Chemical and microbial processes causing anomalous magnetization in environments affected by hydrocarbon seepage

Geophysics ◽  
1991 ◽  
Vol 56 (5) ◽  
pp. 598-605 ◽  
Author(s):  
H. G. Machel ◽  
E. A. Burton
Keyword(s):  
Magnetic Anomalies ◽  
Hydrocarbon Exploration ◽  
Magnetic Mineral ◽  
Hydrocarbon Accumulation ◽  
Magnetic Minerals ◽  
Total Magnetization ◽  
Microbiological Processes ◽  
Surface Exploration ◽  
Mineral Assemblages ◽  
Anthropogenic Processes

(Aero‐)magnetic anomalies have been reported from several commercial hydrocarbon accumulations. However, the processes responsible for such anomalies are relatively poorly understood. This paper conceptually discusses chemical and microbiological processes involved in generating anomalous magnetization related to hydrocarbon accumulations, including hydrocarbon seepage environments. Based on thermodynamic criteria and microbiologic activity, the formation and destruction of magnetic mineral assemblages can be predicted. Under the influence of hydrocarbons, magnetite and pyrrhotite are the most important magnetic minerals formed, and the most abundant magnetic mineral destroyed is hematite. Hence, the invasion of hydrocarbons may result in “positive,” “absent,” or “negative” magnetic contrasts relative to the total magnetization prior to hydrocarbon invasion, depending upon the amounts of authigenic magnetite and pyrrhotite formed relative to the amounts of hematite destroyed. Magnetism may be generated also by natural and anthropogenic processes that have no relationships to an underlying or adjacent hydrocarbon accumulation. Consequently, anomalous magnetization, even if associated with a hydrocarbon accumulation, may or may not be genetically related to it. Magnetic mineral assemblages and the resulting magnetic contrasts, such as those predicted in this paper, have been documented from some hydrocarbon seepage environments. Hence, anomalous magnetization can be used for hydrocarbon exploration in association with other surface exploration methods.

Magnetic data processing for hydrocarbon exploration in the Pannonian Basin, Yugoslavia

Geophysics ◽  
2001 ◽  
Vol 66 (6) ◽  
pp. 1669-1679 ◽  
Author(s):  
Milenko Burazer ◽  
Milinko Grbović ◽  
Vicko Žitko
Keyword(s):  
Magnetic Field ◽  
Oil And Gas ◽  
Pannonian Basin ◽  
Magnetic Anomalies ◽  
Hydrocarbon Exploration ◽  
Oil Fields ◽  
Window Function ◽  
Magnetic Data ◽  
Band Pass Filter ◽  
Magnetic Minerals

Because magnetic minerals may directly indicate the presence of oil and gas deposits, magnetic methods are applied to hydrocarbon exploration in oil‐bearing sedimentary basins. The basic problem in applying these methods is the isolation of weak magnetic anomalies sourced by low concentrations of the magnetic minerals present. These weak anomalies are often masked by much stronger magnetic anomalies caused by underlying magnetic rocks and/or by rocks in the basin sediments. Weak local anomalies can efficiently be isolated by applying selective 1‐D digital frequency filters. The method of filtering has been checked by data obtained using simple models of magnetic sources and using a model representative of the local geology in our study area in the southern Pannonian basin, Yugoslavia, The magnetic field frequency content was analyzed by applying the power spectral density estimation, using the maximum entropy method. The digital filters were designed using the window function method. The best results were obtained by the Kaiser window function for the chosen range of the band‐pass filter. In our study area, me isolated local magnetic anomalies have amplitudes of ±10 nT and trend in an east‐west direction parallel to the predominant structural grain. These anomalies correlate very well with the known oil and gas fields. As an example, filter processing of magnetic anomaly data, combined with the 3‐D seismic data gained in the filtered magnetic field, correlate well with one of the oil fields. The next phase of the project will concentrate on the anomalies occurring outside the established gas and oil fields.

Paleomagnetism and magnetic mineralogy of the Nahant gabbro and tonalite, eastern Massachusetts

1985 ◽  
Vol 22 (10) ◽  
pp. 1425-1435 ◽  
Author(s):  
Patricia A. Weisse ◽  
Stephen E. Haggerty ◽  
Laurie L. Brown
Keyword(s):  
Hydrothermal Activity ◽  
Magnetic Mineral ◽  
Magnetic Minerals ◽  
Ti Oxides ◽  
Apparent Relationship ◽  
Mineral Assemblages ◽  
Intrusive Rocks ◽  
Eastern Massachusetts ◽  
Chemical Remanent Magnetization

Paleomagnetic analyses of the Nahant gabbro and an associated tonalite of eastern Massachusetts reveal five separate populations of paleomagnetic directions. The gabbro and tonalite, exposed in the northernmost portion of the Boston Basin, were selected for this study because of the lack of significant macroscopic evidence of penetrative metamorphism or deformation. The tonalite has not been radiometrically dated; however, an Ordovician date has been reported for the gabbro. Detailed petrographic data indicate that the five populations of paleomagnetic directions obtained for the gabbro and tonalite correspond to five styles of magnetic mineral alteration.A mean alteration index was calculated for each site in the Nahant gabbro and tonalite. There is an apparent relationship between magnetite alteration and normalized intensity at a given demagnetization level, with normalized intensity decreasing as magnetite alteration increases. This relationship illustrates the interdependence between paleomagnetic properties and magnetic minerals. Alteration of these minerals is probably associated with periods of magnetic overprinting.Autometasomatism is believed to have occurred in the gabbro. Fluid flushing initiated plagioclase decomposition, liberating Ca + Al + Si to form Al-rich (4–10 wt.% Al2O3) titanite (sphene, CaTiSiO5). Whether the metasomatism was deuteric or the result of postdeuteric hydrothermal activity has not been determined.The Nahant gabbro poles are interpreted as representing either a deuteric or a metasomatic chemical remanent magnetization (CRM). The tonalite pole is interpreted as a deuteric CRM. The gabbro and tonalite poles are similar to a number of poles from Ordovician to Devonian intrusives in the Acadia terrane, including the St. Stephen and St. George plutons.Petrographic examination of polished sections from the Devonian St. Stephen and St. George plutons reveals evidence for alteration of primary magnetic mineral assemblages. Similarities exist between these alterations and alterations observed in the Nahant suite, most notably, the formation of titanite from Fe–Ti oxides. The repeated determination of similar poles on intrusive rocks in the Acadia terrane suggests there may be some tectonic or geomagnetic significance to poles from intrusives; however, without more data pertaining to the timing of metasomatic events observed in the Nahant suite and the St. Stephen and St. George plutons, the age and significance of these poles cannot be interpreted with certainty.

Ground magnetic survey for the investigation of magnetic minerals at Iboro Village, Abeokuta, south-western Nigeria

2021 ◽  
Vol 20 (2) ◽  
pp. 99-106
Author(s):  
O.I. Popoola ◽  
O.A. Adenuga ◽  
E.O. Joshua
Keyword(s):  
Magnetic Anomalies ◽  
Geological Structure ◽  
Magnetic Survey ◽  
Magnetic Minerals ◽  
Contour Maps ◽  
Ogun State ◽  
Geological Map ◽  
Regional Gradient ◽  
Ground Magnetic ◽  
Iron Ore Deposit

The geological map of the old western region of Nigeria indicates the presence of iron ore deposit at Iboro village Ogun state (7.9983o - 7.99933o N, 3.5790o - 3.5890o E). Hence a ground magnetic survey was carried out at a location at Iboro village so as to delineate the subsurface magnetic anomalies and to know whether the anomalies favour accumulation of magnetic minerals. The survey was carried out using high resolution proton precession magnetometer model G-856X. Eight traverses were run at 5m separations and earth magnetic intensity values were measured at 10m intervals along each traverse; the acquired data were corrected for drift. The residual anomalies obtained by removal of regional gradient from observed data using trend analysis were presented as profiles and maps. The treated data were qualitatively and quantitatively interpreted and the results gave values for the total ground magnetic anomalies that varied between a minimum and maximum peak values of about -33.0 and 30.6nT respectively. Depth to the basement rock was estimated using Peter’s half slope method which gave a maximum depth of about 13m. The contour maps and the total relative graphs present the subsurface picture of the geological structure that is assumed to harbour the metallic minerals through the action of the field towards the concentration of anomalies. It was suspected that the overburden was relatively thin in the study area and the minerals were at a shallow depth.

Paleomagnetic history of the Mealy dykes of Labrador, Canada

1983 ◽  
Vol 20 (12) ◽  
pp. 1818-1833 ◽  
Author(s):  
J. K. Park ◽  
R. F. Emslie
Keyword(s):  
The Other ◽  
Magnetic Mineral ◽  
Magnetic Minerals ◽  
Linear Distribution ◽  
Tectonic Zone ◽  
Magnetic Components ◽  
History Of ◽  
Component C ◽  
A Site

Paleomagnetic analysis of the Mealy diabase dykes of Labrador reveals magnetizations that predate the Grenville event at about 1000 Ma. These dykes intrude the Mealy Mountains anorthositic complex in the Grenville Structural Province. They are well south of the Grenville Front Tectonic Zone, but were apparently never subjected to temperatures as high as 500 °C during their post-consolidation history.Four distinct magnetic components were uncovered by thermal and alternating field treatments and a fifth remained unresolved. The major magnetic mineral present, titanomagnetite, is thought to record two magnetic directions acquired during cooling from magmatic temperatures. These are B (D = 305°, I = −76°; N = 18 sites; κ = 12; α95 = 11°) and A (D = 095°, I = +52°; N = 20 sites; κ = 46; α95 = 5°). Component B has much within-site dispersion. The other two components, C (D = 274°, I = −47°; N = 10 sites; κ = 15; α95 = 13°) and D (D = 292°, I = −74°; κ = 5; α95 = 31°), probably reside in magnetite and pyrrhotite, respectively. Component C, antiparallel to A, was probably acquired at about the same time as A. We suggest that C and A represent the first stable magnetizations retained by the dykes following an extensive period of cooling and re-equilibration of the magnetic minerals. Components B and D, which agree in direction, represent a later stage of cooling.Component B has a pole at 148°E, 34°S (δp = 18°, δm = 19°) in agreement with regional metamorphic poles from the Grenville; A, however, has a pole at 173°W, 23°S (δp = 5°, δm = 7°), which apparently "sees through" the peak in Grenville activity. The A site poles have a linear distribution along the Keweenawan Track and probably relate to an age between 1000 and 1150 Ma.

A SUB-TILL SAPROLITE AND THE OVERLYING SOIL PROFILE NEAR MOUNT ORFORD, SOUTHERN QUEBEC

1984 ◽  
Vol 64 (4) ◽  
pp. 577-585 ◽  
Author(s):  
C. R. DE KIMPE ◽  
M. R. LAVERDIÈRE ◽  
P. LASALLE
Keyword(s):  
Key Words ◽  
Clay Minerals ◽  
Soil Profile ◽  
Mineralogical Composition ◽  
Magnetic Mineral ◽  
Mineral Assemblages ◽  
Extractable Al

A saprolite deposit and the overlying soil profile developed in a glacial diamicton were sampled near Mount Orford, Southern Quebec. The two materials differed mainly by the magnetic mineral and extractable Al contents, by the Fedithionite/Feoxalate ratio and by the mineralogical composition. Illite and chlorite were the dominant clay minerals in the till whereas muscovite and kaolinite were the major minerals in the saprolite. A comparison was also made with another previously described saprolite deposit 4 km away from this one, in which chlorite was slightly transformed to smectite. It is suggested, from the mineral assemblages, that the two saprolites have probably formed at different times, the first one during Tertiary and the second one during an interglacial stage. Key words: Saprolite, glacial diamicton, kaolinite, muscovite, Tertiary alteration

Improving the crosscorrelation method to estimate the total magnetization direction vector of isolated sources: A space-domain approach for unstable inclination values

Geophysics ◽  
2020 ◽  
Vol 85 (4) ◽  
pp. J59-J70 ◽  
Author(s):  
Nelson Ribeiro-Filho ◽  
Rodrigo Bijani ◽  
Cosme Ponte-Neto
Keyword(s):  
Magnetic Field ◽  
Real Data ◽  
Magnetic Anomalies ◽  
Direction Vector ◽  
Total Field ◽  
Magnetization Direction ◽  
Ambient Magnetic Field ◽  
Total Magnetization ◽  
Synthetic Test ◽  
Equivalent Sources

Knowledge of the total magnetization direction of geologic sources is valuable for interpretation of magnetic anomalies. Although the magnetization direction of causative sources is assumed to be induced by the ambient magnetic field, the presence of remanence should not be neglected. An existing method of correlating total and vertical gradients of the reduced-to-the-pole (RTP) anomaly estimates the total magnetization direction well. However, due to the numerical instability of RTP transformation in the Fourier domain, an assumption should be considered for dealing with inclination values at approximately 0°. We have adopted an extension to the standard crosscorrelation method for estimating the total magnetization direction vector, computing the RTP anomaly by means of the classic equivalent layer technique for low inclination values. Additionally, an ideal number of equivalent sources within the layer is considered for reducing the computational demands. To investigate the relevant aspects of the adopted method, two simple synthetic scenarios are presented. First, a magnetic anomaly produced by a homogeneous and isolated vertical dike is considered. This test illustrates the good performance of the adopted approach, finding the true magnetization direction, even for low inclination values. In the second synthetic test, a long-wavelength component is added to the previous magnetic total-field anomaly. In this case, the method adopted here fails to estimate a reliable magnetization direction vector, showing weak performance for strong interfering magnetic anomalies. On the real data example, the application tests an isolated total-field anomaly of the Carajás Mineral Province, in northern Brazil, where the inclination of the ambient magnetic field is close to zero. The obtained results indicate weak remanence in the estimated total magnetization direction vector, which would never be reached in the standard formulation of the crosscorrelation technique.

scholarly journals Heavy mineral assemblages in tills and their use in distinguishing glacial lobes in the Great Lakes region

1979 ◽  
Vol 16 (12) ◽  
pp. 2219-2235 ◽  
Author(s):  
Q. H. J. Gwyn ◽  
A. Dreimanis
Keyword(s):  
Great Lakes ◽  
Mineral Content ◽  
Heavy Mineral ◽  
Heavy Minerals ◽  
Great Lakes Region ◽  
Adirondack Mountains ◽  
Magnetic Minerals ◽  
Grenville Province ◽  
Provenance Areas ◽  
Mineral Assemblages

Two main source areas of heavy minerals in tills have been defined in the Great Lakes region: a source in the Superior and Southern Provinces and another in the Grenville Province. The Superior–Southern source is typified by low heavy mineral content and high epidote percentage in contrast to the Grenville source which has a high content of heavy minerals of which garnet, tremolite, and to a lesser extent sphene and orthopyroxene are characteristic. The Huron lobe tills have a mineral suite characteristic of the Superior–Southern source. Two subsources can be distinguished in the Superior–Southern area; however, they are too limited in extent to be characteristic of major glacial lobes. Two other subsources have been identified in the Grenville provenance area: a western Grenville subsource containing abundant garnet and having a low purple–red garnet ratio; and an eastern Grenville subsource distinguished by high garnet and tremolite content and a garnet ratio generally greater than one. The western and eastern Grenville subsources are the provenance areas for the tills of the Georgian Bay lobe and the Ontario–Erie lobe respectively. A possible third Grenville subsource in the Adirondack Mountains is distinguished from other Grenville sources by a lower heavy mineral content and more abundant orthopyroxene and magnetic minerals. This assemblage may be characteristic of the southern portion of the Ontario–Erie lobe.

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