Publications

Limestone is one of the viable reservoir options available today. Under favorable conditions, limestone reservoirs can exhibit primary porosity, which may be further enhanced by secondary porosity through dissolution or fractures, thereby increasing both porosity and permeability. However, in certain cases, deep burial, deposition, and diagenesis can result in extremely tight limestone geoscientists to rely on secondary porosity to asses reservoir potential. This study aims to evaluate the reservoir properties including shale volume, porosity, permeability, and petrophysical rock type (PRT) of the tight limestone in the upper Baturaja Formation within the “Kirai” field of the South Sumatra basin, a proven hydrocarbon-producing area. A petrophysical evaluation was conducted using an integrated dataset, incorporating well logs, conventional core, petrography, and routine core analysis. The analysis result indicates that the upper Baturaja Formation limestone consists of five distinct rock types (RT-1 to RT-5) classified using the Pore Geometry Structure (PGS) method. RT-1 represents the best reservoir quality, characterized by fractured and vuggy Wackestone/floatstone, with shale volume (Vsh) ranging from 1% to 14% (average: 6%); porosity from 2% to 13% (average: 9%); and permeability from 23 mD to 207 mD (average: 66 mD). RT-2 consists of matrix pores and mouldic floatstone. RT-3 exhibits poor matrix porosity and comprises mouldic wackestone/floatstone. RT-4 consists of tight wackestone/floatstone and/or clay-laminated layers. RT-5 is characterized by tight or clay-rich dispersed wackestone/floatstone. The reservoir quality in this field is expected to improve locally, particularly in the area where open fractures are connected to dissolution pores, including vuggy and mouldic porosity systems.