New Classification of Carbonate Rocks for Reservoir Characterization

Abstract

ABSTRACT Several classifications of carbonate rocks are known to the oil industry. The most recent one is the Dunham Classification, 1962, more than 33 years ago. In these classifications, several descriptive names were used to identify different types of Limestones which have created a lot of confusion specially for non-geologists. Rock typing is one of the main challenges in reservoir characterization. Defining porosity and permeability finger prints for each rock type is a subject of increasing interest in reservoir geology. This will help defining permeability distribution, better understanding of fluid flow behavior which will lead to more realistic • production forecast. Unfortunately, previous classifications of carbonate rocks are not enough to satisfy this objective since conventional rock types have wide range of porosity and permeability. Detailed core description and petrographic studies on the Thamama and Arab Carbonate reservoirs of Abu Dhabi have significantly contributed to the internal architecture, the building blocks and other parameters used in carbonate rock classification. Scanning Electron Microscope, Nuclear Magnetic Resonance and Mercury Injection Capillary Pressure were used to differentiate between the different rock types. The main objective of this work is to find out a practical classification of carbonate rocks to be used in reservoir characterization and to match with the petrophysical and dynamic data. The building blocks of carbonate rocks used in this study are the micrite crystals, the micrite particles and the carbonate grains. The Micrite particle is a new component introduced by the authors to the building blocks of carbonate rocks. It represents an aggregate of micrite crystals ranging in size between 10–20 microns which create inter-particle porosity of approximately 4 microns. This new finding has explained the major deflection on the Ø/K cross plots of the Thamama reservoirs of Abu Dhabi. The new classification proposed in this work is mainly based on pore throat size distribution. The other components used in this classification are the micrite particles, grain/micrite ratio and core porosity and permeability. The proposed classification has explained several unique cases related to mis-matching among descriptive rock types, measured petrophysical data and dynamic data. Significant variation in permeability at constant porosity and the changing capillary pressure and relative permeability behavior for one and the same rock type are good examples.