Sedimentological Control on Reservoir Properties in a Miocene Isolated Carbonate Build‐Up in Central Luconia, Malaysia

ABSTRACT

Reservoir characterization in carbonates has long been a divisive topic between geologists, petrophysicists and reservoir engineers. Permeability prediction in carbonate rocks is usually treated as a petrophysical black box due to the complexity of carbonate reservoirs. This study describes the sedimentology of cores from a well penetrating Miocene carbonate from Central Luconia, offshore Sarawak, Malaysia, and characterizes the petrophysical qualities of the carbonate reservoir using core‐derived sedimentological data. The study also constructs an integrated sedimentology‐based petrophysical rock type classification scheme for carbonate reservoir characterization. This involves integration of core and petrographic description to provide geological context to routine core analysis (RCA) and mercury injection capillary pressure (MICP) data. The results can be applied in the modelling stage to allow sedimentological and diagenetic information to be considered independently. Eleven facies are identified based primarily on biota distribution and rock fabric. The resulting facies form elements of three facies associations, namely, lagoon, reef and slope. Five rock types (RT1 to RT5) are identified on the basis of the cumulative frequency distribution of calculated flow zone indicators (FZI). High quality reservoirs (RT1, RT2 and RT3) in all three facies associations are dominated by floatstone facies with grainstone or packstone matrix, framestone facies as well as packstone facies. Reservoir properties were found to be widely distributed even within individual facies associations due to several factors, particularly the primary depositional fabric and the presence of secondary processes. The similar reservoir quality observed in coraline floatstone with skeletal algal packstone facies, algal packstone and skeletal packstone facies clearly suggests matrix control on reservoir quality in coarser‐grained deposits. The distribution of good reservoir properties is due to the presence of primary porosity and significant secondary porosity in floatstone facies. The significantly lower reservoir quality of skeletal packstone is due to the muddy matrix, which reduced the primary porosity and significant cementation that occurred afterwards.