Litofasies dan Lingkungan Pengendapan Formasi Kujung, Blok ‘GMF’ Lepas Pantai, Cekungan Jawa Timur Utara

Gusti Muhammad Fiqry, Prof. Dr. Ir. Sugeng Sapto Surjono, S.T., M.T., IPU., ASEAN.Eng.

2026 | Skripsi | TEKNIK GEOLOGI

Formasi Kujung merupakan interval karbonat penting pada Cekungan Jawa Timur Utara karena berpotensi sebagai reservoir hidrokarbon. Namun, heterogenitas karbonat menyebabkan persebaran fasies dan lingkungan pengendapannya sulit dikenali secara lateral maupun vertikal. Penelitian ini bertujuan mengidentifikasi litofasies, asosiasi fasies, lingkungan pengendapan, dan sikuen stratigrafi Formasi Kujung pada Blok “GMF”, lepas pantai Cekungan Jawa Timur Utara. Data penelitian meliputi batuan inti, sayatan tipis, sidewall core, mud log, biostratigrafi, seismik, dan wireline log dari tujuh sumur. Metode yang digunakan mencakup interpretasi litofasies, estimasi litofasies pada interval tidak berinti menggunakan Multi Resolution Graph-Based Clustering atau MRGC, validasi elektrofasies, analisis asosiasi fasies, interpretasi lingkungan pengendapan, dan analisis sikuen stratigrafi. Penelitian ini dibatasi pada Kujung Unit 1 dan Kujung Unit 2 sebagai interval utama dalam korelasi stratigrafi dan analisis fasies. Hasil penelitian menunjukkan bahwa Formasi Kujung tersusun atas fasies karbonat dan nonkarbonat yang dikelompokkan menjadi enam asosiasi fasies, yaitu Carbonate Buildup, Slope Debris-Reworked Carbonate, Toe-of-Slope Carbonate, Lower Slope Transitional, Shale-Influenced Toe-of-Slope, dan Upper Slope Facies shale influenced Pada kedua interval utama tersebut, Kujung Unit 1 didominasi karbonat primer sebesar 85,7%, sedangkan Kujung Unit 2 didominasi litologi nonkarbonat halus, lempungan, terigenik, dan organik sebesar 60,4%, dengan penurunan karbonat primer menjadi 34,4%. Integrasi data fasies, elektrofasies, biostratigrafi, dan seismik menunjukkan bahwa Formasi Kujung berkembang pada sistem Lingkungan pengendapan structurally Controlled Rimmed Carbonate Platform. Analisis sikuen stratigrafi menunjukkan adanya transgressive system tract atau TST dan highstand system tract atau HST yang dikontrol oleh paleotopografi basement, ruang akomodasi, pertumbuhan karbonat lokal, dan redeposisi material karbonat.

The Kujung Formation is an important carbonate interval in the North East Java Basin because of its potential as a hydrocarbon reservoir. However, carbonate heterogeneity makes the lateral and vertical distribution of facies and depositional environments difficult to identify. This study aims to identify the lithofacies, facies associations, depositional environments, and sequence stratigraphy of the Kujung Formation in the “GMF” Block, offshore North East Java Basin. The data used in this study include core samples, thin sections, sidewall cores, mud logs, biostratigraphic data, seismic data, and wireline logs from seven wells. The methods include lithofacies interpretation, lithofacies estimation in non-cored intervals using Multi Resolution Graph-Based Clustering or MRGC, electrofacies validation, facies association analysis, depositional environment interpretation, and sequence stratigraphic analysis. This study is limited to Kujung Unit 1 and Kujung Unit 2 as the main intervals for stratigraphic correlation and facies analysis. The results show that the Kujung Formation consists of carbonate and non-carbonate facies, which are grouped into six facies associations: Carbonate Buildup, Slope Debris-Reworked Carbonate, Toe-of-Slope Carbonate, Lower Slope Transitional, Shale-Influenced Toe-of-Slope, and Upper Slope Facies shale influenced. Within these two main intervals, Kujung Unit 1 is dominated by primary carbonate, accounting for 85.7%, whereas Kujung Unit 2 is dominated by fine-grained non-carbonate, argillaceous, terrigenous, and organic-rich lithologies, accounting for 60.4%, with the proportion of primary carbonate decreasing to 34.4%. The integration of facies, electrofacies, biostratigraphic, and seismic data indicates that the Kujung Formation developed within a structurally Controlled rimmed carbonate platform depositional enviroment. Sequence stratigraphic analysis indicates the presence of a transgressive system tract or TST and a highstand system tract or HST, controlled by basement paleotopography, accommodation space, local carbonate growth, and redeposition of carbonate materials.

Kata Kunci : Karbonat, Formasi Kujung, litofasies, MRGC, rimmed carbonate platform, sikuen stratigrafi