1

Palaeomagnetic Dating of Reservoir Rocks- a Review

Abdelaziz L. Abdeldayem

Department of Petroleum Geoscience, Faculty of Geoscience and Petroleum Engineering, Universiti Teknologi Petronas, Bandar

Seri Iskandar, 31750 Tronoh, Perak Darul Ridzuan, Malaysia

Abstract- The study of remanent magnetism, palaeomagnetism,

is reviewed here as a potential geochronologic tool that can be

used to date sedimentary reservoir rocks, particularly

unfossiliferous sandstone, where most oil reserves reside When

sandstone is formed, magnetically susceptible microscopic iron

particles record the direction and magnitude of the Earth’s

magnetic field. This record, therefore, provides information on

the past behavior of the field throughout geologic times, including

palaeo-locations of magnetic pole positions and the geomagnetic

reversal records preserved in rock sequences (magneto-

stratigraphy). Such records provide a time-scale that can be used

as a geochronologic tool to determine the absolute age of a

reservoir rock, hence the depiction of its depositional history and

evolution.

Palaeomagnetic dating can be applied to both rock outcrops

(oriented core samples) and subsurface reservoirs (borehole cores

or magnetic logging). In case of core plugs (surface or subsurface),

stepwise demagnetization is applied to remove unstable magnetic

records and isolate primary records. Both visual and statistical

methods are then used to analyse the entire spectrum of the

residing remanent magnetisation to isolate the primary

component that is supposed to reflect the age of the rock. The

later is then used to calculate the position of the corresponding

palaeomagnetic pole or determine the type of magnetic polarity

based on the standard geocentric axial dipole model for the

average geomagnetic field. The results are then correlated to

corresponding standard records of either the apparent polar

wander path (APWP) or geomagnetic polarity time Scale (GPTS)

to determine the age of the reservoir rock involved. In the case of

magnetic logging, a different approach is applied where the

Geological High-resolution Magnetic Tool (GHMT) is used to

simultaneously record both the magnetic susceptibility and the

Earth’s total magnetic induction down the borehole where the

magnetic reversal sequence is determined by isolating the polarity

of the scalar remanent magnetization. Unlike working with cores

that are sometime incomplete due to poor recovery or suffer from

drilling-induced overprints due to the strong magnetic field

present in the pipe, GHMT permits a continuous clear record of

time reference targeted.

Illustrative examples representing the above-mentioned

palaeomagnetic dating methods will be presented here.

I. CONCEPTS OF PALAEOMAGNETIC DATING

Introduction

Most oil reserves reside in sedimentary rocks, mostly

sandstone, that usually have a broad age window due to lack of

any credible record of index body fossils or limited bio-

stratigraphic dating records. This has always caused a problem

of where to consider them in the geochronological sequence of

a basin.

Palaeomagnetism, as a viable dating tool, has long been

potentially utilized to provide absolute dates for different types

of rocks by correlating their primary Natural Remanent Magnetization (NRM) records to pre-established international

standard records throughout geologic time. This technique is

based on the fact that most rocks, including sandstones and

other reservoir rock types, contain magnetic grains that

become aligned with the direction of the Earth's magnetic field

at the time of their formation. These accessory iron oxides, like

magnetite and hematite that are known as ferromagnetics, not

only have the ability to acquire the orientation and intensity of

the then Earth's magnetic field during rock formation but also

to preserve it in their internal orientation and retain it

throughout geologic time. These wide-scale frozen NRM

records of the Earth's magnetic field have provided an important tool for determining the age and evolution path of

different types of rocks [1, 2, 3]. Most reservoir rocks

comprise relatively high content of magnetic minerals and

measurable magnetism that allow them to be credibly studied

and dated palaeomagnetically [2, 4, 5, 6].

The mechanism by which NRM is acquired depends upon

the mode of formation and subsequent history of the rock as

well as the characteristics of the magnetic minerals it contains.

As for sedimentary reservoir rocks, they acquire a specific type

of NRM that is known as Detrital Remanent Magnetization

(DRM) which results from the alignment of magnetic particles during deposition. There are two other basic types of NRM

that depend on the mechanism of palaeomagnetic signal

acquisition, which are TRM and CRM. The former refers to the

thermoremanent magnetisation acquired when a rock (igneous

or metamorphic) cools below the Curie temperature of its

magnetic minerals, thereby „locking‟ the NRM that may last

for billions of years. The latter, CRM, is the chemical

remanent magnetisation acquired when a magnetic mineral

grows through a critical „blocking‟ volume or grain size at

which the field is locked in and the acquired remanence may

also be stable for billions of years.

The total NRM observed in a particular rock is composed of primary magnetization remain that was acquired during the

formation of the rock, as well as the more recently added

secondary magnetizations. The latter are, by definition, those

magnetizations that were acquired more recently than the

original time of rock formation. Different components of

NRM, which have been acquired at different times in the rock's

history, will reside in different populations of magnetic

mineral grain within the rock, which commonly acquires

different magnetic stabilities. Separation of the different

components of magnetization in a rock is normally achieved

by progressive demagnetization procedures, i.e. magnetic

cleaning. The lower stability components are removed

preferentially during the early stages of the treatment, and the

higher stability components during later stages. Different

magnetic cleaning experiments and stability tests are usually

used to assess the primary nature of the residing NRM. Details

of these methods are beyond the scope of this paper but can be

explored in standard text books [e.g. 1, 2, 4, 6 and 7].

The present review is, therefore, aimed at outlining viability

of palaeomagnetism as a valid dating tool for reservoir rocks

where absolute dates can be obtained by correlating primary NRM records derived from these rocks to established

international standard records of different time scales [e.g. 8].

Dating Potentiality

The fundamental of palaeomagnetic dating is based on the

fact that the main geomagnetic field is subject to changes in its direction and strength on time scales ranging from years to

millennia and even millions of years [9, 10, 11]. The more

useful geomagnetic changes are those resulting from of long-

term fluctuations with a time scale that allows them to be

recorded in every type of natural rocks making them

worldwide phenomena. For instance, it has long been

established that the geomagnetic field inverts its polarity (i.e.,

magnetic north and south poles interchange) at irregular

intervals. Such a random character of polarity reversal has

given it a value as a stratigraphic dating tool and helped in the

establishment of the first geochronological palaeomagnetic

time scale, the Geomagnetic Polarity Time Scale (GPTS), that is globally valid [15] and the development of the branch of

magnetostratigraphy [12, 13, 14]. The time spent in one or the

other polarity state varies from about 104 to 107 years, and the

transition period may take between 103 and 104 years. It simply

orders strata into intervals characterized by the NRM direction

of the rock, being either in the direction of the present Earth‟s

magnetic field (normal polarity) or 180 away from it (reverse polarity).

With the availability of more and more credible recent

palaeomagnetic inputs, precision of standard records of the

Earth's magnetic field reversals through time has improved

considerably and a well-established GPTS is currently

available, particularly for the past 200 Ma where a complete record is preserved in the sea-floor (oceanic crust). This, in

turn, enables potential geochronologic applications on a wide

scale time range [12, 14].

To further strengthen this indispensible stratigraphic dating

tool, recent attempts have been made to astronomically

calibrate the GPTS through the extensive use of

cyclostratigraphy [e.g., 16] to develop what is now known as

the Astronomically calibrated Polarity Time Scale (APTS)

[17]. The APTS is now almost complete for the Neogene [18]

and has been developed for the Late Triassic [19, 20, 21].

Luckily, most hydrocarbon reservoir rocks are covered by this detailed magnetostratigraphy time scale period. Fig. 1 shows

the most recently constructed Phanerozoic GPTS as

implemented by the Subcommission for Stratigraphic

Information of the International Commission on Stratigraphy.

In most applications, polarity stratigraphic is used to

chronologically arrange rock bodies into identifiable units

based on variations in their polarity records. A correlation is

usually sought between observed magnetic polarity zonation in

a strartigraphic section and the GPTS. In essence, the objective

is to determine a pattern of polarity zones that provides a

„fingerprint‟ of a particular interval of the GPTS [4]. If a

distinctive set of reversals observed in a particular rock sequence can be matched unambiguously with the appropriate

part of the GPTS, then the geological age of each of the beds

containing a polarity inversion can be deduced.

On a larger time scale variations of some 106-109 years, the

isolated primary NRM from a rock of specific age can be used

to calculate the position of their corresponding palaeomagnetic

pole based on the standard geocentric axial dipole model for

the average geomagnetic field. Averaging pole positions over a

period of several thousand years yields a mean pole direction

for this period that corresponds to the geographic poles (the

Earth's spin axis). Therefore, all rocks magnetized at the same

time should have the same palaeomagnetic pole position that corresponds to the geographic pole of that age [23]. It has long

been realized, however, that successive palaeomagnetic pole

positions from successions of old rocks on a coherent

continental block do not coincide with the present geographic

pole, but rather lie on a path that gradually shifts away from

the present pole as older and older rocks are considered. As the

Earth's axis of rotation is fixed, the changing positions of the

pole, as a function of time, is due to the motion of the

continental block from which rock samples were obtained.

This path is, therefore, termed the Apparent Polar Wander

Path, APWP, as the movement of the pole reflects the drift of this continental block through time rather than the movement

Fig. 1. Phanerozoic GPTS, modified from "The Concise

Geologic Time Scale" [22, Cambridge University Press) as

implemented by the Subcommission for Stratigraphic

Information of the International Commission on Stratigraphy

(https://engineering.purdue.edu/Stratigraphy/charts/educational.h

tml, 2010).

of the geomagnetic pole [24]. Each coherent rock block has,

therefore, a unique APWP.

In addition to its use for studying plate movements [25],

APWP is also used to determine absolute age of a rock unit

within the same continental block by plotting the computed

palaeomagnetic pole from this rock unit on it. There are over

10,000 palaeomagnetic poles currently available worldwide

with a range in age from the Archean to recent and with a

quality that ranges from excellent to highly questionable.

Palaeomagnetic poles are continuously compiled into the

IAGA Global Palaeomagnetic Database (GPMDB). Since it was first released in 1991, the GPMDB has been successively

updatesd every two years [e.g., 26]. It facilitates assembling

the subset of poles into APWPs for the different continental

fragments. (http://www.ngdc.noaa.gov/geomag/paleo.shtml).

Picking out the right poles from the published data is the main

issue that guides the construction of a reliable APWP. The

general selection criteria used by most palaeomagnetists are

based on those proposed by [27, 28] that have been modified

for particular applications, for example, by [29]. A useful

discussion on these pole selection criteria and the different

construction methods of APWP can be found in [6]. Fig. 2 displays an example of one of the most recently used APWP

for Eurasia using the Palaeomagnetic Euler Poles (PEP)

construction approach [30].

With the acquisition of many good quality palaeomagnetic

poles during the past few decades, APWPs for different

continental blocks have been significantly refined and are

currently used for diverse dating applications. Even when the

rock is found to carry a secondary NRM records it can still be

useful to compare with the APWP and geochronologically

used to depict the post-formation history of the rock [4, 6, 7].

It is relevant to mentioned here that APWP has extensively been used to date the unfossiliferous Nubian Sandstone rock

unit that crops out in many parts in NE Africa and the Middle

East. This unit has a strategic importance as a reservoir and

aquifer rock unit in many parts. It has been assigned a broad

age window ranging from Early Palaeozoic to Late Cretaceous

due to lack of any credible record of index body fossils.

Palaeomagnetism, as a viable dating tool, has been potentially

successful in providing absolute dates for this unit in many

parts [31, 32].

II. APPLICATIONS AND CASE STUDIES

As detailed above, palaeomagnetism can be potentially

utilized to provide absolute dates for both surface and

subsurface rocks. Carrying out a palaeomagnetic study on

surface or subsurface reservoir rock can, therefore, take one of

the two following forms:

- Direct studies on oriented surface or subsurface standard

cylindrical cores; or

- Indirect studies through magnetic logging.

In case of direct studies, surface or subsurface core plugs are sampled and stepwisely demagnetized (magnetic cleaning

using either alternating field „a.f.‟ or heating) to remove the

unstable magnetic records. The entire record/ spectrum of the

residing remanent magnetisation is then traced and analysed,

both visually and statistically, in order to isolate the primary

component that reflects the age of the rock concerned [33].

The results are then correlated to corresponding international

standard records of either the GPTS or APWP to determine the

age of the reservoir rock involved.

As for indirect studies, magnetic logging using the

Geological High-resolution Magnetic Tool (GHMT) is conducted. This tool simultaneously records both magnetic

susceptibility and Earth‟s total magnetic induction with great

precision in a borehole in order to determine the

palaeomagnetic reversals in the sedimentary sequence, hence

its complete magnetostratigraphy record regardless of core

recovery problems. It was developed as a cooperative venture

between Commissariat a l‟Energie Atomique–Laboratoire

d‟Electronique, de Technologie et d‟Instrumentation (CEA–

LETI), Ecole Normale Superieur, and Total Oil Company in

France, and has been operating since 1990 in the Ocean

Drilling Program (ODP) by Schlumberger (ODP Leg 134). It

helped in obtaining high-resolution magnetostratigraphic records at different sites (e.g., 34, 35, 36, and 37). This

magnetic logging tool consists of two sondes:

1. The Susceptibility Measurement Sonde (SUMS) records

signals related to formation magnetic susceptibility.

2. The Nuclear Resonance Magnetometer Sonde (NMRS)

accurately measures the total magnetic induction in the

borehole.

As demonstrated by [38, 39, 40], measurements from both

sondes contribute to the field anomaly in the borehole and are

therefore combined to determine the palaeomagnetic reversals

record, the magnetostratigraphy, by isolating the polarity of the scalar remanent magnetization.

A full coverage of the entire spectrum of current

geochronological applications of palaeomagnetism is not

feasible within the space available here but can be traced

elsewhere [1, 2, 4, 6, 7]. There are tens of available illustrative

examples to demonstrate the viability of palaeomagnetism a

dating tool for different types of rocks. In the following

sections, three representative case examples of the mainstream

applications are presented here to highlight the potential of

palaeomagnetic dating of surface and subsurface rocks using

direct and indirect application approaches. They were selected

Fig. 2. Euraisa APWP constructed using the palaeomagnetic Euler

poles (PEP) approach. Lines show five APWP tracks (0–50, 50–

140, 140–160, 160–180 and 180–250 Ma) represented by great circle

segments. Numbers in rectangles show a track number (after [30]).

to cover different geographical areas with different geological

and temporal settings.

Case Study I

In their detailed study, [41] analysed the palaeomagnetic

characteristics of the Tertiary deposits of the Mae Moh Basin in northern Thailand. Similar to other Tertiary intermontane

basins in this part of Thailand, these deposits comprise

hominoid fossils and Neogene mammal faunas and are rich in

lignite and oil shale deposits. Relative ages provided by

biochronological studies were controversial and neither

absolute ages nor correlations with marine deposits could be

obtained before. Magnetostratigraphy was therefore called on

to provide a precise temporal framework to estimate the timing

and duration of basin formation.

The study was conducted on two sampled sections that

yielded credible polarity reversal records. Results were then combined to those obtained previously by [42] to obtain a

complete magnetostratigraphic record for a composite

sequence of 525m. With the help of available biochronological

constraints, they could identify 15 polarity zones (seven

normal and eight reversed) from the composite section that

were preferably correlated to the period from Chron C5ACr to

C5r.3r of the GPTS of [43] which cover an age range between

14.1 and 12.0 Ma (Fig. 5A). Based on sedimentation rate

calculations and known tectonic subsidence history of the Mae

Moh Basin, alternative correlation with the GPTS (Fig. 5B)

was found less plausible as they yield unrealistic age for the

fossiliferous lignite levels. The obtained high-resolution magnetostratigraphic record has been considered as a reference

for the continental middle Miocene of all Southeast Asia. .

Case Study II

This case study shows how downhole magnetic logging can be used to obtain a high-resolution magnetostratigraphic record

through indirect continuous measurements. The study example

chosen here is that of [44] that concerns the Middle to Late

Miocene sedimentary sequence in the central part of the

Vienna Basin, Austria. Here, the GHMT was used to obtain

high-precision downhole measurements to derive the sequence

of polarity reversals of the traversed Miocene sedimentary

sequence in the central part of the basin.

As clearly shown in figure 4, good agreement between the

biostratigraphical and the palaeomagnetic records could be

achieved for the post-Badenian sequence. Stratigraphic control

with this integrated approach was good in the Sarmatian and

Pannonian, but difficult in the Badenian due to poor measurement signals. The good correspondence of the

magnetostratigraphic results with the GPTS of [18] and the

Central Paratethyan biozones (Fig. 4) strongly supports the

validity of these records.

Such findings enabled [44] to work out variable

sedimentation rates throughout the section and visualize the

infill history of the Vienna Basin as controlled by regional

tectonic activity, but modulated by eustatic influences.

Case Study III

In this study case, it is shown that even when secondary

palaeomagnetic records are dominating in a reservoir rock,

geochronological application of APWP can still be valuable in giving insight into the post formation history of that rock. The

chosen study here is that of [46] who applied a detailed

Fig. 3. (A) Preferred correlation of the composite Mae Moh

sequence with the GPTS of [43]. The letters at the left of the

stratigraphic column indicate the nomenclature applied to the

lignite zones. (B) Alternative correlation of the Mae Moh sequence with the GPTS (after [41]).

Fig. 4. Magnetostratigraphic of the Miocene sedimentary sequence in the

central part of the Vienna Basin and their correlation to the GPTS of [18]

and the Central Paratethyan biozones. Solid lines indicate correlations with

high confidence; dotted lines indicate correlations with less certainty. Grey

areas indicate the error bars for the biostratigraphic analysis. On the

palaeomagnetic log black represent normal polarity, white reversed polarity

and grey uncertain polarity. The intervals with no palaeomagnetic data are

indicated with crosses. Solid arrows on the GPTS indicate the short polarity

subchrons and dashed arrows indicate the location of the polarity

fluctuations within Chron C5n.2n as described by [45]. The main

unconformities in the well are indicated by an undulating line on the

palaeomagnetic log (after [44]).

palaeomagnetic study on the Mississippian Barnett Shale gas

unconventional reservoir and source rock, north central Texas,

and the underlying Ordovician carbonates of the Ellenburger

Group in the Fort Worth Basin. Here palaeomagnetic analysis

of samples from three scribe oriented conventional drill cores

were used to depict the nature and timing of different

diagenetic events in the Barnett Shale. This is done in light of

known records of alteration, especially from basin fluids near

major faults and karsts developed in the underlying

Ellenburger Group carbonates and Precambrian basement

rocks. Through the use of the APWP of North America, the unit was found to contain multiple secondary CRMs with

palaeomagnetic poles that correspond to ages that were related

to multiple diagenetic events elapsing the time from mid-

Pennsylvanian to late Permian (Fig. 5).

These secondary palaeomagnetic components were

interpreted as to have formed due to the effect of externally-

derived fluids that may have migrated from the Ouachita thrust

zone. Both the Ellenburger carbonates and the Barnett Shale

seem to have experienced remagnetizing fluid alteration events

during their histories, and connections can be made in some

cases between the two units. Both units were remagnetized in the early-mid Pennsyvanian period, suggesting that the burial

of the section and its thermal maturity for hydrocarbon

generation remagnetized portions of both units. Both units also

contain a widespread Permian to early Triassic aged set of

remagnetizations, which correspond to maximum burial of the

unit. This interpretation was further supported by the fact that

both units contain highly radiogenic strontium values in these

zones suggesting a possible mixture of externally-derived

radiogenic fluids perhaps from the Ouachita thrust or from

deep basement brines. It was also assumed that internal burial

fluids may have been present and played a role in these alterations [46].

Plots of computed palaeomagnetic poles on the APWP also

helped the identification of two additional diagenetic events in the Ellenburger carbonates that do not exist in Barnett Shale

CRM records; a late Ordovician-Silurian remagnetization

which predates deposition of the Barnett, and a late Triassic to

Jurassic CRM (Fig. 5). This was attributed to the low

permeability of the Barnett shale, particularly after

cementation during the Pennsylvanian and Permian periods,

that did not allow this late fluid to infiltrate into it.

The obtained palaeomagnetic records were further found in

good correlation with the fracture orientation patterns which

enabled their classification accordingly (Fig. 5). Fractures with

northeasterly azimuths were found in rock that contains a

Pennsylvanian and a late Permian CRM and are filled by calcite and sulfates. The early CRM was interpreted to have

formed during burial diagenesis, while the latter was due to

gravity-driven orogenic fluids derived from the uplifted

Ouachita highlands. Northwesterly fractures, on the other

hand, are associated with rock that contains a late

Pennsylvanian to early Permian CRM. These are filled with

calcite, quartz and contain relict hydrocarbons.

III. SUMMARY AND FUTURE PROSPECTS

The wide-scale irregular directional and magnitude changes

of the geomagnetic field with time and the occurrence of

ferromagnetic minerals in virtually all types of rocks have

rendered palaeomagnetism a viable dating tool that can be

applied in a very wide range of geochronological contexts.

This is particularly true for reservoir rocks that are mostly

unfossilferous and difficult to date where absolute dates can be

estimated by correlating isolated primary NRM records to established international standard records through time. Even

when secondary NRM records can only be found,

palaeomagnetism can still be used as a highly potential tool to

trace the diagenetic history and post-formation changes of

these rocks.

Two essential levels of geochronological precision have

been demonstrated; polarity stratigraphy with periodicities of

103-107 years through the use of the GPTS, and the average

geomagnetic pole position with variations of some 106-109

years using the APWP, a wide time scales with a potential

geochronologic applications range. Recent advances in

palaeomagnetism have led to the documentation of a wealth of well-resolved NRM records. This, in turn, helped in placing

the technique alongside with radiometric dating as a standard

geochronological tool. It has been proved as a unique

“fingerprint” for correlating rocks of diverse origin, enabling

an important global time framework. As detailed in this

review, the application of palaeomagnetism in various

geochronologic investigations is now commonplace.

It is hoped that new research findings and improvement of

palaeomagnetic methods will continue to develop to give more

accurate geochronological estimations. Future palaeomagnetic

research activities and accountability place exceptional technical demands upon geochronologic applications. Some of

the sought future prospects are as follows:

1. Refinement of established polarity stratigraphy and

palaeomagnetic pole records is still needed to offer entirely new opportunities for improved dating scales.

Fig. 5. Secondary palaeomagnetic poles from the Mississippian

Barnett Shale gas reservoir and source rock, north central Texas, and

the underlying Ordovician carbonates of the Ellenburger Group in

the Fort Worth Basin plotted on the North American APWP.

Diamond is bedding parallel (stratiform) deposits; dark square is

northeasterly and sub-vertical fractures; triangle is matrix only;

circle is vertical, northwesterly tectonic fractures; light square is

vertical and northeasterly fractures (after [46]).

2. Application of sophisticated statistical techniques, as the

sequence slotting algorithms, need to be developed to

replace the current eye-matching correlation for a better

palaeomagnetic dating assessments.

3. Exploration in depth of the relationship between magnetic

minerals and rock forming processes is urgently needed to

establish, both in descriptive and experimental manner, the

sensitivity of magnetic minerals to post-formation changes.

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