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The East Java Basin stands as a prolific Tertiary Basin, contributing to the production of oil and gas up to 4 BBOE. This mature basin necessitates inventive strategies to unlock its remaining potential, the emphasis lies on leveraging new technology and fostering innovative ideas. This includes enhanced imaging techniques for investigating deeper targets, new perspectives on petroleum plays and geological models, as well as modern engineering solutions tailored to navigate complex subsurface drilling challenges.

This paper presents a study case from quite recent exploration discovery located in the North Madura Platform, offshore East Java Basin. In 2021, the X-1 well was drilled and targeted at the emerging Late Eocene Ngimbang Carbonate Play. The well result proved the presence of the predicted play type of an isolated carbonate build-up and encountered better reservoir quality as compared to surrounding wells within the same play.

Navigating a spectrum of drilling and operational challenges, commencing from the preparatory stages through execution, the technical intricacies were further intensified when drilling into the shallow carbonate and unconsolidated formation, thick shale followed by massive carbonate of the reservoir target. In the face of these challenges, three casing sections were strategically set. The upper section drilled riser-less was meant to encounter unconsolidated formation and losses of Lidah and Wonocolo (or Kawengan) respectively and drilled to Ngrayong Formation. The intermediate section, thick and time-sensitive shale formations of Tuban, Rancak to Kujung Formation, necessitated the application of a High Performance Water Based Mud (HPWBM) to ensure wellbore stability and efficient progress. Drilling a thick and massive carbonate of Ngimbang Carbonate in the last hole section, demanded a proactive approach. Obviously, the challenging total losses occurred once we penetrated the target and drilling continued through the implementation of the Pressurized Mud Cap Drilling (PMCD) mode without having to spot cement plug to combat the losses that can damage the reservoir. Severe karstification was observed from drilling as well as from logs and well-test data. Furthermore, the well stands as the first well to encounter total losses in deep Ngimbang Carbonate Play in the region.

Exploring mature basins, especially in Western Indonesia in this particular context, will offer advantages in terms of data availability and proven petroleum systems. Although the remaining potential resources often require further enhancement of its data and understanding. The recent oil discoveries in the emerging play within the East Java Basin

stand as a testament to success and have the potential to promote exploration initiatives across various mature basins in Indonesia. This discovery was also important to support the Government of Indonesia’s production target of 1 MBOPD and 12 BSCFPD by 2030.