The Principles of SEM-EDS Analysis


Step 1 – BSE Image

Each cuttings, core or thin section sample is divided into a series of fields which are analysed sequentially. Initially, the back scatter electrons (BSE) are measured equating the surface detections to atomic weight, the brightness of the image reflecting chemical composition. The BSE data can be utilised to identify effective porosity, diagenetic and mineral transition phases within particle grains.

Step 2 – Elemental Composition

The cuttings particles are then further analysed by the EDS detectors which collect ~6000-10,000 x-ray counts per analysis step (circa 4-5 million across the whole sample) resulting in approximately 250,000 mineral point counts per sample.

More Detail

A beam of electrons is focused on the sample being studied. At rest, an atom within the sample contains ground state (or unexcited) electrons in discrete energy levels or electron shells bound to a nucleus. The electron beam may excite an electron in an inner shell, ejecting it from the shell while creating an electron hole where the electron was. An electron from an outer, higher energy shell then fills the hole and the difference in energy between the higher energy shell and the lower energy shell may be released in the form of an x-ray.

The number of x-rays emitted from a specimen can be measured by an EDS spectrometer. As the energy of the x-rays are characteristic of the difference in energy between the two shells and of the atomic structure of the element from which they were emitted this allows the elemental composition of the specimen to be measured.

This technique can identify any element present in quantities above 3% total volume with an atomic weight heavier than Boron (Be⁴).

Step 3 – Mineral Composition

The elemental composition in combination with the brightness of the BSE image can be converted in to mineral phases by sophisticated software such as Carl Zeiss’ SmartPI™ software.

This technique can differentiate both crystalline and non-crystalline mineral phases and can identify over 100 minerals including speciating all of the clay volumes.

Step 4 – Presentation of Results

The data produced from these analyses are presented as downhole logs, in tabular format and as pre-defined reports. These data include; bulk mineralogy, lithology, key elemental ratios, elemental proxies for organic content, PorSCAN which includes porosity %, pore size distribution and pore aspect ratio and, RoqFRAC™ for each sample. The RoqFRAC data output is presented as a down hole well log highlighting brittleness and ductility of each sample interval. An example of a typical data rich summary chart can be down loaded from the download section of the website.

PorSCAN derives porosity data from the sample. These data include % effective porosity, individual pore size, shape and orientation data (relevant if sample has been pre-orientated) and pore fabric data showing the distribution of, for example matrix porosity versus fracture porosity.

Finally, mineral maps can be retained for each sample allowing users to further manipulate and extrapolate data which can include the quantification of texture, grain/grain relationships, grain/pore relationships, grain/cement relationships as well as the identification of drilling contaminants, higher end clay species and feldspar phase analysis.

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