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1.1 Introduction The evaluation of shaly formation is considered as oneof difficult tasks as it causes many problems :

The shale affects the evaluation of the reservoirparameters as porosity and water saturation .

Although several models and researches have beenmade for studying shaly formations , most of them arenot fully understood or considered to be illusive .

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The presence of shale in sand tend to reduce the true

resistivity(RT) ,of the hydrocarbon bearing zones andto increase the value of Ro .

Quantitatively a non representative high (Sw

) value iscalculated if clean formation models are used .thepotential of the zone is then underestimated orcompletely masked .

High Vsh might encumber visual detection ofhydrocarbon zones .

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The interpretation problem in shaly formations is in calculatingporosity and saturation values free from shale effects.

Because the shale effect depends on the shale content ,the estimationof Vsh is of prime importance

Qualitatively Vsh indicates whether the formations can be

considered clean or shaly to determine model or approach that wewill use in interpretation

Quantitatively VSh is used to estimate the shale effect on logresponses and if needed ,to correct them to clean formation

responses.

Shale effect all logging tools to some degree ,there are numerousmethods have been developed to indicate presence and estimate shalecontent involving SP ,gamma ray and porosity logs

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1.2 shale content from SP Log Vsh from SP log can be calculated by

WhereEpsp is the shale response in the shaly zone of interest

ESSP is the shale response in an adjacent clean ,thick zone,contains same water salinity as the zone of interest

Restriction of this method :

1. SP is of good quality

2. Other shale indicators are absent

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Factors affect estimation of Vsh from SP log :

1) salinity changes

2) Rw/RMF contrast

3) Hydrocarbon content

N.B. The presence of hydrocarbon in a formationreduces the reading of the SP log which result in ahigher shale content

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1.3 Shale Content From the

Gamma Ray Log The shale volume is related to a shale index ,ISh :

Empirical relationships were used to determine VSh

For tertiary rocks ,the Larionov equation is used :

The Stieber equation is

The clavier et al equation is :

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For older rocks , the larionov Equation is :

An empirical equation can be developed specifically

for a formation or geologic unit of interest .

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