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Stone, Vaughan C. A.1, W. Heiko Oterdoom2, J. Derek Fairhead1 
(1) School of Earth Sciences, Leeds, United Kingdom 
(2) Petronas Carigali White Nile (5B) Limited - formerly of Lundin Sudan BV, Khartoum, Sudan

ABSTRACT: Prioritising Exploration Leads in the Sudan Using Magnetic Alteration Oil Seepage Signatures Recognised in High Resolution Aeromagnetic Data

A coherent low amplitude magnetic aureole (annular positive anomaly) is observed to correlate closely with the Jarayan oilfield in the Muglad Basin. Similar observations in other parts of the world lead us to strongly suspect that micromagnetic anomalies of this specific type arise mainly from redox bacteriological effects triggered by vertical microseepage of pooled hydrocarbons. Unattributed aureoles in the same region are observed to correlate closely with the margins of structural highs interpreted from the gravity and seismic data. These aureoles too are likely by analogy to be closely coupled (subject to seep routes) with pooled hydrocarbon deposits and so provide potentially decisive criteria for prioritising exploration prospects and leads within the Muglad Basin. 
It is probable that the areal extent and amplitude of a seep-related aureole, rather than being proportional to the field's physical size, are more strongly functions of: (i) seep duration, (ii) seep rate, (iii) timing, (iv) overpressure, (v) water-table conditions and (vi) sub-soil geo/biochemistry. Smaller oil fields therefore should generally be no more difficult to detect by this method than larger fields. 
Numerous papers spanning the 1930s to 1990s have been published on this subject, however, little momentum has been gained in recent years for general acceptance of this approach, nor in developing the processing techniques to routinely seek low amplitude aureoles. Whilst the micromagnetic approach to date has perhaps been a missed opportunity, it is recommended that as world oil output drops and smaller oilfields become economic to exploit, this method should now be recognised as a standard exploration tool.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.