Sandstone

Introduction

Sandstones make up 20-25 percent of all sedimentary rocks. They are common in geologic systems of all ages and are distributed throughout the continents. They occur in beds ranging in thickness from a few centimetres to tens of metres. ^[1]

Sandstones consist mainly of silicate grains ranging in size from 1/16 to 2 mm. These particles make up the framework fraction of the sandstone. Sandstones may also contain various amounts of cement and very fine-size (<~0.03 mm) material called matrix, which occupy interstitial pore space among the framework grains. ^[1]

The three-part architecture - framework, cement, matrix - is fundamental to understanding sandstone behaviour as a reservoir rock. Framework grains are the primary particles. Cement fills pore space after deposition, reducing porosity and permeability. Matrix is fine material that was deposited simultaneously with the framework grains, also occupying pore space. A high-quality petroleum reservoir sandstone has abundant framework grains, little matrix, and incomplete cementation - leaving interconnected pore space through which fluids can migrate.

Composition and Analysis

Because sandstone grains are relatively coarse compared to particles in shales, the framework mineralogy can generally be determined with reasonable accuracy using a standard petrographic microscope or backscattered electron microscopy. ^[1]

Bulk chemical composition can be measured by instrumental techniques such as X-ray fluorescence and inductively coupled argon plasma emission spectrometry (ICP). The chemistry of individual mineral grains is commonly determined using an electron probe microanalyzer or an energy dispersive X-ray detector (EDX) attached to a scanning electron microscope. ^[1]

The combination of petrographic microscopy and electron beam techniques gives geologists an unusually complete picture of sandstone composition. Microscopy identifies the mineralogy, grain shape, sorting, and textural maturity. X-ray fluorescence gives rapid bulk chemistry of the whole rock. The electron probe and EDX allow the chemistry of individual grains to be measured, enabling provenance studies that link specific grains back to specific source rock types.

Framework Mineralogy Overview

Only a few principal kinds of minerals and rock fragments make up the bulk of all sandstones. The major framework constituents are quartz, feldspars, and rock fragments. Accessory minerals, defined as those with average abundances below about 1-2 percent, include coarse micas (muscovite and biotite) and heavy minerals - minerals with specific gravity greater than about 2.9. ^[1]

See Sandstone Framework Mineralogy for a detailed account of all framework grain categories, Quartz in Sandstones, Feldspars in Sandstones, Heavy Minerals, and Rock Fragments in Sandstones for individual components.

Classification

Sandstones are classified primarily on the basis of framework mineralogy, using a QFL (quartz-feldspar-rock fragment) triangular diagram. The major classes are quartz arenites, feldspathic arenites, and lithic arenites, with corresponding wacke variants when matrix exceeds ~5 percent. See Sandstone Classification, Quartz Arenite, Feldspathic Arenite, Lithic Arenite, and Volcaniclastic Sandstone for details.

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