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TitleAlteration of organic matter by ion milling
AuthorSanei, H; Ardakani, O H
SourceInternational Journal of Coal Geology vol. 163, 2016 p. 123-131, https://doi.org/10.1016/j.coal.2016.06.021
Year2016
Alt SeriesEarth Sciences Sector, Contribution Series 20160059
PublisherElsevier
Documentserial
Lang.English
Mediapaper; on-line; digital
File formatpdf
Subjectsfossil fuels; ion exchange; organic materials; optical properties; reflectivity; reflectance
Illustrationstables; photomicrographs; histograms
ProgramShale Reservoir Characterization, Geoscience for New Energy Supply (GNES)
AbstractIon milling is increasingly used as a mean of sample surface preparation for scanning electron microscopy (SEM) to provide high-resolution imagery needed for studying the nanoscale structure of mudstones. Application of this technique has introduced new insight into nano-structural properties within organic matter (i.e., organic porosity) as an important reservoir control factor in unconventional shale/tight rocks. The continuous bombardment of the sample surface by ions (e.g.,Gallium, Argon) results in the formation of kinetic heat energy, hence heating the milled surface of the rock.While this technique has proven to provide a spectacular surface for studying the fine structures, the effect of initial heat transfer to the organic matter (OM) is notwell understood. Optical properties ofOM (reflectance and fluorescence spectrometry) respond to the degree of aromatization of organic molecules, caused by thermal alteration. The effects of ion milling on dispersed OM were studied on four different macerals (i.e., solid bitumen, pyrobitumen, zooclasts, and alginate) within mudrocks of different thermal maturity levels. These mudrockswere milledwith a cryogenic broad ion beam(C-BIB) and a focused ion beam (FIB). The random reflectance (Ro), bi-reflectance and the fluorescence red/green quotient (Q) of the macerals were measured and compared before and after the milling processes.
The results show that initial heating caused by ion bombardment results in significant thermal alteration of the OMsurfaces. The C-BIB milling resulted in 10 to 53% increase in the Ro values of the studied macerals, depending on their initial chemical refractoriness. The higher energy FIB milling increased Ro values of the samemacerals by one order of magnitude. The solid bitumen appears to be more susceptible to thermal alteration than more refractory macerals (e.g., zooclasts and pyrobitumen). Ion milling did not shift the fluorescence spectra in immature alginate, suggesting that thermal alteration caused by ion milling is limited to the surface and not the entire body of the maceral. This study also documents significant morphological and possible chemical changes in pyrite due to the ion beam incident thermal alteration in most samples. Since most ion beam SEMstudies focus on organic nanoporositywithin theOM, thermal alteration caused by ion milling may have serious implications for image-based physical analysis of pores. The possibility of increased nanoporosity due to rapid devolatilization of lighter organic compounds in macerals could potentially have a significant impact on organic porosity, especially at lower thermal maturities (i.e., oil and wet gas windows).
Summary(Plain Language Summary, not published)
Ion milling is increasingly used as a means of sample surface preparation for scanning electron microscopy (SEM) to provide high-resolution imagery needed for studying the nanoscale structure of mudrocks. Application of this system has introduced new insight into nanoporosity within organic matter (organic porosity) as an important reservoir control factor in unconventional shale/tight rocks. The continuous bombardment of the sample surface by ions results in conversion to kinetic heat energy, hence heating the milled surface of the rock. While this technique has proven to provide a spectacular surface for studying the fine structures, the effect of initial heat transfer to the organic matter (OM) is not well understood. The present study compares the optical properties of various macerals dispersed in mudrocks (chitinozoan, alginate, solid bitumen, and pyrobitumen) that represent a wide range of thermal maturities, before and after being polished by C-BIB and FIB methods to determine the effects of thermal alteration of OM during the milling process.
GEOSCAN ID298784