Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 Palaeomagnetic Applications in Hydrocarbon Exploration and Production: Introduction PETER TURNER 1 & AMANDA TURNER 2 1School of Earth Sciences, The University of Birmingham, Birmingham B15 2TT, UK 2TERRASCIENCES Ltd, 8 Canfield Place, London NW6 3BT, UK Abrief outline of palaeomagnetism include the classic work of Irving (1964) and the more recent books by McElhinny (1973), Tar- Although the study of magnetism is one of the ling (1983) and Butler (1992). oldest sciences, palaeomagnetism is a relatively Palaeomagnetism and rock magnetism have young geoscience which incorporates aspects of considerable value in hydrocarbon exploration geomagnetism, rock magnetism and geology. Its and production although there have, as yet, application to sedimentary sequences developed been few subsurface palaeomagnetic studies. rapidly in the post-war era as a result of both The main areas in which palaeomagnetic data theoretical and technological advances. The may be useful to the petroleum geologist are: theoretical advances included an ever-increasing palaeomagnetic dating and magnetostratigra- understanding of the Earth's magnetic field and phy; susceptibility logging of cores; magnetic a much more detailed knowledge of the physical fabric studies for palaeocurrent analysis; using properties of those minerals capable of carrying remagnetization to date diagenetic events, in- magnetism (natural remanence) under Earth cluding those associated with hydrocarbon mi- surface conditions. Also, applications to a wide gration; and the identification of magnetic variety of geological problems in many different anomalies associated with hydrocarbon plumes parts of the world led rapidly to a much greater and oil seeps. Basic susceptibility and rem- awareness of the way in which rocks acquired anence measurements are also required for the their magnetization, how it might be altered interpretation of aeromagnetic surveys. during later geological history and how the For the benefit of the non-specialist some of derived palaeomagnetic directions might be the basic concepts in palaeomagnetism will be used to unravel the movement of large crustal dealt with here. These concern the Earth's blocks (apparent polar wandering) and the more magnetic field, magnetic minerals and rem- localized rotation of smaller tectonic blocks. A anence acquisition and the main applications key phase in the development of palaeomag- relevant to hydrocarbon exploration and pro- netism was the recognition that marine magnetic duction. anomalies represented reversals of the Earth's magnetic field. Subsequent systematic sampling and measurement of the ocean basins by the The Earth's magnetic field ODP (ocean drilling project) has resulted in a The Earth's magnetic field roughly corresponds sophisticated geomagnetic polarity time scale to that of a geocentric axial dipole. In this simple (GPTS) from the middle Jurassic to present day. model a magnetic dipole at the centre of the It is beyond the scope of this overview to Earth is aligned parallel to the rotation axis provide more background information but there (Fig. 1). The geographic latitude (h) ranges from are a number of excellent texts which cover all +90 ~ at the north pole and -90 ~ at the south pole aspects of palaeomagnetic theory and appli- and the inclination of the field and geographic cation. For theoretical background there is a latitude are related by: thorough introduction to geomagnetism by tan I = 2 tan h (1) Parkinson (1983); O'Reilly (1984) provides a rigorous and concise treatment of rock and In fact, the Earth's field is more complicated and mineral magnetisms and the techniques of a more accurate model is that of an inclined sampling and measurement are very clearly geocentric dipole with the dipole inclined at an described by Collinson (1983). Textbooks which angle of 11.5 ~ to the rotation axis. This accounts describe the application of palaeomagnetism for about 90% of the field and the other 10% is From TURNER, P. & TURNER, A. (eds), 1995, PalaeomagneticApplications in Hydrocarbon Exploration and Production, Geological Society Special Publication No. 98, 1-5. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 2 P. TURNER & A. TURNER geomagnetic A B north.pole N north magneticpole ~..~._~raphic pole) magnetic equator I ~~---/ geoorahic ,i_ o,o )-~ o0oa,or- P geomagnetic .1~r~'- best-fi{ling I\ / equator X dipole- I X / I \ ./~_.. south magneticpole [I = -90~ Fig. 1. (A) The geocentric axial dipole model. The magnetic dipole M is at the centre of the Earth and aligned with the rotation axis k is the geographic latitude and re is the radius of the Earth and N the north geographic pole. The magnetic field vector H for one location with inclination is shown. (B) The inclined geocentric dipole model. The diagram shows the difference between magnetic poles and geomagnetic poles and also a comparison between the magnetic and geomagnetic equators. The difference between magnetic poles and geomagnetic pole is due to the non-dipole field which accounts for 10% of the total field. (A and B after Butler 1992). attributed to non-dipole contributions. Short- ations of about 2 x 103 A/m. Titanomagnetites term variations of the non-dipole field (<3000 have the ideal formula Fe3_xTixO4(0< x< 1) years), for example the eastward shift in decli- and crystallize in the spinel structure. They are nation since about AD 1800 in the UK, constitute ferrimagnetic and have saturation magnetiz- the geomagnetic secular variation. ations of about 4.8 x 10 -5 A/m. They are thus The relationship tan I = tan k has important magnetically much stronger than haematite - a implications for palaeomagnetic measurements fact which must be taken into consideration of core material. Because most core material is when assessing the rock magnetic composition unorientated, any palaeomagnetic measure- of sedimentary rocks. They typically form from ments will be unorientated in the horizontal igneous melts and in the sedimentary environ- plane and thus have only arbitrary declinations. ment are usually at least partially oxidized to Inclination, however, can be related directly to their non-stoichiometric equivalents, the titano- geographic latitude. Since the magnetic incli- maghemites, which have the ideal formula nation at the equator is 0 ~ the magnetic polarity Fe(3_x)RTixRI-]3(I_R)O4 where [] are vacancies in from core can only be determined if the sampling cation sites of the structure. site is sufficiently far from the palaeoequator There are also some magnetic iron sulphides (say >10~ such as some pyrrhotites (Fe,_xS where 0<x<0.13 and greigite (Fe3S4). Most other Magnetic minerals and remanence iron-bearing minerals in the sedimentary en- acquisition vironment are paramagnetic or diamagnetic. These include the iron carbonates (paramag- There are only a relatively small number of netic) and pyrite (diamagnetic). Of particular minerals which are capable of carrying natural interest to the petroleum industry are magnetite permanent magnetization and are therefore of spherules (cc. 50 Ixm) which may be the oxi- importance in palaeomagnetic studies. These dation products of framboidal pyrite and magne- include the iron oxides, including haematite tosomes, small magnetite crystals which formed (et-Fe203) the titanomagnetites and the titano- in magnetotactic bacteria. These are especially maghemites (Fig. 2). In general, haematite and significant because of their microbial origin and ferric oxyhydroxides are magnetically weak the possible link to hydrocarbon environments antiferromagnets with saturation magnetiz- and also because they lie in the single domain Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 INTRODUCTION 3 TiO 2 1 ~" Fe 2 TiO 4 ulvospinel , [maghemitization ~ ~Se/.,.. "//t@,.~ . TMI subsolvus Fe 0 r precipitation, ~iC--I~Fe304 r- m~i-i-i-i-i-i-i-i-i-~a~n, "] ~'Fe203 wust~te ~ oxidation of silicates, .~/~/fl Isupergene alterationl haematite alteration of clay minerals | martitization J maghemite serpentinization Fig. 2, The FEO-TiO2-FeO3 ternary diagram of rock magnetism. Points in the triangle correspond to oxide compounds or mixtures of compounds containing FeE+, Fe3+ and Ti4+. The point TM60 represents the idealized titanomagnetite with composition Fe2.4Ti0,604. Some of the paths which may be followed by the magnetic mineral fraction of crustal rocks in the course of geological history are indicated (after O'Reilly 1984). size range (<1 ~m) and are thus palaeomag- relaxation time may be greatly reduced and such netically very stable. grains may contribute to unstable magnetiz- A key concept in palaeomagnetism is the time ation. When the relaxation time is very short dependence of magnetization. All magnetic (several seconds to a few minutes) the magnetiz- particles have a relaxation time (.r) which is ation is said to be viscous. Viscous remanent determined by grain volume, coercivity, satu- magnetization (VRM), because of its short ration magnetization and temperature as fol- relaxation time, will align itself in the ambient lows: magnetic field direction, which may be the geomagnetic field. "r -= 1/C exp (vJsBc/21T) (2) Equation 2 also explains why thermal demag- Where C is a frequency factor of c. 101~s -1, v is netization is of such fundamental importance in the volume of the grain of coercivity, Be, and palaeomagnetism. By progressively increasing spontaneous magnetization, Js, and k is