Detecting and Quantifying Kerogen, Bitumen, and Organic Porosity From SEM Images

Abstract

Scanning electron microscopy (SEM) of ion-milled rock surfaces is crucial to understanding organic matter (OM) distribution and organic-hosted porosity. Secondary and backscatter electron images have been acquired on 12 organic rich shale samples. These samples span a range of mineralogy and TOC. Most samples were in the oil generation window. Ion beam polishing was used to create flat, smooth surfaces. Organic matter imaging was done with low electron beam energy to obtain high resolution and contrast. This allowed for the discernment of two distinctly different organic matter phases, one darker than the other. Darker areas in SEM images are lower in density and these darker areas likely represent bitumen or early oil generation, whereas the lighter areas (more dense) are kerogen. Open pore space in these images appears black to dark gray and is easily quantified. Using various image enhancement tools, the area or volume of both light and dark OM were quantified. Also quantified in this project was the SEM-based porosity associated with organic matter (PAOM). Finally, programmed pyrolysis and LECO TOC tests were run on the samples for comparison to the organic matter types and volumes observed in the SEM analysis. The distribution and shape of the darker organic matter phase and the PAOM are similar. For example the open pores were mostly surrounded by a perimeter of the denser OM, and the same can be said for the less dense, darker OM. Stated differently, the mineral grains are “coated” in the dense OM and most of the porosity and less dense OM is found in the central area between mineral surfaces. There is also a rough trend between the dark OM volume and the S1 value from pyrolysis. Applications The main application is to understand the effect of bitumen and dead oil on rock permeability. Both are potentially mobile in-situ but very difficult to remove completely by laboratory methods. By understanding geometric distributions relative to kerogen, digital simulations can be run to remove both and quantify the effect on permeability. Conclusions The less dense OM phase is likely in transition to hydrocarbon filled porosity. This material may be mobile at in-situ conditions, as suggested by its correlation to S1. Technical Contribution Previous work in this field has focused mainly on quantifying general organic matter, minerals, and pores. This project shows that different types of organic matter can be quantified separately.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017