Prospektivitas Reservoir Berdasarkan Indeks Kualitas Pada Formasi Belumai, Lapangan FR dan IZ, Cekungan Sumatra Utara

Farhan Izzat, Dr. Sarju Winardi, S.T., M.T.; Ir. Jarot Setyowiyoto, M.Sc., Ph.D. IPU, ASEAN Eng.

2026 | Skripsi | TEKNIK GEOLOGI

Formasi Belumai merupakan salah satu reservoir produktif dengan litologi yang heterogen di Lapangan FR dan IZ, Cekungan Sumatra Utara. Sejak ditemukan, beberapa lapangan pada cekungan ini telah mengalami penurunan produksi. Litologi yang heterogen dan penurunan produksi yang terjadi menjadi tantangan untuk dilakukan penelitian untuk mengetahui fasies litologi yang berkembang dan korelasi antarsumur, mengevaluasi properti petrofisika reservoir, serta mengidentifikasi interval reservoir prospektif berdasarkan Indeks Kualitas Reservoir (RQI), sehingga mendukung strategi pengembangan lapangan ke depan.

Data yang digunakan dalam penelitian meliputi wireline log, deskripsi batuan inti, sidewall core, serta data Routine Core Analysis (RCAL). Penentuan fasies litologi menggunakan metode Multi-Resolution Graph-Based Clustering (MRGC). Analisis petrofisika dilakukan dengan metode deterministik, di mana volume serpih dihitung dengan model non-linear Larionov Tertiary (1969), porositas dengan metode Bateman-Konnen (1977) melalui Diagram Volan, permeabilitas dari regresi linear data batuan inti, serta saturasi air menggunakan metode Indonesian Equation.

Berdasarkan hasil penelitian, diketahui bahwa Formasi Belumai tersusun oleh tiga fasies litologi: Batupasir, Batupasir Lempungan, dan Serpih, dengan fasies Batupasir dan Batupasir Lempungan berperan sebagai reservoir. Korelasi antarsumur menunjukkan bahwa Lapangan FR memiliki kualitas reservoir yang lebih baik dibandingkan Lapangan IZ, di mana kedua lapangan tersebut secara struktural dipisahkan oleh sebuah sesar. Perbedaan hasil ini dikontrol oleh jarak terhadap suplai sedimen asal selatan-tenggara, di mana Lapangan FR yang berada lebih dekat dengan suplai sedimen didominasi oleh perkembangan fasies channel bar dan amalgamated channel yang tebal. Sebaliknya, Lapangan IZ yang terletak lebih jauh ke arah barat laut mengalami perubahan fasies lateral menjadi lempungan dengan nilai volume serpih yang lebih tinggi. Berdasarkan nilai RQI dan kemenerusannya, reservoir Sand B diidentifikasi sebagai reservoir paling prospektif karena memiliki nilai RQI tertinggi dengan kategori fair hingga good, serta sebaran lateral yang konsisten tanpa interupsi zona impervious di kedua lapangan. 

The Belumai Formation is a productive reservoir characterized by its heterogenous lithology in the FR and IZ fields, North Sumatra Basin. Since its discovery, several fields within this basin have experienced a decline in production. To address the challenges of lithological heterogeneity and declining production, this study evaluates the developing lithofacies and well correlations, assesses reservoir petrophysical properties, and identifies prospective intervals using the Reservoir Quality Index (RQI) to support future field development strategies.

The data utilized in this study comprise wireline logs, core descriptions, sidewall cores, and Routine Core Analysis (RCAL) data. Lithofacies identification was performed using the Multi-Resolution Graph-Based Clustering (MRGC) method, validated by core data. Petrophysical analysis was conducted deterministically, where shale volume was calculated using the non-linear Larionov Tertiary (1969) model, porosity was determined via the Bateman-Konen (1977) method using the Volan Diagram, permeability was derived from core data regression, and water saturation was calculated using the Indonesian Equation.

Based on the results of the study, the Belumai Formation consists of three lithofacies: Sandstone, Argilliceous Sandstone, and Claystone, with the Sandstone and Argilliceous Sandstone facies serving as the primary reservoirs. Well correlation shows that the FR Field exhibits superior reservoir quality compared to the IZ Field, with the two fields structurally separated by a fault. This variation is controlled by the distance to the source of sediment originating from the south-southeast. The FR Field, located closer to the source, is dominated by the development of thick, homogeneous channel bar and amalgamated channel facies. Conversely, the IZ Field, situated further to the northwest, undergoes a lateral facies change characterized by higher shale volume values. Based on RQI values and reservoir continuity, the Sand B reservoir is identified as the most prospective zone due to its highest RQI values, ranging from fair to good categories, and a consistent lateral distribution without any impervious zone interruptions across both fields.

Kata Kunci : Reservoir Quality Index (RQI), Reservoir Petrophysics, Belumai Formation, Multi-Resolution Graph-Based Clustering (MRGC)