Rock Type Identification Based on the Critical Porosity Value in Sandstone; Case Study from Kutai Basin
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Abstract
The quality of rock type is highly influenced by detail pore geometry of rock. However, detail pore geometry is influenced by rock texture such as grain size, grain sorting, and degree of angularity. These factors cause variation in the quality of rock. Every rock types have similar pore geometry and pore structure, thus it can be assumed that each sample data in a single rock type have similar packing and critical porosity values. In this paper, the pore geometry, pore structure and critical porosity were used for rock grouping.
The concept of pore geometry and pore structure which refer to Wibowo and Permadi (2013) and the concept of critical porosity which refer to Nur (1995) were used to identify the rock type. A mathematical relationship in the form of power law model between pore geometry and pore structure (Wibowo and Permadi, 2013) is used to describe the element of rock volume which have similar characteristics of the fluid flow and namedas rock type. Nur et al. (1995) stated that critical porosity value is influenced by both lithology and internal characteristics of rock. The critical porosity herein is defined as the value of porosity that border the sedimentary rock and suspesion.Therefore, it can be shown that good quality of rock type will have greater range of critical porosity values.
About 314 sandstone samples from Kutai Basin were used for this study. The physical properties of rocks were measured in the laboratory, including porosity, permeability, P and S wave velocity (Vp and Vs). It can be shown that each rock type which is characterized by similar of microscopic geological feature have a different range of critical porosity values. The result of rock grouping using critical porosity values is not significantly different from the result which is identified using rock type chart that refer to Wibowo and Permadi (2013). Based on the value of the critical porosity, rock samples can be grouped properly for each rock type.