Gravity and Magnetic Methods in Petroleum Exploration

Gravity and Magnetic Methods in Petroleum Exploration

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ONGC a Wealth Creator

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Geophysics Geophysics in i n Petroleum Exploration

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Geophysical Geophysical methods meth ods

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Gravity prospecting

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Magnetic prospecting

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Geophysics in Petroleum Exploration

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• Geo Geoph phy ysics ics prov provid ide es tool tools s for for study tudyin ing g struc tructu turre and composition of earth’s interior • Physica Physicall measure measurements ments are made at the surface surface • Existenc Existence e and propertie properties s of earth’s earth’s crust, crust, mantle, mantle, and and core core are determined primarily by geophysical tools

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• Potential field methods • Gravity • Mag agne neti tic c • Elect lectri rica call • Mag Magnet neto-t o-tell elluri uric c • Non-potential field method • Seismic

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Gravity prospecting

�� • Gravity surveys carried out in search of oil are designed for reconnaissance of large, previously unexplored areas • Gravity method provides presence of sediments in a basin rapidly and economically • Field observed in gravitational prospecting is a composite of contributions from all depths within the usual range of exploration interest • Quantity actually observed is not earth’s true gravitational attraction but its variation from one point to another • Variations in gravity observed depend upon lateral changes in density of earth materials in the vicinity of measuring point

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• Rocks have density which may differ from those that are laterally adjacent • An anomaly in earth’s gravity can be related to a buried geological feature e.g. salt dome, diapir • It is possible to map boundaries and estimate depth distribution of sedimentary basins • Gravity surveys are useful in initial exploration of water covered shelf areas where no geological information is available at all.

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r m 1

m 2

Theory behind gravitational prospecting is based on Newton’s law of force of attraction between two bodies 

��

� where m 1 & m 2 are the masses of two bodies placed at a distance of r apart.

If

�  �� �  �    � 

G is called universal gravitational constant and is the force exerted in between two bodies of unit mass placed unit distance apart. In cgs system G is 6.670 x 10-8. 11

�� Gravitational acceleration Acceleration of a mass � due to attraction of a mass � at a distance  away   ��   ��� i.e. acceleration is the force acting on a unit mass. In case of earth �   (mass of the earth)     (radius of earth)     �� It is the conventional quantity used to measure gravitational field acting on any point. In cgs system unit of acceleration is cm per sec. per sec i.e. cm/sec 2 . It is called as gal �   � ��   �� 

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In case of earth • Homogeneous • Spherical • Non-rotating



�   (mass of the earth)    (radius of earth)     ��

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In gravity prospecting for hydrocarbon exploration, variation in gravitational acceleration are measured which are very small. Therefore, milligal is genarally used as the most suitable unit for measuring gravitational acceleration. ∆g =

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g 2 – g 1

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Earth’s gravitational field and its relation to gravity exploration • Gravity measurements made in exploration work show only variation in gravity from one place to another. • Gravitational attraction due to earth itself is significant only as far as it varies laterally over the earth’s surface. • Only such variations are significant in evaluating the gravity effect of buried bodies. • Any variation in gravity is a positive gravity anomaly if it is higher than from that over the area surrounding it and negative when lower.

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Gravity high over a mass

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Gravity low over a valley

�� gmax= 983,217.72 mgal (pole)

North Pole

gmin = 978,031.85mgal (equator)

s m K 6 5 3 6

Latitude

South Pole

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6378 Kms

Total gravity field on the earth is controlled by •

Mass distribution within the earth

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Ellipsoidal shape of the earth

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Centrifugal force due to rotation of earth

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Gravity Corrections

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Standard Gravity Corrections • Instrument drift correction Compensates for drift in the instrument's spring

• Earth tide correction Corrects for tidal effects of moon & sun

• Latitude correction Incorporates the variation of the Earth's gravity with latitude

• Free air correction Accounts for the variation in gravity due to elevation relative to sea level

• Bouguer correction corrects for the attraction of material between the station and sea level

• Terrain correction Removes the effect of topography to a radial distance of 166.7 km

• Isostatic correction Removes long-wavelength variations in the gravity field related to topography

• Eötvös correction For a moving platform 20

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a) Bouguer correction 22

b) Terrain correction

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Absolute gravity

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Basic Principle A: Protons of atom originally aligned with natural field B: External coil is energized with a DC current resulting in a strong field that aligns protons. C: Current turned off and protons precess back to alignment with external field, generating AC current in coil Larger fields → higher frequencies

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Magnetic prospecting may be carried on ground, from aircraft and from ships. Field techniques are of course different for the three types of surveys. On land, magnetic observations are usually made at fixed positions. In aerial and marine surveys, magnetic fields are recorded continuously from sensors.

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• Diurnal correction • Geomagnetic correction • Elevation correction

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It is a local variation in earth’s magnetic field resulting from variations of the rocks

Magnetic Anomaly = Observed mag. field - Computed mag. field (IGRF)

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In magnetic survey the objective is to ascertain the presence of sedimentary basins and to map their approximate boundaries. Direct interpretation A basin is characterized by smooth contours and low magnetic relief in the magnetic contours. Boundary between zones with appreciably different degrees of relief can indicate the presence of a major basement fault.

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Indirect interpretation Attempt is made to match the observed anomaly with that calculated for a model by iterative adjustments to the model.

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• Magnetic anomaly of a finite body contains +ve & -ve elements arising from the dipole nature of magnetism. • Intensity of magnetization in a body controls the shape of its magnetic anomaly. • Bodies of identical shape can give rise to different magnetic anomalies.

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Earth’s magnetic field magnetic anomalies occur in local field from magnetic rock below surface (similar to gravity anomalies)

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Earth’s magnetic field removal of magnetic material from near surface causes negative anomaly (example is normal faulting)

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SEDIMENTARY SECTION

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Magnetic prospecting

�� • Magnetic prospecting is used to explore for both oil and minerals. • It gives information to determine depth to basement rocks, locate and define the extent of sedimentary basins. • This information is of importance in previously unexplored areas such as continental shelves newly opened for prospecting. • Sedimentary rocks exert a very small magnetic effect compared to igneous rocks. • Virtually all variations in magnetic intensity result from topographical or lithlogic changes associated with the basement or from igneous intrusives. • Today, all magnetic surveys are done from air or from ships due to speed, economy and convenience. 42


�� Magnetic force r P

P 0

If two poles of strength  �   are placed at a distance r apart, the magnetic force  between them will be 

� μ �

The constant μ known as the permeability, depends upon the magnetic properties of the medium in which the poles are situated. μ  � for air or vacuum

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Magnetic field If �  �   �μ � Magnetic field strength at a point is defined as the magnetic force exerted on a pole of unit strength placed at that point. In cgs system the unit of magnetic field is orested and in mks system it is tesla which is 104Oe. In magnetic prospecting, variation in magnetic field are very small. Therefore, gamma defined as 10-5Oe is most commonly used unit in geophysical work.

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Magnetic rocks have almost always acquired their polarization from the earth’s field. If the rock is igneous, its direction of magnetization will be that of the earth’s field at the time it cooled from its initial molten state to a temperature below the Curie point. This is called thermo remnant magnetization. If the rock is sedimentary, any orientation of its magnetic grains during deposition would have been in alignment with the field that existed when the deposition occurred. This is depositional remnant magnetization.

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Total magnetization M consists of • Remnant magnetization Mr • Induced magnetization Mi M =Mr + Mi

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Magnetic field of earth depends on • Latitude • Longitude • Time

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Relative gravimeters

Wordon gravimeter 25

CG-5 gravimeter

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Proton-precession magnetometer

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�� • Geological interpretation of gravity data is not easy as it involves more uncertainties than interpretation of seismic data. • Gravity maps resembles so much to structural maps that one can wrongly identify gravity contours as indicative of structure. • In evaluating gravity maps it is important to keep in mind the true nature of the contours. It is to be noted that 1. The field observed at any point is the summation of gravitational attractions of all subsurface causatives but our objective is to obtain information on the individual sources. 2. Lack of uniqueness in gravity field from a subsurface source means that an infinite number of different configurations can result in identical gravity data at the surface. 26


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Extraction of anomalies associated with individual sub-surface source involves filtering operations. Two types of problems are often encountered in gravity interpretation where anomalies must be separated from one another. 1. When the source with larger dimensions is a regional geologic feature, such as a basin or geosyncline and the smaller is a local feature, such as an anticline or salt dome. The first anomaly can be considered to have a low spatial frequency (equivalent to a large lateral extent or a long wavelength) and the second a high spatial frequency (corresponding to a short lateral distance or wavelength).

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2. When two sources of approximately same size and buried at about same depth are close together, field appears to come from a single source. Resolution of such individual sources is not always possible.

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Direct interpretation Determines possible mass distribution from gravity data This approach is impractical because of the inherent limitations associated with the ambiguity.

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Indirect interpretation This consists of a model for the buried source and predicting the gravity effect from it at the surface. Observed and computed gravity are compared and model is modified by iterative procedures, to minimize the difference. The model that gives the best fit is then considered to be the most probable one.

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Gravity and Magnetic Methods in Petroleum Exploration

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