Diverse Geological Settings Research Articles

Determination of paleoheat-flux using multiple thermal indicator tomography

Multiple thermal indicator tomography is a non-linear inversion procedure which permits simultaneous determination of geological, chemical kinetic, and thermal parameters in the burial and thermal histories of sediments. The procedure is based on inverting measurements with depth in a well of multiple thermal indicators, each of which must have experienced the same thermal history. For two wells from diverse geological settings, tomography is used to assess the paleoheat-flux variations with time most consistent with all the thermal indicator data. Resolution and sensitivity of the results are also addressed. For the case history of well X in the North Sea, with both vitrinite reflectance and sterane isomer thermal indicators, a generally decreasing heat flux from past to present is obtained roughly halving every 50 Ma. The activation energy for the sterane isomer is also determined simultaneously to be 42±20 kJ/mole. For the second case history of the Irma Mclean well in the onshore Gulf of Mexico, data from four thermal indicators are available: vitrinite reflectance, sterane and hopane isomers, and sterane aromaticity. A general decrease in paleoheat-flux from amount 2.8 heat flux units (HFU) at 100 MaBP to 1.25 HFU at the present day is obtained, the activation energy for the sterane isomer is simultaneously determined to be around 50±20 kJ/mole, while the corresponding values are around 80±20 kJ/mole for the hopane isomer, and around 65±20 kJ/mole for sterane aromatization.

Deep long-offset transient electromagnetic surveys for crustal studies in the U.S.A.

Controlled-source electromagnetic methods are often thought of as being used for detailed studies of Earth conductivity at shallow depths, such as is appropriate for minerals exploration. However, for the past quarter of a century, the Colorado School of Mines has placed considerable effort into the development of similar techniques for exploration deeper into the Earth's crust. The main difference between these techniques and shallow methods is primarily that the source moment is generally one to two orders of magnitude greater for the former. These deep electromagnetic sounding studies allow for a considerable flexibility in survey design so that optimum results may be obtained in diverse geological settings. Four time domain electromagnetic sounding surveys in different geological settings across continental U.S.A. are reviewed. The surveys on the U.S. West Coast, in the Pacific Northwest, New York state, and southern Colorado, provide insight into not only the diversity of the technique but also its ability to gather knowledge of deep electrical structures.

Elemental analysis of selected Indian monazite ores by energy dispersive X-ray fluorescence /EDXRF/ spectroscopy

Analysis by Energy Dispersive X-ray Fluorescence spectroscopy of monazite ores from Kerala /Chavara and Manavalakurichi/, Orissa /Chattrapur/ and Tamil Nadu /Tirunelveli/ has been carried out for the determination of their elemental composition using109Cd /annular/ and241Am /disc/ radioisotope sources. The elements Y, Zr, Mo, Pb, Th and U were analyzed using a109Cd source, and the elements La, Ce, Pr, Nd, Sm, Gd and Dy were analyzed using the241Am source in side source geometry. Quntitative results on these 13 elements present in these ores were obtained by the EDXRF technique. It was seen that despite the diverse geological settings, there is remarkable similarity in the elemental composition of these ores, although some trace elements do show certain variations from sample to sample. These results are presented and discussed in this paper.

Sandstone porosity as a function of thermal maturity

Sandstone porosity decreases in the subsurface as a power function of thermal maturity: {phi} = A(M){sup B}, where {phi} is porosity and M is a measure of thermal maturity representing integrated time-temperature history; A and B are constants for a given sandstone of homogeneous properties but vary between data sets. The commonly observed exponential dependence of sandstone porosity upon depth follows as a special case from this power-function relation when temperature increases linearly with depth. The consideration of sandstone porosity in terms of time-temperature exposure offers advantages in the comparison of porosity data from diverse geologic settings, the recognition of unusual porosity within a sandstone sequence, and the prediction of porosity ahead of the drill and at times in the geologic past.

Trace element characteristics, REE patterns and partition coefficients of zircons from different geological environments—A case study on Indian zircons

Twelve zircon separates from diverse geological settings in peninsular India have been analyzed for their Zr, Hf, Sc, REE, Th and U contents employing neutron activation techniques. The whole rock samples of acid charnockite and a related rock of syenitic composition were also analyzed for these elements after separation of zircons. The study reveals that 1. a) the zircons exhibit a wide range of variation in elemental abudances and their ratios, 2. b) their chondritic normalised REE patterns exhibit negative or positive Ce and negative Eu anomalies in addition to smooth HREE enriched patterns without anomalies, and 3. c) there are significant differences in the REE partition coefficients of zircons from the charnockite and the associated syenite. The overall elemental abundances and REE patterns of the zircons could be explained in terms of crystallochemical factors, bulk compositional differences of parent melts and paragenetic histories.

The Transiel method applied to hydrocarbon exploration: a statistical study of its results

With the current growth of the application of electromagnetic methods to petroleum exploration, the question arises: what is their actual contribution? This paper provides a statistical evaluation of the results of 59 Transiel surveys carried out in many diverse geological settings.

Elemental abundances in ultramafic rock and their relation to the upper mantle

Neutron activation and spectrographic data on 58 ultramafic rocks, representative of diverse geographic and geologic settings, indicate considerable differences in abundances of Al, Si, Ti, Co, Cr, Fe, Sc, Mn, Cu, V, and Ni. These wide differences in composition vitiate any calculation of mean elemental abundances for ultramafic rocks taken as a single homogeneous group and indicate gross chemical differentiations either within the mantle or between the mantle and the ultramafic rocks.