J. P. Land Associates, Inc.

The Basis Of Magnetic Prospecting For Sedimentary Structures

Though there's still much to be learned about the geology associated with sedimentary magnetic anomalies, sixty years of Micromagnetic survey has taught us that:

• Each sedimentary formation has magnetic properties different from the younger and older formations immediate to it.
• Though the magnetic susceptibilities of sedimentary rocks are low by comparison with metamorphic and igneous rocks, they are measurable.
• The structural displacement of near-surface sedimentary formations places formations with differing magnetic properties in contact creating measurable high-frequency distortions of the Earth's magnetic field.
• Secondary mineralization occurs in the sedimentary column above a hydrocarbon reservoir due to reduction and oxidation processes triggered by the upward microseepage of light hydrocarbons escaping the trap.
• Depending on the chemical elements involved, one result of such geochemical action is that, within the alteration zone, the iron minerals are converted to forms that are either more magnetic or less magnetic than before and a local magnetic susceptibility anomaly is created. The typical magnetic expression of local sedimentary structure and/or geochemical alteration is a complex of high frequency wavelets thought to be the responses of shallow mineralized faults, structural closures and structure-related stratigraphic traps.
• Within a given area, the varying intensities of the high-wavenumber magnetic anomalies is thought to reflect the degree or amount of geochemical alteration having taken place and the percentage by volume of magnetic or non-magnetic minerals produced.

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