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GeologyPetrophysics

Core Description

Grain Size, Sorting and Texture

Clastic grain sizes and carbonate rock types are documented as a continuous vertical curve, which reflects the variations in the depositional energy of the sediments (Figure 1). By convention, grain size increases to the left, copying the typical gamma ray response in a sand/shale sequence.

Idealized graphic log
Figure 1: Idealized graphic log

The grain size is ascertained by comparison with a grain size comparator chart (Figure 2), or a visual pattern. A high quality hand lens magnifier and a reflecting light binocular microscope are essential for accurate grain size descriptions.

Grain size comparator
Figure 2: Grain size comparator

Textural characteristics of the rock include its grain size, grain size distribution and the degree of shaliness. These characteristics influence the porosity, permeability and the interstitial water saturation. Another commonly used scale for assessing grain size in clastics is the Wentworth Scale (1922). This scale, shown in modified form in Figure 3, links medium, fine or other reported grain sizes to a specific grain diameter.

Grain size comparator Wentworth
Figure 3: Grain size comparator (after Wentworth)

The detrital clay content and the sorting and roundness of grains are factors in the textural maturity of sandstones. Folk’s (1974) model for textural maturity is well respected and followed by many in the oil industry (Figure 4).

Folk's classification system
Figure 4: Folk’s classification system
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