to unravel the paleomagnetic history of the rocks.


6.2.1 Magnetic Anomalies


We know the direction of the Earth’s magnetic field from the compass needle pointing north. Te Earth’s field looks like that of a magnetic dipole – a huge blueprint of your refrigerator magnet, but which is located in the Earth’s core. All rocks on Earth, as well as you and me, are bathed in this magnetic field and respond with induced magnetic fields. But since we do not have much magnetic material in our bodies, the fields of the rocks will dominate. Geophysics uses a


parameter called ‘magnetic susceptibility’, which expresses the strength of the response of a rock to an imposed magnetic field – an easy alternative to estimating the amount of magnetic minerals in a rock sample. High magnetic susceptibility will give a strong induced magnetic field and vice versa. Tese induced magnetic fields, together with the remanent magnetic fields, cause very small deviations to the Earth’s magnetic field strength, known


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Guest Contributor: Christine Fichler, NTNU


as magnetic anomalies. Tey are measured by intricate yet backpack-sized instruments, typically mounted on aeroplanes for hydrocarbon exploration applications. Onshore and offshore areas are mapped line by line and the results processed to create maps of magnetic anomalies, which present the sum of the induced and remanent magnetic anomalies of every single magnetic mineral in the subsurface; in other words, they reflect the distribution of magnetic susceptibility and remanence.


6.2.2 The Magnetic Choir


In order to understand the recorded anomalies at the surface, imagine that the magnetic minerals form a choir and you are the magnetometer ‘listening’ to the choir from the front. You will easily hear the nearest singers, but listening to those further away gets more and more difficult with increasing distance – only if somebody far away is singing very loudly with a megaphone, will you hear them. Te shallowest magnetic rocks in the subsurface are nearest to the magnetometer and will as such generate the strongest anomalies and show a lot of details. With increasing depth, the anomalies become smaller in amplitude and lose details, as illustrated in the left part of Figure 6.8. Te right-hand image of Figure


6.8 illustrates magnetic rocks in sediments and crust and their associated magnetic anomalies: in our choir analogy, the large anomalies (grey) from the crystalline rocks are the ‘singers with megaphones’, while the tiny, small scale anomalies (red) from


just a very few structures in the sediments are the


‘singers in the front’. Te most important thing to


understand about sedimentary anomalies is that they are small


Figure 6.7: Unusually large magnetite crystals from Bolivia. These crystals measure to over 1 cm on edge, 1.5 cm tip-to-tip.


in amplitude and as such are only detectable if located in the uppermost kilometre of the subsurface. Furthermore, mapping them needs high resolution data, which means


Rob Lavinsky/Wikipedia


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