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FOBEX 1: All About Sandstone

    Sandstone, a clastic sedimentary rock that is made up about 25 percent of all sedimentary rocks, distributed in every continent of earth in the world that means they are the common rocks in geologic systems of all ages. It consists of mainly silicate grains with sand size or 1/16 to 2 mm weathering debris, and they may also contain some various amounts of cement with very fine size ( < 0,03 mm) and they are called as matrix. This kind of litology is usually associated with beaches, deserts, and delta environment that its material is accumulated of eroded material in terrestrial.

   The major mineral composing sandstone is Quartz, Feldspars (including K-Feldspars such as orthoclase, microline, sanidine, and anorthoclase) and also plagioclase feldspars (from albit to anorthite), clay minerals and fine micas. Also rock fragments are the main composition of sandstone, those rock fragments can be derived from any weathering process of igneous rock, metamorphic rock, or sedimentary rock. Beside that, there still can be any accessory minerals, and any chemical cements such as carbonate minerals.

   Classification that is usually applied for sandstone is Pettijohn’s (1949). It is based on the mineral composition of Quartz and Feldspars, Lithic or rock fragments, and matrix abundance.

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Figure 1. Pettijohn’s Classification

(http://archives.datapages.com/data/sgs_wb_utf/data/0011/0001/figs/fg6.jpg)

    Pettijohn classification also explains degree of sandstone maturity. The maturity of any sandstone can be measured by its composition. Sandstone composing abundance of matrix is usually immature which can be related to its depositional process. The provenance of this material is usually composed of unstable mineral, such as feldspar so when this material is eroded and transported by water or air, this material is less resistent and turning to muddish. In reverse, if the provenance composing of stable mineral, such as : quartz, this material is more resistent and hardly turning to muddish. Usually the composition explaining about depositional process can be related to textural maturity ( appeared in figure 2 ). Textural mautrity consists of 3 criterias, there are : grain size, shape, and sorting. Usually, sandstone having high degree of maturity is composed of larger grain size with well rounded and good sorting. The roundness of sandstone maturity explains how a material will be abraded during transportation process, and it reflects the distance of transportation process, the more distant the material transported, the material will be more abraded during transportation process and its shape will be rounded, hardness and resistivy of minerals also affect the degree of roundness, such as : quartz and zircon are rounded less readily during transport than unstable minerals will be rounded easily during transportation history.

     According to its sphericity and sorting, the degree of sphericty is influenced by shape of provenance material, the material will be more sphere when the ancient provenance has high sphericity, so that it will affect mechanism of transportation and settling velocity, the spheres material will be transported by rolling  transportation, when the non-sphered material will be transported by sliding transportation and harder to settle easily. The last parameters of textural maturity is sorting framework grains, sorting framework grains is related to cleaning process of mud and settilng of similiar grain size caused by mass media transport, such as : water and wind, so the material tend to be deposited in similiar grainsize and relatively uniform.

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Figure 2. The relation between roundness and sphericity

(http://www.obsidianlab.com/terminology.html)

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Figure3. Parameters affecting degree maturity of sandstone (Boggs, Sam Jr., 2006 )

     Based on those parameters, the maturity of sandstone can be applied to reservoar qualiity caused depositional process affecting the size of pore to accumulate hydrocarbon. However, not only affected by about the porosity of sandstone, we know that sandstone can be used as good reservoir because of its porosity to reserve hydrocarbon. Porosity of hydrocarbon alson can be triggered out by post depositional process, such as : fracture caused by structural geology ( faults and folds ). However, post depositional process not only causing the pore larger, it can cause pore smaller and isolated because of packing and cementation process.

Created By :

Ryan Syahputra Wiranto

( Member of Course and Workshop Division )

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