Tag Archives: Polarised Light

Optical Microscopy

We make extensive use of optical microscopy for both collection of petrography data (textural and modal analysis) and also for more descriptive work.

We are well equipped with research grade, Zeiss optical microscopes capable of :

  • Transmitted plane and cross polarised light
  • Reflected illumination (for investigation of reflective minerals)
  • Epifluorescence microscopy

All the microscopes are equiped with digital cameras (with live view imaging on attached PC’s), and we have stepping stages (but driven using our own, in-house, software) that we use for modal analysis and acquisition of images for preparation of digital thin-sections and deep zoom images.

Textural Analysis

What is it and why might you need it ? 

Textural Analysis provides basic data on sandstone grain size, which almost invariably exerts some degree of control on final reservoir quality (either directly, or indirectly depending upon the degree of diagenetic overprinting).   The grain size data is also useful for calibration of core grain size in heavily cemented sediments, where original grain size is not always easy to determine in core, and cannot be measured accurately using bulk approaches (e.g. seive analysis or laser particle sizing).

We provide the raw data, as well as summary statistics including averages, sorting and other measures of spread and skew – including systematic grain size classification (in ½Φ bins).

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Petrographical Descriptions

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.

SEM Image showing clays on a grain surface, associated with quartz overgrowths and pyrite euhedra.
SEM Image showing clays on a grain surface, associated with quartz overgrowths and pyrite euhedra.

Most of the descriptive work is carried out using optical (plane-polarised light) light microscopy.  However, additional detail will usually be added using scanning electron microscopy and other microscopy methods as required.   Any additional micrcoscopy observations are incorporated directly into the sample descriptions (and not presented as separate descriptions).

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.

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Differential Wettability

This post in a LinkedIn group that I follow, linking to this article,  got me thinking a bit about wettability.  I was struck by the comment that “conventional wisdom says that clays are always water-wet”.   The article makes good use of pretty advanced analysis and imaging technologies to show that this is not (always) the case.   We can show evidence for similar effects using the rather more mundane thin-section petrography, and can add that we very commonly see illite-cements, in particular, that are very heavily oil stained (implying some degree of hydrocarbon wettability) while other mineral surfaces are oil-free.

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