Classification of Microporosity in Carbonates: Examples from Miocene Carbonate Reservoirs of Central Luconia, Offshore Sarawak, Malaysia
Authors: Rahman Md Habibur , Pierson Bernard J. , Wan Yusoff Wan Ismail 
Published: May 2016

Abstract
Miocene carbonate reservoirs may contain appreciable amount of microporosity in grain, cement or matrix. Depending on the crystallometry and morphometry of micrite particles, microporosity can vary both in amount and in nature. Different kinds of micrite microtextures may produce different types of microporosity. This study proposes a classification of micrite microtextures and microporosity. Four different types of micrite microtextures are classified in this study; these are rounded micrites, subrounded micrites, microrhombic and polyhedral micrites, and compact anhedral micrites. Four different types of micropores are classified based on their pore sizes. These are very fine, fine, medium, and coarse micropores. The classification of microporosity allows its occurrence in carbonate reservoirs to be explained depending on micrite microtextures. The classification of micropores in terms of their micrite microtextural occurrence lead us to define the micropores that are contributing to the permeability of the reservoirs and allow fluid flow in carbonates. The classes of micropores together with the corresponding micrite microtextures of their occurrences help to define the potential reservoir rocks in terms of micrite microtextures.

Introduction
The complexity in pore systems of carbonates is mostly due to the biological origin of carbonate sediments and their chemical reactivity. Since more than half of the earth's hydrocarbon reserves are contained within pore systems in carbonate reservoirs, studying these pore systems has always been an integral part of hydrocarbon exploration. Large uncertainties in petrophysical properties of carbonates are due to wide variations in pore type, pore shape, and interconnectivity (Knackstedt et al., 2008).
Porosity in carbonate rocks can be divided into two main pore systems: macropore, and micropore system. These pore systems are divided on the basis of the pore size and their relative contribution to fluid flow in the rock. Micropores are important in that they can provide a pathway connecting microporous grains to one another and to macropores (Cantrell and Hagerty, 1999). Different definitions of microporosity have been proposed and applied in the literature (Figure 1). For our study we defined micropores as all pores that are less than 10 microns in maximum diameter. Microporosity can occur in grain, matrix or cement of carbonate reservoirs. The most widely used classifications of porosity in carbonates are those of Choquette and Pray (1970) and Arve Lønøy (2006). Choquette and Pray (1970) classified pores according to the fabric selectivity of the sediments, whereas Arve Lønøy (2006) classification is texturally derived mostly from Choquette and Pray (1970) and incorporates pore-size differentiation. A classification scheme for micropores that can relate to the abundance, occurrence, and distribution of microporosity, could provide a framework for better understanding of flow-related properties of microporous Miocene carbonate reservoirs of offshore Sarawak. The study of microtextures of micrites is a key factor for understanding the abundance, occurrence, and distribution of microporosity in carbonates.