Diagenesis of the Central Luconia Carbonates : The Roles of Late and High Temperature Corrosive Fluids in Enhancing Reservoir Quality

Ali, Mohammad Y.

The Central Luconia Miocene carbonates have been known as one of the largest LNG producers in the world. However, their reservoir diagenesis is still poorly understood. A comprehensive diagenetic study has been conducted in integrated manner to investigate the cementation history, palaeo-diagenetic fluid systems, and the potential of late and high temperature corrosive fluids in enhancement of pores.

The results show that the reservoirs have undergone a complex diagenetic evolution. Six stages of calcite cementation (Cal-1 to Cal-6), four stages of dolomitization (Dol-1 to Dol-4), one stage of dedolomitization (Ded-1), three phases of dissolution, and several minor late burial events such as fluorite and anhydrite replacement, pyritization and kaolinite bridging have been recognized. Each stage is characterized by different crystal habit, cathodoluminescent characteristic, and isotopic signature indicating their precipitation took place at different temperatures and diagenetic environments. The early surface to shallow burial calcite (Cal-1 to Cal-4) and dolomite (Dol-1 and Dol-2) were mainly precipitated in phreatic and possible mixing water environments at relatively low temperatures (<50^oC). The late calcite (Cal-5 and Cal-6), dolomite (Dol-3 and Dol-4) and dedolomite (Ded-1) were precipitated at higher temperatures (85^o to 130^oC). The late stages of dolomite (Dol-3 and Dol-4) with narrow distribution of ∂18O (-5.29 %o to -6.03 %o PDB) and ∂13C (0.64 %o to -3.65 %o PDB) isotope values have been interpreted to have been precipitated from dolomitizing fluids that may associate with deep burial and hydrothermal conditions at homogenization temperature (Th) ranging from 125^o to 130^oC, while the melting temperature (Tm ice) ranging from -2.6^o to -3.3^oC, corresponding to a salinity of 4.34 to 5.4% weight NaCl equivalent. This interpretation is also supported by the presence of large saddle-like dolomite and high temperature minerals at the deeper part of the reservoirs.

It is concluded that pore dissolution partially took place during the subaerial exposure and emergence of the buildups, early dolomitization and stylolitization. The reservoir properties were further enhanced by hydrothermal related corrosive fluids precipitated at high temperatures (>130^oC) that possibly migrated upwards from deep-seated areas underneath prior to hydrocarbon migration.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013