Traditionally pore throat size distribution data is measured using mercury intrusion porosimetry (or MICP). However, this is not always possible /appropriate, so we can also offer pore size distribution data generated from analysis of scanning electron microscope (SEM) images. This methodology is applicable to any lithology (and is well-suited to very fine-grained sediments such as micritic limesonte / chalk). Data can also be collected from fragments of a specific lithology within cuttings samples, extending the “reach” of capillery-pressure type measurements into uncored intervals.
SEM images are collected systematically over an area of, or of random fields within, a polished thin-section. Following segmentation of the images into pores and “grains” (everything else), detailed information on pore volumes and pore sizes is collected.
These results can be used alongside other measures of pore volume and pore size when modelling permeabilities (e.g. helium porosities, mercury intrusion data, and nuclear magnetic resonance data).
The strategy for image collection is dependant upon the nature of the sample and its pore system. The SEM magnification is set according to the range in pore size we are attempting to characterise. Depending upon the samples, we may collect images from random, non-contiguous fields over the sample area, or regularly distributed, overlapping fields (that can also be stitched into deep zoom photomontages).
Image analysis routines are used to normalise images to consistent grey-scales and to remove any shading effects. Pores are then segmented from grains and, where required, more advanced routines are used to separate / subdivide touching / connected macropores from one another.
Pore sizes and areas are measured and the results processed to provide total pore areas and pore size distribution data.
Results provided include:
Summary data (pdf format; including summary and plot shown as below),
Detailed results (individual pore measurements including pore area, diameter, and other parameters, in xlsx format), and
either all the original collected images or stitched photomontages (as appropriate, in .tif and/or .jpg format).
We offer sample descriptions at a range of level of detail, from summary descriptions focussed on a specific set of feature(s) in a sample or sample set, up to completely-comprehensive characterisation of all aspects of the sample.
There is always a temptation to “scrimp-and-save” a little on this aspect of petrographical studies, but this is the time where the real detail and intricacies of samples can be investigated and documented.
Scanning Electron Microscopy (SEM) can be carried out as a stand-alone service, or in support of other petrographical analyses / descriptions. We can analyse a range of sample types including small rock chips (“stub” samples) and polished thin-sections, as well as other materials.
Our main uses for SEM imaging and analysis are:
Detailed characterisation of clay mineralogy and other microcrystalline components (and their associated microporosity)
Elucidation of paragenetic relationships
Investigation of zoning and chemical variability within cements.
Systematic / automated collection of images for pore image analysis
Preparation of petrographical montages for deep zoom imaging
Because we have Scanning Electron Microscope capabilities in-house, we can offer a rapid turnaround / hotshot analysis of samples. (Many geological materials can be analysed with a minimum of preparation – the samples only need to be dry and, in the case of samples containing liquid hydrocarbons, light cleaning is also required ).
When I’m not being a geology nerd and using our excellent microscopy facilities for work, I’m also a keen amateur archer. Most of the time I’m probably better at petrography than I am at archery. However, now and again I have a good day and can shoot arrows into something resembling a good group. On a really good day I “Robin Hood” myself and shoot an arrow into the back of another one already in the target.
When we bought our bench-top SEM a few months ago, I thought it would be interesting to revisit this sample, which is one that I spent a lot of time looking at when I was a PhD student ( … back in the day … sigh). I also read a piece in today’s paper that gave me pause to think about just how much I (try not to) take for granted in my day-to-day work .