Geology March 2002 Vol 51/1 []

CONTENTS & ABSTRACTS

In English. Summaries in Estonian

Proceedings of theEstonian Academy of Sciences.

Geology



Volume 51 No. 1March 2002

Special issue on geophysics





Preface; 3–4

Alla Shogenova

Thermal conductivity ofCambrian siliciclastic rocks from the Baltic basin;5–15

ArgoJõeleht, Kalle Kirsimäe, Alla Shogenova, Saulius Šliaupa,

Ilmo T. Kukkonen, VitaRasteniene, and Angelina Zabele

Abstract. The thermal conductivity of Cambrian siliciclastic rocks, mainlysandstones and silt­stones, was studied together with their chemicalcomposition and other petrophysical properties in Estonia, Latvia, and Lithuania.The thermal conductivity is mainly controlled by porosity and rock composition.It was found that the Wiener and layered mixing and geometric mean modelsapproximated best the thermal conductivity vs. porosity relationship ofquartz-rich rocks within the measured porosity range (0–30%). The matrixthermal conductivity is positively correlated only with the silica contentwhich reflects the content of quartz, whereas other rock-forming mineralsdecrease the matrix thermal conductivity. The thermal conductivity ofsiliciclastic rocks correlates well with porosity-dependent parameters (wetdensity, apparent resistivity, elastic properties), but has no relation tomagnetic properties.

Key words: thermal conductivity, petrophysical properties, siliciclasticrocks, Cambrian, Baltic basin.

Revision of aeromagneticdata: ground magnetic investigations of altered sedimentaryrocks (Hatrurim basin, Israel);16–32

Boris Khesin and Sophia Itkis

Abstract. Aeromagnetic surveyrevealed two local anomalies with a magnitude of up to 20 nT within theHatrurim basin, where altered sedimentary (mainly carbonaceous) rocks of theHatrurim Formation subjected to combustionmetamorphism (CM) are widespread. Aeromagnetic data analysis and groundmagnetic prospecting show that the CM model cannot relate all the magneticcharacteristics. On one hand, high-amplitude (up to 4000 nT) high-wavenumber ground anomalies could be a by-productof the combustion (surface) metamorphism. On the other hand, poor correlationbetween magnetic susceptibility  values (reached up to4000 ´ 10–5 un. SI) and metamorphic grade of Hatrurim Formationrocks, between magnetic anomalies and/or metamorphic grade/thickness of therocks may suggest involvement of an additional source or presence of highremanent magnetization.

Key words: magneticprospecting, altered sedimentary rocks, combustion metamorphism, Hatrurimbasin, Israel.

Porosity structure of Icelandic basalt;33–46

Omar Sigurdsson and Valgardur Stefansson

Abstract. Systematic sampling of igneous rocks from extinct geothermalsystems in the Icelandic crust has resulted in a database covering most typesof geothermal reservoir rocks presently under exploitationin Iceland. At present, the database includes more than 500 samples spanningthe basaltic to rhyolitic composition and hydrothermal alteration fromliterally fresh rocks to greenschist facies rocks. The database is used tostudy the relationship between different reservoir parameters, such as thermalconductivity and porosity, alteration, and permeability. The present paperdescribes the relationship between porosity and permeability and theinformation that can be derived from this data set. It is found that theaverage difference between effective and total porosities is about 2% and thatthis difference is independent of porosity over the range 0–50%. Thisdemonstrates that the whole pore volume in these igneous rocks is effective forfluid flow through them. The permeability measured with liquid brine issystematically lower than that obtained by measurements using air or gas. Brinepermeability is on average three times lower than gas permeability measured onthe same sample. We explain this discrepancy as a result of the formation of aconnate liquid film that adheres to the rock in combination with capillaryforces that reduce and block portions of the flow paths for the liquid flow.Reasonable conversion of gas permeability to brine permeability can be obtainedby taking into account the effect of immobilization of part of the brine. Thissupports the view that a film of connate brine causes the difference in theobserved permeability values. The bundle capillary tube model seems to be thebest model describing the porosity structure of the Icelandic rocks, whereas theKozeny–Carman model shows very weak correlation to the data. The calculatedtube diameters of the bundle capillary tube model are in the range0.03–80 mm, while laboratory measurements on only a few samples gave values in the range0.05–100 mm. The calculated diameters of the majority of the samples are less than 0.7 mm, or smaller than the wavelength of visible light.

Key words: igneousrock, porosity structure, permeability, basalt, capillary tube model, rockdatabase, geothermal systems.

Gravity and magnetic modelling of anomaloussources of complex configuration; 47–59

Mykola Yakymchuk, IgnatKorchagin, Tatjana Mikheeva, Marina Orlova, and Yuriy Yakymchuk

Abstract. The automated fittingmethod was applied to the modelling of gravity and magnetic anomalies. Thistechnique allows us to use for interpretation the observed components ofgravity and magnetic fields and their linear transformations, the geoidanomalies, and the course gradient data of magnetic fields. The software isbased on the optimization of the sum of square differences between measured and modelled field components. Theuniversal approximation procedure for parametrizationof anomalous sources allows us to describe effectively different elements ofthe modelled space. The optimization procedure for the minimization ofobjective functions includes gradient-based algorithms and singular valuedecomposition technique. The technology is tested on practical geologicalproblems.

Key words: gravity, magnetic,modelling, optimization, automated fitting.



Instructions to authors; 60–62

Copyright Transfer Agreement; 63