Lingkungan Pengendapan dan Sikuen Stratigrafi Formasi Menggala, Lapangan Sakai, Cekungan Sumatera Tengah

SERULI EKA UTAMA, Ir. Jarot Setyowiyoto, M.Sc., Ph.D., IPU, ASEAN Eng.

2021 | Skripsi | S1 TEKNIK GEOLOGI

Lapangan Sakai merupakan lapangan minyak yang terletak pada Sub-cekungan Rangau, Cekungan Sumatera Tengah. Lapangan ini dikategorikan sebagai lapangan tua. Produksi minyak bumi lapangan ini terus mengalami penurunan secara alamiah. Upaya pengembangan lapangan diperlukan untuk mengoptimalisasikan produksi cadangan minyak bumi yang tersisa. Penelitian ini merupakan studi awal dalam melakukan pengembangan lapangan ini. Penelitian ini bertujuan untuk menentukan fasies, lingkungan pengendapan, dan menerapkan sikuen stratigrafi untuk mengetahui distribusi fasies penyusun Formasi Menggala yang berperan sebagai reservoar utama Lapangan Sakai. Data yang digunakan pada penelitian ini berupa batuan inti dari 1 sumur, log sumur dari 13 sumur, dan laporan biostratigrafi. Analisis batuan inti dilakukan mengetahui litofasies. Hasil analisis batuan inti dengan didukung laporan biostratigrafi menjadi dasar dalam menginterpretasi lingkungan pengendapan. Litofasies-litofasies yang memiliki keterkaitan secara genetis dikelompokkan menjadi asosiasi fasies yang mewakili suatu sub-lingkungan pengendapan. Integrasi hasil analisis batuan inti dan data log sumur dalam analisis elektrofasies dilakukan untuk mengetahui asosiasi fasies pada sumur yang tidak memiliki data batuan inti. Distribusi asosiasi fasies diketahui dengan melakukan korelasi bawah permukaan dengan prinsip sikuen stratigrafi. Hasil analisis batuan inti menunjukkan Formasi Menggala tersusun oleh litofasies ccSS, cbcSS, nvSS, dan ncvSS yang terendapkan pada lingkungan sungai teranyam dan secara vertikal mengalami perubahan menjadi litofasies wlvSS, cfSS, ncSS, bhSLSS, nccSS, dan bSL yang terendapkan pada lingkungan estuari pasang surut. Berdasarkan hasil analisis elektrofasies Formasi Menggala tersusun oleh asosiasi fasies braided channel, crevasse splay, tidal channel, tidal bar, dan tidal flat. Suksesi sedimen Formasi Menggala dapat dibagi menjadi dua system tract, yaitu lowstand system tract 1 (LST 1) dan transgressive system tract 1 (TST 1). LST 1 didominasi oleh braided channel, yang mengalami perubahan menjadi crevasse splay, tidal channel dan tidal flat pada bagian tenggara Lapangan Sakai. TST 1 didominasi oleh tidal channel yang secara vertikal mengalami perubahan menjadi tidal flat dan tidal bar.

Sakai field is located in Rangau Sub-basin, Central Sumatera Basin. This field has been classified as marginal field. Oil production from this field continuous naturally decline. Some approaches to field development are required to optimize the production of the remaining reserves. This research is preliminary study to develop this field. Objectives of this research are to determine facies, depositional environment, and applying sequence stratigraphic concepts to determine facies distribution of the Menggala Formation which is the main reservoir of Sakai Field. The data that have been used for this research are core from 1 well, well log from 13 wells, and biostratigraphic reports. Core analysis was performed to determine lithofacies. Core analysis results supported by biostratigraphic reports became the basis for interpreting depositional environment. Genetically related lithofacies are grouped into facies associations which represent a specific depositional environment. Integration of core analysis results and well log data in the electrofacies analysis was carried out to determine facies associations in wells that do not have core. Facies association distribution was known by performed subsurface correlation with sequence stratigraphic framework. Core analysis shows that Menggala Formation is composed of lithofacies ccSS, cbcSS, nvSS, and ncvSS which are deposited in the braided river and vertically have changed into lithofacies wlvSS, cfSS, ncSS, bhSLSS, nccSS, and bSL which are deposited in the tide dominated estuary. Five facies associations were identified based on electrofacies analysis: braided channel, crevasse splay, tidal channel, tidal bar, dan tidal flat. Menggala Formation sediment succession can be divided into two system tracts, LST 1 and TST 1. LST 1 is dominated by braided channel deposits, which changed into crevasse splay, tidal channel, and tidal flat deposits in southeast flank of Sakai field. TST 1 is dominated by tidal channel deposits, which vertically alternates into tidal flat and tidal bar deposits. The data that have been used for this research are core from 1 well, well log from 13 wells, and biostratigraphic reports. Core analysis was performed to determine lithofacies. Core analysis results supported by biostratigraphic reports became the basis for interpreting depositional environment. Genetically related lithofacies are grouped into facies associations which represent a specific depositional environment. Integration of core analysis results and well log data in the electrofacies analysis was carried out to determine facies associations in wells that do not have core. Facies association distribution was known by performed subsurface correlation with sequence stratigraphic framework. Core analysis shows that Menggala Formation is composed of lithofacies ccSS, cbcSS, nvSS, and ncvSS which are deposited in the braided river and vertically have changed into lithofacies wlvSS, cfSS, ncSS, bhSLSS, nccSS, and bSL which are deposited in the tide dominated estuary. Five facies associations were identified based on electrofacies analysis: braided channel, crevasse splay, tidal channel, tidal bar, dan tidal flat. Menggala Formation sediment succession can be divided into two system tracts, LST 1 and TST 1. LST 1 is dominated by braided channel deposits, which changed into crevasse splay, tidal channel, and tidal flat deposits in southeast flank of Sakai field. TST 1 is dominated by tidal channel deposits, which vertically alternates into tidal flat and tidal bar deposits.

Kata Kunci : Sakai field, Menggala Formation, lithofasies, depositional environment, facies association