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Mapping Chemical Heterogeneity of Shale with Micro-FTIR

Yanyan Chen1, Maria Mastalerz1,2, and Arndt Schimmelmann1
1Department of Geological Sciences, Indiana University, 1001 East 10 St., Bloomington, IN 47405-1405, USA
2Indiana Geological Survey, Indiana University, 611 North Walnut Grove Ave., Bloomington, IN 47405-2208, USA; [email protected]

Shale’s fine-grained texture, presence of diverse organic matter types, and complex mineralogical composition create analytical challenges that can be met only by using highresolution techniques. In situ micro-FTIR (Fourier Transform Infra-Red spectroscopy) is a novel, powerful and nondestructive tool to investigate and map the chemical heterogeneity of finely dispersed organic matter and minerals in shales. In this study, four New Albany Shale (Devonian and Mississippian) samples of different maturation levels (vitrinite reflectance [Ro] values ranging from 0.58 to 1.41 vol. %) were analyzed by micro-FTIR mapping. Relative abundances of organic matter and minerals (carbonates, quartz, and clays) were mapped across selected micro-scale regions based on carefully screened characteristic peaks.

The observed proportions of general mineral groups (clays, carbonates, and quartz) and organic matter across micro-scale regions are consistent with corresponding bulk compositional percentages. In addition, distributions of organic matter revealed in chemical maps provide indirect evidence to help evaluate organic porosity of shales. Organic matter domains show high interconnectivity in two early mature samples (Ro 0.58 and 0.68%). However, the interconnectivity of organic matter dramatically decreases in the late mature sample (Ro 1.15%), but interestingly shows a reversal of interconnectivity increasing in the postmature sample having Ro of 1.41%. Porosity in organic matter accounts for a significant portion of total porosity in our studied shale samples. We suggest that increased interconnectivity of organic matter domains results in higher connectivity of pores, forming migration paths for oil and gas in shale. Therefore, in situ micro-FTIR is not only a powerful asset for investigating heterogeneity of shales, but, in combination with complementary porosimetric techniques, strengthens our understanding of porosity systems in shales.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013