Characteristics of Eocene Carbonate Reservoir – A Case Study of Ngimbang Carbonate in North Madura Platform, East Java Basin.

Exploration in the East Java Basin began in 1888 when first discovery was found in the Kuti field from the Lidah play. Fast forward to 2022, there has been a total of 143 discoveries accumulating to total resources of at least 2.2 Bbl oil and 13 TCF gas from more than 600 exploration wells drilled in the basin. More than half of the resources are contributed by the Kujung Play. As Kujung exploration matured, other exploration started to focus on the deeper plays such as the Eocene Ngimbang Carbonate, an underexplored play with sizeable potential. A newly drilled well in North Madura Platform well has succeeded to prove that Ngimbang carbonate contains excellent porosity and permeability as shown by the well test result and the total loss experienced during drilling. Cased hole logging results further proved the high porosity of the carbonate with values of up to 25-30%. Seismic analysis and well data analysis show that the Ngimbang carbonate experienced 3 stages of carbonate growth; Stage 1 – Initiation Phase, Stage 2 – Aggradation Phase, and Stage 3- Backstepping Phase. Based on the well log and petrography thin section analysis, the secondary porosity in Ngimbang carbonate was caused by intensive dissolution-karstification related to subaerial exposure and fracturing and dissolution due to the later diagenesis phase. The secondary porosity developed varies depending on the depositional environment setting and carbonate growth phase, with average of 10-15% porosity range within the shelf-edge to shelf depositional environment, and average of 20-25% (up to 35% max) on the distal isolated carbonate. The unique intense karstification in the isolated carbonate was contributed by tectonic subsidence rate variation which fluctuated the accommodation space formed, and global sea level falls and eustatic fluctuations during the Eocene-Oligocene periods which further promoted major subaerial exposures and contributed to the episodic karstification and secondary porosity generation.