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January 01, 1990


In any petroleum province in the world where there are sand/shale sequences, the geologist, engineer or log analyst confronts toe “shaly sand problem.” The shaly sand problem is really a question of identifying, from logs, the degree to which the presence of clay degrades reservoir quality, because clay affects both porosity and permeability, If too much shale or clay is in a reservoir, production of hydrocarbons is suppressed by excessive reduction in permeability. However, a small amount of clay disseminated in toe pores can be helpful for trapping interstitial water, permitting hydrocarbon production from reservoirs with high water saturation.

Most hydrocarbon-bearing sandstones contain laminated or dispersed clay minerals (Fertl, 1982). As a consequence, any assessment of subsurface strata or reservoir (unless it has been conclusively established otherwise) should begin with the assumption that clay may be affecting log-derived measurements of porosity and water saturation.

An understanding of the nature of clay and of the log data corrections needed if significant amounts of clay are present in a reservoir helps the dedicated learner avoid serious misjudgments in both hydrocarbon development and exploration work. While it is true that the mathematical formulas and adjustments necessary to obtain toe correct information from logs in shaly sand reservoirs are laborious and time-consuming, the end result of better formation evaluation is more than worth the effort.

Computers are an invaluable tool for performing the complicated formulas of shaly sand analysis, but their use does require a thorough knowledge of shaly sand models, Otherwise,

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AAPG Continuing Education Course Notes Series

Log Evaluation of Shaly Sandstones: A Practical Guide

George B. Asquith
George B. Asquith
Texas Tech university
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American Association of Petroleum Geologists
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Publication date:
January 01, 1990




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