Diagenetic and Porosity Development and its Implication towards Hydrocarbon Reserves in Upper Cibulakan Formation, North West Java

Direct examination of rock samples is highly required for reservoir studies. The data are collected from Lower-Middle Miocene carbonates of the Upper Cibulakan Formation. The data were then analyzed through the thin section analysis, scanning electrone microscope (SEM), and core description. The formation is the part of the northwest Java Basin, specifically in Jatibarang sub-basin, extended in offshore and onshore land. The main objective of this study is the distribution of porosity, its development, diagenetic process, as well as their connection to hydrocarbon exploration. Carbonates in Upper Cibulakan Formation is characterized by its local occurrence and intercalation with marine shale sediments and dominated with dolomite minerals. The facies analysis identified the facies association consist of grainstone, wackestone-packstone, packstone-grainstone, and mudstone-wackestone. Type of porosity found in the carbonates are intraparticle, interparticle, vuggy, shelter, channel, and fracture. Fracture porosity dominated the porosity and contribute to the 11.3% of the average of porosity. On the other side, microporosity played a significant role in the reservoir and identified from almost the half of the analyzed core plug, and supply the number of 45.1% from the total of the analyzed core plug. Both of the facies and porosity characters concluded the carbonates diagenesis which separated into three regimes, from syndepositional phase, shallow phreatic phase, and burial. Dolomitization come off in shallow phreatic phase along with the starting of the forming of the hydrocarbon. The formation of hydrocarbon ended in burial phase. The data furthermore support the existence of carbonates reservoir in the formation.