Karakterisasi Geometri dan Distribusi Lateral Carbonate Build-Up Berdasarkan Analisis Fasies Seismik Pada Formasi Kujung Unit I, Cekungan Jawa Timur Utara

Yasmine Aurora Sabilla Nada, Ir. Rahmadi Hidayat, S.T., M.Eng., Ph.D., IPP.; Ir. Salahuddin Husein, S.T., M.Sc., Ph.D., IPM.

2026 | Skripsi | TEKNIK GEOLOGI

Heterogenitas batuan karbonat menyebabkan geometri tubuh karbonat Formasi Kujung Unit I berkembang secara kompleks sehingga penentuan batas dan geometri carbonate build-up masih menyisakan ketidakpastian. Penelitian ini bertujuan mengidentifikasi karakter refleksi seismik horizon top dan bottom Formasi Kujung Unit I serta menganalisis geometri dan distribusi lateral carbonate build-up di Cekungan Jawa Timur Utara. Data yang digunakan meliputi data seismik 3D sebagai data utama, dua lintasan seismik 2D sebagai kontrol korelasi, serta log sumur, well report, dan deskripsi cutting Sumur LA sebagai kontrol stratigrafi. Interval Formasi Kujung Unit I ditetapkan pada kedalaman 5.574 hingga 6.620 ft MDRT, kemudian diproyeksikan ke domain waktu melalui well to seismic tie sebelum penarikan horizon dilakukan. Hasil penelitian menunjukkan bahwa horizon top dicirikan oleh reflektor beramplitudo tinggi yang memisahkan konfigurasi mounded hingga chaotic di bawahnya dari refleksi paralel di atasnya, sedangkan horizon bottom dicirikan oleh reflektor beramplitudo sedang hingga tinggi yang memisahkan refleksi paralel hingga subparalel Formasi Kujung Unit II. Teridentifikasi empat fasies seismik, yaitu patch reef, pinnacle reef, carbonate progradation slope, dan carbonate inter-reef deposits, serta 32 geometri carbonate build-up dengan diameter 236 hingga 1.097 m, luas 0,045 hingga 0,935 km², dan ketebalan 1.197 hingga 1.307 ft. Sebaran tubuh karbonat terkonsentrasi pada bagian tengah, timur, dan utara daerah penelitian dan dikendalikan oleh topografi purba di atas Tuban Trough, variasi ruang akomodasi, serta proses backstepping.

The high heterogeneity of carbonate rocks causes the geometry of the Kujung Formation Unit I carbonate bodies to develop in a complex manner, so that the delineation of carbonate build-up boundaries and geometry still carries uncertainty. This research aims to identify the seismic reflection character of the top and bottom horizons of the Kujung Formation Unit I and to analyse the geometry and lateral distribution of the carbonate build-ups in the North East Java Basin. The data used comprise 3D seismic data as the primary dataset, two 2D seismic lines as correlation control, and well logs, a well report, and cutting descriptions from Well LA as stratigraphic control. The Kujung Formation Unit I interval was defined at a depth of 5,574 to 6,620 ft MDRT and subsequently projected into the time domain through a well to seismic tie before horizon picking was carried out. The results show that the top horizon is characterised by a high amplitude reflector separating an underlying mounded to chaotic configuration from overlying parallel reflections, whereas the bottom horizon is characterised by a moderate to high amplitude reflector separating the underlying parallel to subparallel reflections of the Kujung Formation Unit II. Four seismic facies were identified, namely patch reef, pinnacle reef, carbonate progradation slope, and carbonate inter-reef deposits, together with 32 carbonate build-up geometries with diameters of 236 to 1,097 m, areas of 0.045 to 0.935 km², and thicknesses of 1,197 to 1,307 ft. The carbonate bodies are concentrated in the central, eastern, and northern parts of the study area and are controlled by the antecedent topography above the Tuban Trough, variations in accommodation space, and backstepping processes.

Kata Kunci : carbonate build-up, fasies seismik, Formasi Kujung Unit I, geometri karbonat, Cekungan Jawa Timur Utara.