Can FIB-SEM-EDS Provide Accurate Assessment of Shale Microporosity?

Abstract

Assessment of microporosity is a critical factor in determining the reservoir potential of unconventional shale plays. Focused Ion Beam Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy (FIB-SEM-EDS) is a destructive, site-specific, 3D microanalytical technique which has found extensive use in materials science and, increasingly, in the geological sciences. The newest high-resolution FIB-SEM instruments can image particles and porosity with nanometer-scale resolution, and the EDS attachment allows compositional verification of imaged materials. A FIB-SEM-EDS 3D microanalysis of Neal Shale core samples was conducted to determine the sources of porosity within the shale at different levels of kerogen maturity. The Neal Shale is the organic-rich interval of the Floyd Shale within the Black Warrior Basin of Alabama and Mississippi which has recently been evaluated as a potential hydrocarbon exploration target. A combination of secondary electron (SE) and back-scattered electron (BSE) images was used to analyze the 3D network of organic matter, porosity, and pyrite in the shale samples. Contrary to expectations, no increase in organic matter porosity was observed with increasing thermal maturation. Instead, the organophillic porosity development was found to be linked to bitumen migration/maturation, suggesting that organic porosity development was more significantly affected by the chemical composition of the organic matter, rather than by thermal maturity level. A kinetic model for porosity development based on TOC, kerogen type, and hydrogen index was used to estimate Neal Shale porosity resulting from kerogen decomposition and compared to porosity measurements obtained by FIB-SEM-EDS analysis. Kinetic model results indicated that the Neal shale should have developed as much as 4.82% kerogen porosity at the highest maturity levels. However, the maximum organic porosity measured by FIB-SEM-EDS in this study was 1.12% at a sample depth of 2,830 ft, and organic matter interpreted to be kerogen was not observed to develop any organophillic porosity. Although the maximum observed organic porosity value is nearly identical to the kinetic model calculation of 1.15% kerogen porosity at sample depth 2,820 ft, at all other sample depths porosity is highly overestimated by the kinetic model. Possible reasons for the discrepancy between measured and calculated porosity will be discussed.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016