The M-1x exploration well has successfully identified substantial hydrocarbon discoveries within the Sepinggan Deltaic Sequences (SDS) and Sepinggan Carbonate Sequences (SCS). However, investigating siliciclastic reservoirs in the shallow part of the SCS in the Mahakam PSC has been hindered by challenges associated with carbonate intervals. In this study, recent seismic reprocessing data is utilized to enhance structural predictions and reservoir characterizations, providing valuable insights for future well development.

A comprehensive suite of logging acquisitions, including pressure-fluid analysis and drill stem tests (DST), was implemented to validate the properties and extent of a mixed siliciclastic-carbonate reservoir system. Although carbonate reefs and platforms dominate the interval, this study primarily focuses on the siliciclastic layers, comprising stacked sandbars occasionally interspersed with channel formations across the shelf. Utilizing recent seismic data and attributes such as variance, impedance, and spectral decomposition, an improved understanding of horizon interpretations and fault properties, encompassing their geometry, throw, and extension, was achieved. This integrated approach, combining structural understanding, sedimentological context, and seismic attribute analysis, provides a comprehensive perspective on the distribution of siliciclastic reservoirs bounded by faults and carbonate reefs.

A detailed 3D grid geological model was constructed to visualize the reservoir distribution while considering fault geometry. This model facilitates the estimation of hydrocarbon in place and enables the identification of the influence of structural components on facies patterns and reservoir distributions. Uncertainty analysis was performed on parameters including alternative horizon interpretations, facies distribution, porosity, water saturation, and fluid contacts. Over 10 wells were meticulously selected for the development plan, utilizing a combination of static geological modeling and dynamic reservoir simulation, with the primary aim of optimizing the understanding of reservoir characteristics and achieving optimal well placement.

In conclusion, this study provides a comprehensive investigation into the hydrocarbon potential of mixed siliciclastic and carbonate environments in the South Mahakam area. The enhanced understanding of structural and sedimentological interpretations of reservoir characteristics and distribution will significantly contribute to uncertainty assessment during field development and the exploration of new areas for prospects, both within and beyond the current production field.

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