Mineralogy and petrography of Ordovician and Silurian shales
Source and reservoir rocks of unconventional gas accumulations, commonly called “shales”, are mostly mudstones and claystones.
Mudstone is a clastic sedimentary rock built of cemented mud. Claystone is a sedimentary rock in the form of consolidated clay.
The main components of mudstones and claystones are:
- quartz (silica),
- clay minerals,
- carbonate minerals (calcite, dolomite),
- other, in negligible quantities.
In terms of origin, most shale rock's constituent minerals are either:
- allogenic – clasts of rocks and minerals are brought into sedimentary basins, in which the shales are formed, from external sources (predominantly as detritic terrigenous material derived from rock weathering on land). Common allogenic constituents are: quartz, feldspars, mica, heavy minerals,
- autigenic – formed by sediment precipitation from water or changes occurring in deposited sediments or rocks (diagenesis – e.g. cementation, recrystallisation). Carbonates are the most common autigenic minerals.
Moreover, shale rocks may contain calcareous, silicious or phosphatic remains of animal skeletons.
How the mineral composition and petrography of shales are investigated?
The mineral and petrographic composition of shale rocks is investigated by means of petrographic analysis.
Petrographic analysis is used to determine the following characteristics of the tested rocks:
- mineral composition,
- origin of particular constituents,
- texture (layout and distribution of grains, the degree in which they fill the rock spaces),
- structure (grain size and shape),
- pore space characteristics (size, type and interconnections of empty voids and micr-fractures).
Shale rocks are frequently very heterogenous at a small scale.
Standard examinations of shale rocks include the determination of the mineral composition using:
- XRD (X-ray diffraction),
- petrographic thin section studies (using both transmitted light and cathodoluminescence – CL), and
- SEM (scanning electron microscope) analyses.
XRD analyses of mineral composition are intended to establish the presence of specific minerals (qualitative analysis) or their percentage share (quantitative analysis). In some of the samples detailed quantitative XRD studies of clay minerals are made.
It is important to know the mineral composition because it affects mechanical properties of the rock and, consequently, its susceptibility to fracturing.
XRD studies are supplemented by geochemical analyses, including the determination of the percentage share of:
- main elements: (such as silica - Si, aluminum - Al, iron - Fe, calcium – Ca),
- trace elements (e.g. molybdenum - Mo, nickel - Ni, lead - Pb, vanadium – V), and
- rare earth metals (lanthanides)
Since shale rocks are composed of very fine grained mineral fractions, traditional petrographic thin section studies have inherent limitations due to achievable levels of magnification.
Scanning electron microscope (SEM), the most useful complementary tool, offers very high levels of magnification. The SEM allows for:
- imaging the distribution of individual minerals in the rock (backscattered electrons [BSE] images of polished thin sections),
- the determination of the chemical and (consequently) mineral compositions in a micro-area (analysis of selected grains or crystals), and
- mapping the distribution of particular elements using X-ray detectors or electron microprobe.
FIB (focused ion beam) SEM, a further development of traditional SEM, offers very high magnifications (at nanometric resolution) that enable examinations of micro-textures and micro-structures, including detailed imaging of micro- and nano-porosity, a vital factor for natural gas retention in shale rocks.
author: Marek Jasionowski