Diagenesis of the Central Luconia Carbonate Platforms: The Roles of Early Dolomitization and Late Hydrothermal Fluids in Enhancing Deep Reservoir Properties

Abstract

The Central Luconia Miocene carbonate platform represents one of the largest regions of Liquified Natural Gas (LNG) production in the world. Although several studies have been conducted, the reservoir diagenesis of this gas-producing region remains poorly understood. To address this issue, a comprehensive and systematic diagenetic study has now been undertaken. Methodologies used included petrography, X-ray diffractometry (XRD), scanning electron microscopy (SEM), backscattered electron microscopy (BSEM), and cathodoluminescent microscopy (CL). Other technologies included elemental analysis using electron probe microanalyzer (EPMA), fluid inclusion microthermometry (FIM), and stable C, O, S, and Sr isotope analyses. The resulting datasets have been integrated so that the paleodiagenetic fluid flow, cementation history, and potential late-stage high-temperature hydrothermal corrosive fluids can be assessed with respect to the effect on reservoir potential.The results show that the reservoirs have undergone a complex diagenetic evolution over time. Six stages of calcite cementation (Cal-1 to Cal-6), four stages of dolomitization (Dol-1 to Dol-4), and one stage of dedolomitization (Ded-1) have occurred. Three phases of major dissolution and several minor late burial diagenetic events, such as fluorite and anhydrite replacement, pyritization, and kaolinite bridging have also been recognized. Each stage is characterized by different crystal habits, cathodoluminescent characteristics, elemental compositions, and isotopic signatures, indicating their precipitation took place at different temperatures and diagenetic environments. The early surface to shallow burial calcites (Cal-1 to Cal-4) and dolomites (Dol-1 to Dol-2) were mainly precipitated in marine, phreatic, and possible mixing water environments at relatively low temperatures (<50° C). The late calcites (Cal-5 and Cal-6), dolomites (Dol-3 and Dol-4), and dedolomite (Ded-1) were precipitated at higher temperatures (85–130° C). The late stages of dolomite (Dol-3 and Dol-4) have narrow distribution of δ18O[−5.29 to −6.03‰ Peedee Belemnite (PDB) scale], and δ13C (0.64 to −3.65‰ PDB) isotope values have been interpreted as precipitating from dolomitizing fluid that may be associated with deep burial and hydrothermal conditions. Fluid inclusion homogenization temperatures (Th) range from 125° to 130° C, and the melting temperatures of ice (Tm) range from −2.60° to −3.30° C, corresponding to a salinity of 4.34 to 5.41% weight NaCl equivalent. This interpretation also is consistent with the presence of large saddlelike dolomite and high-temperature minerals in the deeper part of the reservoirs.Three main phases of dissolution that enhanced the porosity occurred during the subaerial exposure of the platforms. The reservoir properties were enhanced further by early dolomitization, followed by hydrothermal-related corrosive fluids at high temperatures (>130° C) that possibly migrated upward from deep-seated areas underneath the reservoir via faults prior to hydrocarbon migration. This late diagenetic fluid flow was constrained by porous and nonporous layers formed during deposition and early diagenesis. These fluids created high porosity (up to 40%) and permeability (exceeding 1000 mD) within the hydrocarbon reservoirs.