403 272 - 9590

220-19 St. SE, Calgary, AB, T2E 6P5

Order Form

Tips

Assorted Geological Tips

– Texture, Cement, and Fossils

Understand key characteristics of geological samples including grain size, sorting, cementation, matrix, fossils, and accessory minerals. Learn how these features influence porosity, permeability, and the interpretation of depositional environments.

TEXTURE – GRAIN OR CRYSTAL SIZES

  • Size classification should be based on a modified Wentworth scale.

  • Do not try to record size grades without reference to a standard comparator.

  • Shape involves both sphericity and roundness:

    • Sphericity: Comparison of surface area to a sphere of the same volume

    • Roundness: Sharpness of edges and corners of a fragment

      • Angular: Edges and corners sharp; little or no wear

      • Subangular: Faces mostly untouched; edges and corners rounded

      • Subrounded: Edges and corners rounded to smooth curves; areas of original faces reduced

      • Rounded: Original faces almost completely destroyed

      • Well rounded: No original faces remain

SORTING

  • Sorting describes uniformity of shape, roundness, specific gravity, mineral composition, and size.

  • Well-sorted samples indicate consistent depositional energy, while poorly sorted samples suggest variable conditions.
  •  

Cement and Matrix

  • Cement: Chemical precipitate deposited around grains and in interstices of sediment

  • Matrix: Small individual grains filling interstices between larger grains

  • Cement is deposited chemically; matrix is deposited mechanically.

  • Chemical cement is uncommon in sandstone with clay matrix.

  • Common cementing materials: silica and calcite

  • Dolomite and calcite: deposited as crystals in interstices and aggregates in voids

  • Anhydrite and gypsum: more commonly associated with dolomite and silica

  • Additional cementing materials: pyrite, siderite, hematite, limonite, zeolites, phosphatic material

  • Silt acts as a matrix; clay may cause porosity loss by compaction or swelling

  • Argillaceous material can be evenly distributed in siliclastic or carbonate rocks

  • Coarse-grained sandstones generally have greater permeability than finer ones when cement is similar

FOSSILS AND ACCESSORIES

  • Microfossils, small macrofossils, or fragments are used for correlation and environmental interpretation

  • Geologists should distinguish: foraminifera, ostracods, chara, bryozoa, corals, algae, crinoids, brachiopods, pelecypods, gastropods

  • Record presence and relative abundance; use slides or photos to illustrate principal microfossils

  • Accessory constituents may indicate depositional environment

  • Common accessories: glauconite, pyrite, feldspar, mica, siderite, carbonized plant remains, heavy minerals, chert, sand-sized rock fragments
  •  

POROSITY AND PERMEABILITY

  • Among the most important observations in sample examination

  • Porosity and permeability determine fluid flow and reservoir quality