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The grain-size distributions of oil-reservoir rocks may be classified in four groups: arkose, low-rank graywacke, orthoquartzite, and quartzose graywacke types. Limestone and the rare high-rank graywacke oil-reservoir rocks are omitted from consideration.
In the Miocene low-rank graywackes of South Trinidad, B.W.I., the amount of favorable oil-reservoir rocks in the 1,000 cubic miles of Miocene sediments may be recognized as about 12% on the basis of their peculiar grain-size distribution. Similarly, by plotting size (median diameter) against sorting (phi percentile deviation) in these rocks it can be shown that graywacke oil sands are generally among the best sorted sediments and in any graywacke series the oil-reservoir rocks are better sorted than their associated barren sediments. By using this size-sorting relationship it can be shown that the most abundant oil-reservoir rocks are low-rank graywackes which vary in average size from 0.250 mm.-0.0625 mm. Outside these limits the finer-grained sediments are generally watered, while t e coarser are also barren or may be gas and tar reservoirs, as noted by P. G. Nutting.
By using variations in grain-size distribution as indicators of the varying proportions of mineral constituents, particularly the clay fraction, it can be shown that there is a predictable relationship between clay content and the self-potential curve of the electric log. Similarly the grain-size distribution when coupled with its implications in terms of proportions of mineral constituents may be used to recommend improvements in completion procedures in terms of gravel pack and slotted-liner practice.
Published work on the effects of grain size on porosity and permeability of reservoir rocks clearly indicates that lack of consideration for the inter-relationship between size and mineral composition had led to contradictory results. Recent work emphasizes that it is necessary to consider the petrography of the rock, the composition of the fluid, and their mutual interaction, before laboratory analysis can be used to predict reservoir behavior in more than an approximate manner.
The relationship between grain-size distribution and fluid saturation has been investigated in the laboratory, and results in the field show a parallel relationship; in both cases there appears to be a traceable relationship between grain-size distribution and oil-water saturation in sandstone reservoirs.
On the basis of these findings it seems clear that grain-size distribution, suitably qualified by its inter-relationship with proportions of mineral constituents, is a useful measure of petrographic properties of reservoir rocks and forms a basic quantitative measure for empirical comparison between petrographic and reservoir characteristics in oil-producing reservoir rocks.
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