Porosity cut-off is a major key parameter in evaluation of reservoir dimensions. Through the use of this parameter formation rocks regarded as ‘reservoir(s)’ are distinguished from ‘non-reservoir’ ones. This makes determination of this parameter to have a direct impact on estimations of reservoir volumes and in turn volumes of hydrocarbon accumulation. Error in the determination may influence commerciality of the hydrocarbon accumulation. Traditionally, determination of porosity cut-off values is made through relationship between rock porosity and permeability. Although some suggested the use of supporting data such as clay volume and porosity types or computational methods such as artificial neural network, this method is often regarded as unsatisfactory due to different intrinsic features between the two. Relationship between rock pore throat size – obtained from mercury injection on core samples – and permeability is on the other hand more direct in nature. This study actually attempts to integrate this more theoretically consistent relationship into the old porosity – permeability relationship. The integration is essentially made through multi-variable regression analysis using data from eight (4 sandstone and 4 limestone) oil and gas fields in Indonesia. Results from the study have shown significant improvement in porosity – permeability relationship resulting in more reliable porosity cut-off estimates. The method, which procedure is presented in this article is hoped to have helped petrophysicists in reducing and overcoming high uncertainties in estimating porosity cutoff.

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