Petrographic Analysis and Facies Interpretation of The Mundu Formation Carbonates: Implications For Reservoir Potential

Carbonate reservoirs account for more than half of the world's hydrocarbon reserves; however, their quality is highly heterogeneous and is influenced by petrographic characteristics.The Mundu Formation in East Java represents one of the important carbonate units in the East Java Basin petroleum system.This study investigates the petrographic characteristics of limestone from the Mundu Formation in Gunung Pegat, East Java, to evaluate its carbonate reservoir quality.Fieldwork included geological mapping, stratigraphic profiling, and rock sampling, followed by petrographic analysis of seven thin sections (LP5, LP7, LP11, LP14, LP16, LP17, LP19).Thin sections were stained with Methylene Blue and examined under Plane Polarized Light (PPL), Cross Polarized Light (XPL), and a -530 gypsum plate compensator.The results reveal three main lithological types-wackestone, packstone, and grainstone-with porosity values ranging from 12% (fair) to 47% (excellent), as reported by Koesoemadinata (1980).Facies interpretation indicates deposition in inner and outer back-reef lagoon environments, with the highest reservoir quality observed in high-porosity grainstone.Additional petrophysical analysis on 14 samples reveals permeability values ranging from 42.119 to 361.086 mD, categorized as good to very good.These findings emphasize that lithological variability, depositional facies, and diagenetic processes are the key controls on carbonate reservoir quality in the Mundu Formation.The study provides valuable insights for carbonate reservoir modeling and hydrocarbon exploration in East Java.