Page 52 - Petrophysics

26 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

Porphyroblasts: crystals created during metamorphism that are larger
than the mineral grains in the rock.
Pyroclasts: viscous magma containing gas erupting at the surface; the
gas expands rapidly, blowing the plastic magma into fragments high in
the air. Pyroclasts less than 2 mm in size are called ash, between 2 mm
and 64 mm it is lapilli, and when the size is greater than 64 mm pyroclasts
are known as blocks or bombs.
Pumice: formed from a froth of small bubbles in magma which has
erupted suddenly. It is light, glassy, and floats on water.
Sedimentary rocks: composed of the weathered fragments of older
rocks that are deposited in layers near the earth’s surface by water, wind,
and ice.
Shale: composed of clay particles less than 1/256 mm. Not gritty when
tested by biting. Exhibits fissility.
Siltstone (mudstone): composed of particles between 1/256 and
1/16 mm in size. Noticeably gritty to the teeth.
Tuff a deposit of volcanic ash that may contain as much as 50%
sedimentary material.
Vitreous (glassy): variously described as greasy, waxy, pearly, or silky.

REFERENCES

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Oct. 1983. National Technical Information Service, US Department of
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