Petrography

Petrography

Thin section petrography is one of the key analytical tools in sedimentology.  It offers the greatest “bang for the buck” when characterizing clastic, carbonate and evaporate systems and can also yield information on associated igneous and metamorphic lithologies.

Thin sections of rock samples (Outcrop, core, SWC, cuttings) are typically prepared at 3x1” or 3x2”, with samples vacuum impregnated with coloured resin.  The following mineral stains are applied to discriminate the minerals present:

  • Carbonate mineral phases (Calcite / Fe Calcite, Dolomite / Fe Dolomite)

  • K-feldspar

Description of the thin section components can be either through a visual estimate, or quantitative through point counting.  Image analysis can also be undertaken to determine the abundance of certain components, particularly porosity.

Sample intervals for analysis are determined by the rock type, with carbonates requiring a much denser sample spacing in order to fully characterize the reservoir section.

Petrographic analysis can provide information on:

  • Rock Texture (grain size, sorting, clast roundness and grain contacts / compaction, sedimentary structures & bioturbation

  • Detrital grain and matrix composition

  • Fossil & microfossil assemblage

  • Sediment provenance

  • Depositional facies / microfacies (particularly for carbonates)

  • Microfossil dating of some groups

  • Authigenic cements and their distribution and relationships

  • Porosity type(s), abundance and distribution

  • Pore evolution

Chamosite Ooid Packstone

A silty argillaceous chamosite ooid packstone typical of transgressive deposits in the Lower Goru of Pakistan

Sandstone with Primary & Secondary Pore Systems

Selective dissolution of K-feldspar framework grains in the subsurface has produced secondary intragranular macropores. Original grain outlines are preserved by early Fe-chlorite grain pellicles.  Late authigenic feldspar has locally reduced the secondary macropores.

Carbonate Porosity

Interparticle porosity (I) is patchily preserved and is lined by early isopachous freshwater calcite cements (FC), overlain by a later generation of coarser grained calcite (LC). Note that selective leaching of High Magnesium Calcite foraminifera (F) post dates these cements and is of inferred burial origin.

Large Benthic Foraminifera Rhodoid Packstone / Grainstone

Mud poor packstone / grainstone containing a diverse assemblage of foraminifera types, including Amphisteginids, Miogypsinids and aligned discoid foraminfera (Heterosteginids).  The sample displays moderate levels of interparticle and intraparticle macroporosity.