Efficacy of the SRM/MBS Approach for Analyzing Intra-Sedimentary Magnetic Anomalies

Abstract Magnetic methods have long been used to assist in the analysis of a prospective sedimentary basins in terms of the overall size, shape and structure. Less frequently has magnetic data been used for "pin pointing" hydrocarbon concentrations. The SRM/MBS method, as developed by Robert Foote, provides a tool for reducing exploration risk by providing direct targeting information. The method's efficacy has been demonstrated by statistical methods; however, the scientific basis on which this method is based needs to be examined. The method depends on the magnetic properties of the rocks in the sedimentary section being changed by the presence of hydrocarbons at depth and that these changes will produce magnetic anomalies that are distinguishable from anomalies produced by the magnetic basement and other effects. These two criteria are met in the real world and thus the SRM/MBS method has potential. Using the SRM/MBS method, as part of an exploration program, has the potential of reducing risk by focusing on areas where there is a higher probability of hydrocarbon concentration.

Table of Contents Introduction Changes in Magnetic Character within the Sediments above Petro-Reserves Background on Magnetic Methods The Sedimentary Residual Magnetic (SRM) Anomaly and Magnetic Bright Spot (MBS) Method Conclusions References More Information on SRM/MBS

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Introduction

The Sedimentary Residual Magnetic (SRM) anomaly and Magnetic Bright Spot (MBS) is demonstrated, in a number of papers and articles by Robert Foote. Through statistical analysis of the success rate of using Sedimentary Residual Magnetic (SRM) anomalies to focus exploration Foote has presented a compelling case, to develop confidence in this method we must look at the science that it is based on. The SRM method depends on two points;

The magnetic properties of the rocks in the sedimentary section may be changed by the presence of hydrocarbons at depth and Changes in sedimentary magnetic properties result in magnetic field changes that are detectable and distinguishable from the magnetic field anomalies produced by magnetic basement and other effects.

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Changes in Magnetic Character within the Sediments above Petro-Reserves

Of particular interest here is the production or enhancement of magnetic minerals in the shallow sedimentary package above concentrations of trapped hydrocarbons. The underlying assumption is that hydrocarbons are generated and/or trapped at depth and leak in varying quantities to the surface and produce, through geochemical interaction, magnetic minerals in the sediments. The existence of seeps has long been established as fact (Horvitz, 1939, 1985; Jones and Drozd, 1983; Price, 1986). Enrichment of magnetic mineralization due to hydrocarbon migration (Foote, R. S., 1996, Machel, H.G., 1996) is also a well know phenomenon.

Bacteria and other microbes play a profound role in the oxidation of migrating hydrocarbons. Their activities are directly or indirectly responsible for many of the diverse surface manifestations of petroleum seepage. These activities, coupled with long-term migration of hydrocarbons, lead to the development of near-surface oxidation-reduction zones that favor the formation of hydrocarbon-induced chemical and mineralogical changes. This seep-induced alteration effect has led to the development of a varied number of geochemical exploration techniques. Some detect hydrocarbons directly in surface and seafloor samples, others detect seep-related microbial activity, and still others measure the secondary effects of hydrocarbon-induced alteration using magnetic techniques (Schumacher, 1996; Saunders et al., 1999). The figure 1 (below) shows a generalized model of hydrocarbon microseepage and hydrocarbon-induced effects in the sedimentary .

Figure 1: Generalized model of hydrocarbon microseepage and hydrocarbon-induced effects in the sedimentary.

Figure 2: Mechanisms for seeps

The presence of magnetic materialization has been observed in drill results with direct correlation with producing well and negative correlation with dry wells (Foote, R. S., 1996) Thus we can expect magnetic material to overly hydrocarbons trapped at depth. The question remains can one detect these concentrations with some degree of confidence using magnetic survey techniques?

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Background on Magnetic Methods

The earth has a magnetic field and we can think of this as if the earth had inside it a simple "bar magnet" aligned to produce a north and south pole. If we remember playing with magnets as a child we will recall how iron filings would line up in lines emanating from one pole of the magnet and returning in the other. In a perfect earth we can imagine these lines passing uniformly from the earth's north pole to the south pole. In any one place the field lines would enter into or emanate from the earth at some angle (know as magnetic inclination) with uniform line density. Magnetometers effectively measure the density of the magnetic field lines at any point and we call this measurement the magnetic field strength. If the earth were of one homogeneous material the magnetic field strength (field line density) would vary smoothly, diminishing towards the magnetic equator (remember the bar magnet field lines) to a value about half that at the poles.

The earth is, however, non-homogeneous. In addition there are a group of minerals, particularly those containing iron or nickel, that have a special property (known as magnetic susceptibility) that allows the magnetic field to pass through them more easily than free space or other material. These minerals are called magnetic minerals. A concentration of these magnetic minerals forms a place where the magnetic field can pass more easily and thus the field lines "crowd in" to take advantage of the easy passage. This has the effect of increasing the field line density over those areas of concentration of magnetic material. If a magnetometer is used to measure the field strength along a line crossing one of these areas of concentration one would observe an increase in field strength (higher field line density due to "crowding") over the zone of increased magnetic mineralization. Thus producing a "magnetic anomaly".

Rocks and sediments have varying amounts of magnetic minerals in them. In broad terms igneous and metamorphic rocks (basement rocks) have higher magnetic mineral concentrations then sediments. Thus one would expect and indeed we observe that the largest "magnetic anomalies" over sedimentary basin originate from the variations in magnetic mineral content in the basement rocks. These anomalies can be tens or even hundreds of times greater than those that might be produced by magnetic mineral concentrations in the overlying sediments.

At this point one asks "how can we, first identify relatively small intra-sedimentary magnetic anomalies in a "jumble" of much stronger basement originating anomalies and second on what basis do we separate the two types of anomalies"?

The physical properties of the behavior of magnetic fields provided the solution. As you move away from a concentration of magnetic material the concentrating effect on the magnetic field lines diminishes and the lines move further apart thus producing a weaker effect. Depending on the shape of the magnetic body this can be anywhere between a linear decrease for a half space shaped concentration (i.e. if you double the distance between the magnetometer sensor and the source, you half the intensity to a anomalous effect) to a cubic relationship for relatively small sphere shaped bodies (i.e. If you double the distance you diminish the anomalous field strength by eight times). In practice the factor is usually between 1/d2 and 1/d2.5 (where d is the change in distance) so deeper sources will produce weaker magnetic anomalies. More important, this effect results in a change in the overall shape of the anomalies with deeper sources producing broader shaped anomalies than the anomalies produced by shallower sources (see figure 3). In geophysical jargon we say that the deeper magnetic bodies produce anomalies of a longer wavelength than shallower ones.

Figure 3: Cartoon illustrating the shapes of anomalies caused by deep and shallow sources

Methods have been developed for distinguishing and separating the magnetic anomalies cause by deep sources from those originating in the sedimentary section. These methods are based on analyzing the wavelength (flatness/shape) of the magnetic anomalies and using filtering (linear) and non-linear methods to separate the "shallow" anomalies from the "deeper" ones.

Frequency Domain filtering techniques have been used (Hopkins,R , and Urquhart W.E.S. ,1990) and perfected for many years to separated and enhance different aspects of magnetic anomaly data. Using these techniques one can remove the effect of deeper sourced magnetic anomalies leaving the near surface intra-sedimentary anomalies.

Once the shallow (high frequency - short wavelength) anomalies have been isolated they can be studied and categorized. There are many qualitative methods to do this most involving some sort of map presentation. The SRM/MBS method as developed by Robert Foote analyses the data in a quantitative way and is thus a more sound and repeatable method of exploration.

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The Sedimentary Residual Magnetic (SRM) Anomaly and Magnetic Bright Spot (MBS) Method

In this approach a method for the removal of the "basement-caused" variations in the magnetic data is developed, which allows the "basement-removed" profile to be presented as an approximate horizontal line at the vertical scale of 1.0nT/inch without data averaging. Anomalous regions along a profile line are defined as the Sedimentary Residual Magnetic (SRM) anomalies. Anomalous regions are then identified by the clustering of line-to-line SRM anomalies. These SRM anomalous regions existing over two or more adjacent lines develop the Magnetic Bright Spot (MBS)

The MBS anomalies vary in size and intensity. A formula (quantitative approach) is developed to provide for classification of the MBS anomalies into (8) intensity levels, which is the final rating. The Anomalies are corrected for sea depth and normalized to a water depth or 2000ft. The final rating is then used to prioritize exploration planning.

A number of articles and papers ( Foote, R. 1992, 1996, Rose and Associates, 2003) have been written demonstrating the efficacy of the SRM/MBS technique based on statistical analysis relating to the correlation of Magnetic Bright Spot (MBS) anomalies and producing wells and perhaps more importantly the negative correlation between MBS and non-producing and dry wells as well as non-producing areas.

The Rose and Associates independent study of MBS results in the Gulf of Mexico found:

"Based solely on prospect, discovery and MBS anomaly counts, there is a significantly higher actual success rate for drilled prospects associated with MBS anomalies than for prospects where no MBS anomaly is evident. Three hundred seventy eight (378) drilled prospects where identified in the study area. The results are characterized either as discoveries or dry holes."

The following table summarizes the results.

MBS anomalies are evident on 56% of the discoveries. The critical factor to note is that 97% of the MBS anomalies are associated with discoveries. This is over twice the 41% success rate for discoveries without MBS anomalies. Clearly there is statistical validity to the process.

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Conclusions

The two points, on which the Sedimentary Residual Magnetic (SRM) anomaly and Magnetic Bright Spot (MBS) method is based:

The statistical analysis over existing fields demonstrated the efficacy of the SRM/MBS technique. Thus there is basis in science for the approach. The test studies show that in practice the method will enhance the success rate of an exploration program where the SRM/MBS method in incorporated into the methodology.

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References

Foote, R. S., 1996, Relationship of near-surface magnetic anomalies to oil- and gas-producing areas, in D. Schumacher and M. A. Abrams, eds., Hydrocarbon Migration and Its Near-Surface Expression: AAPG Memoir 66, p. 111-126.

Hopkins,R , and Urquhart W.E.S. ,1990 Enhancement and interpretation of aeromagnetic data from the Beaufort Sea, Mackenzie Delta Region:" 60th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 677-680.

Horvitz, L., 1939, On geochemical prospecting: Geophysics, vol. 4, p. 210-228.

Horvitz, L., 1969, Hydrocarbon prospecting after thirty years, in W.B. Heroy, ed., Unconventional Methods in Exploration for Petroleum and Natural Gas: Dallas, Southern Methodist Univ. Press, p. 205-218.

Machel, H.G., 1996, Magnetic contrasts as a result of hydrocarbon seepage and migration, in D. Schumacher and M.A. Abrams, eds., Hydrocarbon Migration and Its Near-Surface Expression: AAPG Memoir 66, p. 99-109.

Ross and Associates, 2003, Gulf of Mexico Study in Cs Solutions SRM/MBS Technology Information Book.

Saunders, D.F., K.R. Burson, J.J. Brown, and C.K. Thompson, 1993, Combined geological and surface geochemical methods discovered Agaritta and Brady Creek fields, Concho County, Texas: AAPG Bulletin, vol. 77, p. 1219-1240.

Schumacher, D., 1996, Hydrocarbon-induced alteration of soils and sediments, in D. Schumacher and M.A. Abrams, eds., Hydrocarbon Migration and Its Near-Surface Expression: AAPG Memoir 66, p. 71-89.

Thrasher, J.A., D. Strait, and R.A. Lugo, 1996a, Surface geochemistry as an exploration tool in the South Caribbean, in D. Schumacher and M.A. Abrams, eds., Hydrocarbon Migration and Its Near-Surface Expression: AAPG Memoir 66, p. 373-384.

Thrasher, J.A.,. Fleet, S.J. Hay, M. Hovland, and S. Duppenbecker, 1996b, Understanding geology as the key to using seepage in exploration: the spectrum of seepage styles, in D. Schumacher and M.A. Abrams, eds., Hydrocarbon Migration and Its Near-Surface Expression: AAPG Memoir 66, p. 223-241

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