Grain size frequency and cumulative weight percentages, mean grain size, standard deviation, and skewness provide the geologist with information necessary for studies assessing rock complexity and heterogeneity, and in some cases, depositional environment. Although grain size distribution data are important to the exploration geologist, they are not normally available in sufficient quantity or on a timely basis.

The new computerized Particle Size Analyzer (PSA) enables the explorationist to obtain grain size distribution information rapidly and economically on sidewall and conventional cores. The porosity and the measured grain size distribution, which includes silt and clay sizes, are used to "index" the quality of the reservoir rock. This quality grading or "indexing" is then used to evaluate changes in formation water saturation and/or electric log resistivity, thereby better identifying hydrocarbon productive intervals and oil-water contacts.

Data generated from conventional core samples in the Mitchell sand of the Rodessa Formation in Saint Mary field, Lafayette County, Arkansas, illustrate the use of particle size data for improved well evaluation.

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