Sedimentology of the Ngrayong formation and its Sandstone Reservoir Characterization :: Along Braholo River, Ngampel Village, Blora District Central Java Province, Indonesia

NGOC, Nguyen Thi Bich, Dr.Ir. Subagyo Pramumijoyo, DEA

2005 | Tesis | S2 Teknik Geologi

Formasi Ngrayong telah dipelajari sejak lama. Meskipun demikian, lingkungan pengendapan Formasi Ngrayong selah di determinasi secara berbeda dari satu peneliti ke peneliti lainnya. Oleh karenanya, Formasi Ngrayong menjadi menarik untuk dipelajari lebih dalam sentang lingkungan pengendapannya. Pengulunan stratigrafi telah dikerjakan sepanjang sungai Braholo, dari desa Ngampel hingga desa Sendangharjo untuk mendapatkan kalom litologi penampang vertical. Berdasarkan sekuen stratigrafi zona Rembang, Formasi Ngrayong terletak selaras distas Formasi Tawun, sedangkan Formasi Bulu memumpang tidak selaras diatas Formasi Ngrayong. Lingkungan pengendapan Formasi Ngrayong serta karakterisasi reservoar batu pasiunnya dipelajari dari segi sedimentologi. Sedimentologi menekankan pada foraminifera, fosil jejak, analisis ukuran butir, analisi pertrografi (sayatan tipis), serta provenan. Porositas dan permeabilitas batupasir Formasi Ngrayong juga diukur untuk evaluasi kualitas reservoar. Porositas dan permeabilitas diukur dengan alat “consolidation permeameter”. Analisis ukuran butir menunjukan karakterisasi batupasir yaitu halus sampai menengah (2.03 - 3.14 phi), sortasi cukup baik hingga baik (dengan simpangan baku berhiasan dari 0.36 sampai 1.0 phi), membulat tanggung hingga membulat, merupakan sedimen super-mature dan batulemgpung pasiran mencirikan sortasi buruk (dengan simpangan baku berhiasan dari 1.09 sampai 1.75 phi), merupakan sedimen immature. Lingkungan pengedapan batupasir menandakan lingkungan sand-bar, dan batulempung pasiran mencirikan lingkungan lagoon. Dengan cara ploting diagram QFL, provenan batupasiran Formasi Ngrayong berasal dari blok kontinental. Selain itu, komposisi batupasir dan mineral berat (apatit, rutil, kyamit, zirkon, hematit, magnetit, dan turmalin) mencerminkan endapan Formasi Ngrayong berasal dari batuan beku plutonik dan batuan metamorf. Porositas berhiasan dari 31 persen sampai 44 persen. Pemeabilitas berhiasan dari 61 milidarey sampai 372 milidarey. Porositas dan permeabilitas ini sangat baik dan memungkinkan batuopasir Formasi Ngrayong menjadi batuan reservoar. Akan tetapi, permeabilitas lebih tinggi dari 1500 milidarey berdasarkan hasil analisis ukuran butir. Hasil studi ini menunjukkan bahwa perubahan muka air laut relatif telah mempengaruhi proses dan produk sedimentasi di Cekungan Jawa Timurlaut selama kala Miosen Tengah. Pengendapan Formasi Ngrayong terdiri dari enam siklus. Setiap siklus terdiri dari suatu deret lapisan-lapisan batugamping pasiran – batupasir kuarsa – batulempung pasiran atau serpih. Fosil Foraminifena ( Amphistegina lessonii, Elphidium sp, Quinqueculina sp, Asterorotalia gaimadii, Lepidocyclina sp, Cyclocypues sp) dan fosil jejak (Thalassinoides) menunjukkan bahwa lingkungan pengendapan tepian tengah (middle shelf) hingga tepian dalam (inner shelf). Umur Formasi Ngrayong yaitu Miosen Tengah. Litologi, fosil dan tekstur pengendapan mencirikan bahwa Formasi Ngrayong terbentuk pada lingkungan transisi. Khususnya pada lapisan batupasir kuarsa dan batugamping diendapkan pada lingkungan sand-bar, serta batulempung pasiran dan serpih diendapkan pada lingkungan lagoon. Akan tetapi, maka air laut mengalami perubahan selama proses pengendapan Formasi Ngrayong. Kenam memingirkan telah terjadi pendalaman ke arah atas hingga penampang sekuen tengah dan pendangkalan kembali ke arah atas hingga puncak dari sekuen. Kesimpulan itu ditunjukkan dalam tesis ini.

The Ngrayong Formation has been studied for a long time. However, depositional environment of Ngrayong Formation has been determined differently from various author. Therefore, Ngrayong Formation was attracted to study deeper about its depositional environment. The measure section was carried out along Braholo River from Ngampel to Sendangharjo village for vertical profile lithologic column. According to stratigraphic sequence of Rembang zone, the Ngrayong Formation conformably overlies the Tawun Formation. Meanwhile, the Bulu Formation unconformably overlies the Ngrayong Formation. Depositional environment was studied in term of sedimentology of Ngrayong Formation and its sandstone reservoir characterization. Sedimentology emphasized on foraminifera, trace fossil, grain size analysis, petrography analysis (thin section), provenance. Porosity and permeability of the sandstone of Ngrayong Formation were also measured to evaluate reservoir quality. Porosity and permeability were measured by consolidation permeameter. The grain size analysis showed that the sandstone is fine to medium grain (2.03 - 3.14 phi), moderately well-sorted to well-sorted (with standard deviation range from 0.36 to 1.0 phi), subrounded to rounded, supermature sediments, whereas, the sandy claystone is characterized as poorly sorted (with standard deviation range from 1.09 to 1.75 phi), and immature sediments. The depositional environment of sandstones could be sand bar environment and lagoon environment for sandy claystones. With QFL ternary plot, the provenance of sandstone of Ngrayong Formation came from continental blocks. Moreover, the composition of sandstone and heavy mineral (apatite, rutile, kyanite, zircon, hematite, magmatite, and tourmaline) reflected the sediments of Ngrayong Formation came from plutonic igneous rock and metamorphic rock origin. Porosities range from 31 percents to 44 percents. Permeabilities range from 61 milidarcies to 372 milidarcies. These porosities and permeabilities are very good to consider the sandstones of Ngrayong Formation as reservoir rocks. However, permeabilities were higher than 1500 milidarcies by grain-size analysis method. Results of the study show that relative sea level changes have affected the processes and products of sedimentation in the Northeast Java Basin during the Middle Miocene. The Ngrayong Formation is composed of six cycles. Each cycle consists of a series of sandy limestone - quartz sandstone - sandy claystone or shale layers. Base on foraminifera fossils (Amphistegina lessonii, Elphidium sp, Quinqueculina sp, Asterorotalia gaimadii, Lepidocyclina sp, Cyclocypues sp) and trace fossils (Thalassinoides), the depositional environment of Ngrayong Formation is in inner shelf to middle shelf environment. Moreover, the lithologies and sedimentary textures of the Ngrayong Formation points to deposit on a transitional environment as indicated by presence of quartz sandstone and limestone beds, and sandy claystone or shale respectively, particularly in sand bar environment and in lagoonal environment. The sea level change has also been ascertained during the depositional processes of Ngrayong Formation. There were upward deepening up to the middle sequence section and shallowing again upward to the top of the sequence. The age of Ngrayong Formation is Middle Miocene (N7-N9). The conclusive proofs were shown in this thesis.

Kata Kunci : Petrograf,Lingkungan Pengendapan,Formasi Ngrayong, Depositional environment, paleontology, petrography, grain size analysis, heavy mineral, provenance, reservoir quality of sandstone, Ngrayong Formation