QEMSCAN® by FEI is an automated petrography system based around a scanning electron microscope (SEM). The addition of Energy-dispersive X-ray (EDX) spectroscopy detectors and an extensive mineral database enables the identification of minerals and other compounds in-situ.
The sample surface is scanned along a grid pattern, where each node is identified as a mineral, other compound or as pore space. The selected grid resolution defines a mineral map as a matrix of pixels, with each containing detailed chemical information.
- Objective and reproducible mineral identification
- Robust bulk mineralogical fraction data
- Grain size and pore size distribution
- Grain and pore space morphology
- A wide range of resolutions from 1.5 mm to 1 µm
- Over 12 000 mineral analyses points per minute
- 24-hour unattended operation
QEMSCAN can be applied to a wide range of rock types, including sandstone, carbonates and mudrocks. These can be prepared from a range of formats:
- Core plugs
- Drill cuttings
- Heavy mineral separates
- Outcrop rocks
- Existing polished thin sections
The ability to work on drill cuttings is a particular advantage over conventional petrography as the QEMSCAN software can digitally remove drilling mud to reveal the original in-situ porosity.
The QEMSCAN process
The system scans a 10 x 10 mm or 4 x 4 mm surface area, divided into a field grid. For each field, the electron beam steps across the surface, collecting data at points spaced 50, 10 or 4 μm apart in both the X and Y directions (the stepping interval). The beam excitation volume within the material is roughly 1 cubic μm or less depending on the material.
The system first takes an intensity reading of back-scattered electron (BSE) to gauge the density of the material. The electron beam dwells at points where the BSE reading is above a certain threshold to allow the X-ray detectors to record a spectrum. This spectrum is then fed to the Rocktype mineral identification library. The lower intensity areas are recorded as background or porosity. The values are extrapolated to form field images which are stitched together to produce the mineral map.
The electron beam scans each field to produce a backscatter electron (BSE) image. Areas filled with resin, composed primarily of carbon, return low BSE values and are excluded from the EDX analysis.
Regions with high BSE values are scanned again with a more powerful electron beam using a 15 kV accelerating voltage. The electron beam dwells on each analysis point while the Energy Dispersive X-ray (EDX) detectors record x-rays. Once sufficient x-ray counts are recorded the beam moves to the next analysis point.
The analog signal from the EDX detectors is fed to a signal processing unit that builds an x-ray spectrum curve for each point.
The QEMSCAN phase classification routine matches the observed x-ray spectrum against an extensive library of known mineral phases and selects the best match. The mineral library is based on scans of an extensive Rocktype collection of physical mineral samples.
A detailed mineral map is constructed using the identified phase for each point. Areas with a low BSE signature are mapped as background and porosity. Related phases are merged into common mineral groups for reporting.
The digital data set is finally analysed and output as modal mineralogy, mineral association and grain morphology.
Fully digital petrography
A traditional workflow uses optical petrography to describe the matrix and pore space morphology and an instrument like XRD for bulk chemical and mineralogical data. A QEMSCAN analysis generates both types of information in a single run with a resolution and numerical detail not available otherwise. The data is spatially associated which reveals the interaction between morphology and mineralogy.
Even when results from optical petrography are delivered in digital form (e.g. a spreadsheet or database) it is still produced through a subjective manual process. Optical petrography results only contain a few hundred quantified data points, subject to the operator's interpretation. Data from other instruments like XRF and XRD do give objective, quantified data but only bulk figures for the sample as a whole, without contextual detail.
By contrast, QEMSCAN data is multi-dimensional, detailed and fundamentally quantitative. Each sample yields up to millions of data points, each rich in chemical data. When averaged this provides bulk figures as with XRD but uniquely it also gives you detailed mineral maps yielding structure, distribution, morphology and association.
Digital data makes it easier to collaborate with other teams in your own business and digital workflows such as log calibration or reservoir modelling should generally be based on robust digital datasets. Rocktype provides both detailed minerals maps and extensive tabular data that you can mine long into the future.