The Tangguh Field in eastern Indonesia is a giant gas field complex having a total gas in place of more than 20 tcf. The gas accumulation is distributed across seven (7) discovered fields namely Vorwata, Wiriagar Deep, Roabiba, Ofaweri, Kepe-Kepe, Wos and the Ubadari Field. As the global pursuit of decarbonization intensifies, bp has developed a Carbon Capture, Utilization, and Storage (CCUS) strategy in Tangguh to mitigate the release of carbon dioxide (CO₂) emissions and to support incremental gas production. Tangguh produced gas contains up to 14% CO₂. The CCUS initiative involves injecting the produced CO₂ back into the reservoir using the Tangguh Enhanced Gas Recovery (EGR) programme.

Of paramount importance to the successful long-term storage of CO₂ in Tangguh (and also across the globe) is the integrity of the geological caprock sealing the reservoir storage container, that safeguards the long-term storage of CO₂ against potential leakage. This study presents a detailed investigation into the integrity of the geological caprock at Tangguh, with a focus on Tangguh Main reservoirs (re: Jurassic Sandstone) which will be targeted as injected reservoirs. This is to enhance the security and viability of large-scale carbon storage initiatives. In Tangguh, the main caprocks are late Jurassic marine shales. These vary laterally: The facies change from East to West, and this also includes variation in the mineralogy (carbonate mineral content). This facies and mineralogy variation impacts the properties of the shale and therefore the caprock’s capacity and integrity.

Understanding the seal requires integration of a range of different datasets. bp and its partners have invested significant time and effort to deliver an integrated assessment of the caprock quality and integrity to support it carbon capture, sequestration, burial and EGR plans. This multi-faceted approach investigated the quality of these caprocks by integration of top-down and bottom-up analyses. Top-down analyses included fluid geochemistry and lab analysis to understand the capillary entry pressure, grain size analysis (Laser Particle Size Analysis), mineralogy content (XRD), stratigraphic correlation, and pore-pressure modelling. Investigations of the fluid geochemistry and mud gas were undertaken to determine the column height capacity of surrounding fields. Biostratigraphy plays an important role in the correlation of the laterally varying shale facies.

The top-down data mentioned above are the key inputs to and support the calibration of the basin model. The basin model was critical to the understanding of the burial history and its implication for the caprock behaviour through geological time. The basin model helped to visualize the pressure history and permeability variation through time. It also helped visualize the structural history of the basin, and the variation in structural relief and tilting through time.

The seal capacity of the late Jurassic caprock in the Bintuni basin has evolved through time due to burial induced compaction, changes in effective stress and diagenesis. There is evidence for vertical migration through the caprock in the geological past; however, at present day conditions, even the lowest quality caprock in Tangguh can support a vertical dry gas column of at least 1,200 ft. This study has given significant confidence that the caprock will form an effective seal for the long-term storage of CO₂ in the subsurface.

Keywords: Tangguh, Jurassic, Gas Field, Seal Capacity, Basin Model, CO₂, Geochemistry, CCUS, EGR, sequestration