Petrographical analyses is an essential service providing estimates of pore size distribution, grain size, sorting, porosity, and mineral abundances, fabric, rock classification, porosity varieties and abundances.

Results include a determination of shale volume, shale distribution, porosity varieties and abundances with a discussion of diagenesis, diagenetic sequence, porosity origins and development, controls on porosity and permeability. Point count data is presented in tabular form with colour photomicrographs accompanied by detailed descriptions.

Core or chip samples are cleaned in a vapor phase bath in an attempt to remove solvable excess residual hydrocarbon. The samples are then impregnated with blue epoxy to identify porosity and preserve textures, polished and mounted onto a glass slide. The samples are then ground down to a thickness of 30 microns and stained with a combined carbonate stain of Alizarin Red-S (for calcite) and potassium ferricyanide (for
ferroan carbonate).

Thin section microphotograph, blue colour is porosity, other colours are different minerals

Finally, a second glass slide is glued on the polished surfaces. The prepared thin sections are examined petrographically. Individual minerals are identified by their stained colour and crystal structure (if visible). 

Thin section "visual porosity" is called effective porosity in petrographic reports. It is usually less than effective porosity from logs or core analysis because it does not include microporosity inside grains or rock fragments. Petrographic reports sometimes refer to core porosity as "total porosity". This is confusing, as the common usage of "total porosity" is the sum of effective porosity (core porosity or log analysis porosity) plus clay bound water. Consider the context.

The petrographic data summary, including framework mineralogy, diagenetic minerals and cements, textures, grain size range, and average porosity and permeability, is provided in tabular form.

Grain size distribution histogram - tabular data for each sample will also be supplied

Macro fracturing are usually visible with blue epoxy. To look for possible micro fractures, a second set of thin sections are made from the same samples. These are injected with fluorescence Rhodamine-B red  epoxy. These samples are examined and photographed under ultra-violet (UV) light.

More detailed images may be made by scanning electron microscopy (SEM) to assess pore geometry and grain texture.



Example of a complete petrographic point count analysis on four thin section samples. Note that the same mineral can occur in several catagories. For comparison of log analysis results to petrographic data, the sum of each mineral quantity must be found. For example, quartz (+ chert) adds up to 54% by weight and dolomite is 21% in Sample #1. Visual porosity (reported elsewhere on the report summary was only 0.40% compared to core porosity of 8.4 %. This indicates that most of the porosity is microporosity, not visible in microphotographs.

XRD data for the same 4 samples. Note that Sample #1 shows 50% quartz (vs 51% on TS) and 31% dolomite (vs 21% on TS). Total clay is 5% vs 10% on thin section report. These differences are typical of the possible variations in values, so we don't get too excited about an exact match with log analysis results. The Clay Fraction numbers break down the minerals in the <3 micron float sample and shows that only 27% of the clay sized particles are clay minerals, the balance being non-clay minerals. 

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