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The analysis result of grains frequency curve and relation between grains size to cumulative have shown medium grains at Gilimanuk (Core drilling-1) and coarse grains at Ketapang (Core drilling-2). In general both of them are showed by pattern uniformity, which is represented by the similar of curve pattern. On the grain size of -2 phi as medium gravel with percentage between 6.47 to 35.88%, while core drilling -2 on the size of -2 phi between 6.86 to 61.11%.The average grains size of core drilling -1 are gravel about 21.3%, sand 60.2%, silt 5% and clay about 0.4% while core drilling-2 are characterized by 44.3%, sand 26.8%, silt 24.6% and clay about 0.6%. These result shows that at location of Core drilling -1 is dominated by sand where as at location of Core drilling -2 is dominated by gravel. These situation can be interpreted that the sediment at core drilling -2 location have influenced by strong marine current which can transport the large amount of gravel size compare to the location of Core drilling-1 which is dominated by sand. Based on the relation shape of grains size curve versus cumulative frequency shows that the sediment of Core drilling-1is interpreted as a beach sand deposits and only one sample which shows as a river sand deposits which was found at depth 0 – 3 m depth. In general, the sample of Core drilling –2 shows that the pattern of sediment tend as a beach sand deposits and only one sample which shows the combination between coastal deposits and river deposits ( 4 – 5 m depth). From this sample, the coarse to fine grains is deposited by coastal media and fine grains material (about 10%) is deposited by river media. The sample of river deposits is found as lamination because the only one which is created from combination between coastal and river depos its from all sample of core drilling-2.Keywords: core drilling, grain sediments, media transport, environmental deposition Hasil analisis menggunakan kurva frekuensi butiran serta hubungan antara besar butir terhadap kumulatif menunjukkan dominasi ukuran butiran sedang di daerah Gilimanuk (Bor-1) dan kasar di daerah Ketapang (Bor-2). Pada kedua daerah tersebut, secara umum memperlihatkan pola keseragaman, yang ditunjukkan oleh pola kurva yang sama. Pada ukuran butir -2 phi (kerikil sedang) pada Bor-1 berjumlah antara 6,47 – 35,88%, sedangkan pada Bor-2 pada ukuran -2 phi berjumlah antara 6,86 – 61,11%.Kandungan rata-rata butiran pada Bor-1 adalah: kerikil 21,3%, pasir 60,2%, lanau 5% dan lempung 0,4%, sedangkan Bor-2 adalah: kerikil 44,3%, pasir 26,8%, lanau 24,6% dan lempung 0,6%. Hasil ini menunjukkan bahwa pada Bor-1 didominasi oleh pasir dan pada Bor-2 didominasi oleh kerikil. Hasil ini dapat memberi gambaran bahwa pada Bor-2 berarus lebih kuat karena mampu menstranspor butiran kerikil dalam jumlah yang lebih besar dibandingkan dengan Bor-1 yang didominasi oleh pasir. Berdasarkan beberapa kurva hubungan antara besar butir vs frekuensi komulatif pada Bor-1 menunjukkan endapan pantai (beach sand), hanya satu contoh menunjukkan endapan sungai (river sand), yaitu contoh B1 (0 – 3m). Pada Bor-2, secara umum menunjukkan pola yang lebih mendekati endapan pantai (beach sand), hanya satu contoh menunjukkan kombinasi endapan pantai dan endapan sungai (river sand), yaitu contoh B2 (4 – 5 m). Pada contoh ini, butiran berukuran kasar sampai halus diendapkan oleh media pantai dan ukuran halus dengan persentase sekitar 10% merupakan endapan sungai. Contoh endapan sungai adalah pada B2 (4 – 5 m) ini merupakan endapan sisipan karena satu-satu terbentuk dari kombinasi pantai dan sungai dari seluruh contoh pada Bor-2.Kata kunci: pemboran inti, butiran sedimen, media transport, lingkungan pengendapan
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Seismic method is one of the most frequently applied geophysical methods in the process of oil and gas exploration. This research is conducted in Nias Waters, North Sumatra using one line 2D post-stack time migration seismic section and two wells data. Reservoir characterization is carried out to obtain physical parameters of rocks affected by fluid and rock lithology. Seismic inversion is used as a technique to create acoustic impedance distribution using seismic data as input and well data as control. As final product, multi-attribute analysis is applied to integrate of inversion results with seismic data to determine the lateral distribution of other parameters contained in well data. In this research, multi-attribute analysis is used to determine the distribution of NPHI as a validation of hydrocarbon source rocks. In that area, there is a gas hydrocarbon prospect in limestone lithology in depth around 1450 ms. Based on the results of sensitivity analysis, cross-plot between acoustic impedance and NPHI are sensitive in separating rock lithology, the target rock in the form of limestone has physical characteristics in the form of acoustic impedance values in the range of 20,000-49,000 ((ft/s)*(g/cc)) and NPHI values in the range of 5-35 %. While the results of the cross-plot between the acoustic impedance and resistivity are able to separate fluid-containing rocks with resistivity values in the range about 18-30 ohmm. The result of acoustic impedance inversion using the model based method shows the potential for hydrocarbons in the well FYR-1 with acoustic impedance in the range 21,469-22,881 ((ft/s)*(gr/cc)).
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Core drilling had been carried out in three locations such as in Brondong Village (BH-01), Pasekan Village (BH-02), and Karangsong Village (BH-03). Those three cores are similar in lithology consist of clay. They are correlated based on fragment content, such as fine sand lenses, mollusk shells, rock and carbonate materials which discovered from different depths. Single side band of shallow seismic reflection recorded paleochannels in E sequence at the north and the west of investigated area. It’s predicted the north paleo channels were part of Lawas River or Tegar River, while the west paleo channels were part of Rambatan Lama River. Microfauna content of all those three cores indicated that from the depth of 0.00 meter down to 25,00 meters are Holocene/Recent, from 25,00 meters to the bottom are Pleistocene which were deposited in the bay to middle neritic environment.
Key words: Core drilling, seismic interpretation, Cimanuk Delta
Pemboran inti dilakukan di 3 (tiga) lokasi yaitu di Desa Brondong (BH-01), Desa Pasekan (BH- 02), dan Desa Karangsong (BH-03). Ke tiga inti bor itu mempunyai kesamaan litologi yaitu lempung. Korelasi dilakukan berdasarkan kandungan fragmen seperti lensa-lensa pasir halus, cangkang moluska, material batuan dan material karbonat yang terdapat pada kedalaman yang berbeda. Seismik pantul dangkal saluran tunggal merekam adanya alur purba pada runtunan E di bagian utara dan barat daerah penelitian. Diduga alur purba di bagian utara merupakan bagian dari tubuh Kali Lawas atau Kali Tegar, sedangkan di bagian barat sebagai bagian dari tubuh Kali Rambatan Lama. Kandungan mikrofaunanya menunjukkan ketiga bor inti mulai kedalaman 0,00 meter sampai 25,00 meter berumur Holosen/Resen, sedangkan dari 25,00 meter sampai batas bawah pemboran berumur Plistosen yang diendapkan pada lingkungan teluk hingga Neritik Tengah.
Kata kunci: Bor inti, penafsiran seismik, Delta Cimanuk.
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The study area is fluviatile deposit which consists of yellow to light gray loose sand, iron oxide, and kaoline spreading from the land shallowing to sea direction. Concentration grade for determining of gold and silver content used a total analysis of the hot mixture HF-HNO3-HClO4 with measurements "Atomic Absorption Spectrofotometry".(AAS). Highest concentration of gold and silver found at sample GCTS-31 that gold 0.096 ppm and silver 2.284 ppm, in muddy sand type sediment, at 3.1 meters depth and arround 1,250 meter length from coast line. While the lowest concentration of the gold was 0.025 ppm found at sample GBTS 14 in very fine sand type sediment located at 1.07 meter depth and approximately 320 meters length from coast line. The lowest concentration of the silver was founded at sample GBTS 25 that is 0.860 ppm in sandy mud sediment type located at 2.49 meters depth and approximately 3,000 meters length from the coast line. The gold placer is may derived from weathering of Pambuang Formation which spreads in the west and north of Sampit Bay, and transported by Mentaya River and longshore current system.
Key word : sediment samples, laboratory analyses, gold, silver and Sampit Bay.
Daerah penelitian merupakan endapan fluviatil terdiri dari pasir lepas berwarna kuning muda sampai abu-abu pucat, oksida besi, dan kaolin yang memiliki sebaran makin mendangkal ke arah laut. Kadar konsentrasi untuk menentukan kandungan emas dan perak menggunakan analisis total campuran HF-HNO3-HClO4 dengan metoda AAS. Analisa besar butir digunakan untuk memisahkan sedimen fraksi kasar sedangkan analisa pipet dilakukan terhadap sediment fraksi halus. Kadar emas dan perak tertinggi terdapat pada contoh GCTS -31, yaitu emas 0,096 ppm dan perak 2,284 ppm, pada jenis sedimen lumpur pasiran, di kedalaman 3,1 meter dan sekitar 1.250 meter dari garis pantai. Sedangkan kadar emas terendah yaitu 0,025 ppm terdapat pada contoh GBTS 14 pada jenis sedimen pasir sangat halus di kedalaman 1,07 meter dan sekitar 320 meter dari garis pantai. Kadar perak terendah dijumpai pada contoh GBTS 25 yaitu 0,860 ppm dengan jenis sedimen pasir lumpuran pada kedalaman 2,49 meter dan sekitar 3.000 meter dari garis pantai. Sumber emas plaser diperkirakan dari lapukan Formasi Pembuang yang memiliki sebaran cukup luas di sebelah barat dan utara Teluk Sampit dan ditransportasi oleh Sungai Mentaya dengan anak-anak sungainya, serta sistem arus memanjang pantai.
Kata kunci : contoh sedimen, analisis laboratorium, emas, perak dan Perairan Teluk Sampit.
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Weathering is a geological phenomenon that is often an important considered because of its destructive properties, mainly in subsurface. Many parameters are used to measure the presence of weathering indications. This study focuses on testing of magnetic susceptibility (c), compound oxide content and Fe3O4/Fe2O3 ratio that have been selected as an indicators of weathering process. This study explains in detail the quantitative analysis of weathering based on these parameters in basaltic andesite rocks found in coastal atmospheric areas. The results obtained for weathered rocks, magnetic susceptibility, compound oxide content such as CaO and the Fe3O4/Fe2O3 ratio has decreased significantly. In the coastal atmosphere, the weathering of basaltic andesite rocks also marked by the distribution of magnetic minerals which tend to be in the domain of pseudo single domain (PSD) or single domain (SD). Thus it can be concluded that both of low frequency magnetic susceptibility (clf), CaO and Fe2O3 content and also the Fe3O4/Fe2O3 ratio can be used as weathering level indicators.Keywords: weathering, andesite, magnetic susceptibility, Fe3O4/Fe2O3 ratio, coastal atmosphere. Pelapukan merupakan fenomena geologi yang sering menjadi pertimbangan penting karena sifatnya destruktif terutama pada bawah permukaan. Banyak parameter yang digunakan untuk mengukur adanya indikasi pelapukan. Penelitian ini berfokus pada pengujian suseptibilitas magnetik (c), kandungan senyawa kimia dan rasio Fe3O4/Fe2O3 yang dipilih menjadi parameter sebagai indikator pelapukan. Penelitian ini menjelaskan secara rinci analisis kuantitatif pelapukan berdasarkan kedua parameter tersebut pada batuan beku basaltik andesit yang terdapat pada area atmosfer pantai. Hasil penelitian didapatkan untuk batuan lapuk, suseptibilitas magnetik, kandungan senyawa kimia CaO dan rasio Fe3O4/Fe2O3 mengalami penurunan. Pada atmosfer pantai batuan basaltik andesit yang mengalami pelapukan juga ditandai oleh distribusi mineral magnetik yang cenderung berdomain pseudo single domain (PSD) maupun single domain (SD). Dengan demikian dapat disimpulkan bahwa baik suseptibilitas magnetik low frekuensi (clf), kandungan senyawa kimia CaO dan Fe2O3 maupun rasio Fe3O4/Fe2O3 dapat dijadikan sebagai indikator tingkat pelapukan.Katakunci: pelapukan, andesite, suseptibilitas magnetik, rasio Fe3O4/Fe2O3, atmosfer pantai.
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The gas field, initial named W-ITB Field, is located at the southwestern part of the East Java sedimentary basin in Madura Strait area. W-ITB Field was discovered by W-ITB#1 well in 2006. In W-ITB#1 well gas reservoir layer was just only found at Selorejo and Mundu Formation, on the other hand, on W- ITB#2 the gas reservoir is not found in Mundu Formation. Determination of reservoir characteristic including the distribution and quality at W-ITB Field, was done by using 3D geological modelling both for structure and stratigraphy. This model was executed based on integration of well data (petrophysics) and cross section seismic interpretation. The results, at Zone 2 and Zone 3 for vertical V-shale distribution shows as a good quality reservoir (0-15%). Laterally distribution, area at southwest of W-ITB 1 well has low V-shale or chatagorized as a good quality reservoir. While, porosity distribution, zone 1 and zone 2 have better reservoir (29-35% V-shale value) than Zones 3 and 4. NTG distribution result indicates that zone 2 and 3, with high value means a good reservoir. Due to only two exploration well, to guide lateral distribution, so that acoustic from seismic data is used for porosity distribution.
Key words: modelling, reservoir, characteristic, V-shale, porosity, quality, Madura Strait
Lapangan gas dengan nama inisial W-ITB terletak di bagian barat daya cekungan sedimen Jawa Timur yang termasuk di wilayah Selat Madura. Lapangan ini ditemukan dari Sumur W- ITB#1 pada tahun 2006. Pada sumur W- ITB#1 Lapisan reservoir yang mengandung gas hanya dijumpai pada Formasi Selorejo dan Mundu, namun dari hasil sumur W- ITB#2, lapisan reservoir gas dalam Formasi Mundu tidak diperoleh. Penentuan karakteristik reservoir termasuk distribusi dan kualitasnya di Lapangan W-ITB dilakukan dengan pemodelan geologi 3-Dimensi baik secara struktur dan stratigrafi dengan berdasarkan pada integrasi data sumur pemboran dan penampang seismik yaitu analisis petrofisik dan interpretasi seismik. Berdasarkan pemodelan 3-Dimensi, pada Zone-2 dan Zone-3 untuk distribusi V-shale secara vertikal merupakan zone dengan kandungan reservoir yang baik dengan nilai V-sh 0 – 15%. ecara distribusi lateral, wilayah di bagian barat daya dari sumur W- ITB#1 memiliki kandungan V-sh yang rendah atau dikategorikan reservoir dengan kualitas baik. Adapun pada distribusi porositas, Zona-1 dan Zona-2 mengandung reservoir yang lebih baik dengan nilai 29- 35% daripada Zona-3 dan Zona-4. Hasil distribusi NTG mengindikasikan bahwa Zona-2 dan Zona-3 dengan nilai tinggi mengandung reservoir yang baik. Karena hanya memilki dua sumur eksplorasi, untuk memandu distribusi lateral maka hasil impedansi akustik dari data seismik digunakan untuk distribusi porositas.
Kata kunci: pemodelan, reservoir, karakteristik, V-serpih, porositas, kualitas, Selat Madura
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The main goal of seismic exploration is to get an accurate image of subsurface section so it can be easily interpreted. Pre Stack Depth Migration (PSDM) is such a powerful imaging tool especially for complex area such an area where strong lateral velocity variations exist. The main challenge of PSDM is the need of accurate interval velocity model.In this research, Dix Transformation, coherency inversion, and tomography are used for initial interval velocity model, and then tomography is used for interval velocity model refinement. We compare also between seismic image resulted from PSDM and PSTM to determine the best method. The seismic data that processed in this paper is derived from north western part of Australian Waters.
Kata kunci: Pre Stack Depth Migration, Dix Transformation, coherency inversion, tomography.
Tujuan utama dari eksplorasi seismik adalah menghasilkan citra yang akurat dari penampang bawah permukaan sehingga diinterpretasi lebih mudah. Pre Stack Depth Migration (PSDM) merupakan suatu metode yang memberikan hasil peningkatan kualitas citra seismik pada daerah kompleks dimana terjadi variasi kecepatan lateral yang signifikan. Salah satu syarat penting yang harus dipenuhi agar hasil PSDM lebih optimal adalah model kecepatan interval yang akurat. Dalam penelitian ini Transformasi Dix, inversi koheren, dan tomografi digunakan untuk memenuhi syarat tersebut. Perbandingan hasil penampang seimik PSDM dan PSTM dilakukan untuk menentukan metode terbaik. Data seismik yang diolah dalam tulisan ini berasal dari wilayah Perairan Baratlaut Australia.
Kata kunci: Pre Stack Depth Migration, Transformasi Dix, inversi koheren, tomografi
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Some deepwater multiple attenuation processing methods have been developed in the past with partial success. The success of surface multiple attenuation relies on good water bottom reflections for most deepwater marine situations. It brings the bigger ability to build an accurate water bottom multiple prediction model. Major challenges on 2D deepwater seismic data processing especially such a geologically complex structure of Seram Sea, West Papua – Indonesia are to attenuate surface related multiple and to preserve the primary data. Many multiple attenuation methods have been developed to remove surface multiple on these seismic data including most common least-squares, prediction-error filtering and more advanced Radon transform.Predictive Deconvolution and Surface Related Multiple Elimination (SRME) method appears to be a proper solution, especially in complex structure where the above methods fail to distinguish interval velocity difference between primaries and multiples. It does not require any subsurface info as long as source signature and surface reflectivity are provided. SRME method consists of 3 major steps: SRME regularization, multiple modeling and least-square adaptive subtraction. Near offset regularization is needed to fill the gaps on near offset due to unrecorded near traces during the acquisition process. Then, isolating primaries from multiples using forward modeling. Inversion method by subtraction of input data with multiple models to a more attenuated multiple seismic section.Results on real 2D deepwater seismic data show that SRME method as the proper solution should be considered as one of the practical implementation steps in geologically complex structure and to give more accurate seismic imaging for the interpretation.Keywords : multiple attenuation, 2D deepwater seismic, Radon transform, Surface Related Multiple Elimination (SRME). Banyak metode atenuasi pengulangan ganda dikembangkan pada pengolahan data seismik dengan tingkat keberhasilan yang rendah pada masa lalu. Keberhasilan dalam atenuasi pengulangan ganda permukaan salah satunya bergantung pada hasil gelombang pantul pada batas dasar laut dan permukaan pada hampir seluruh survei seismik laut. Hal tersebut menentukan keakuratan dalam membuat model prediksi pengulangan ganda dasar laut dan permukaan air. Tantangan utama dalam pemrosesan data seismik 2D laut dalam khususnya struktur geologi kompleks seperti Laut Seram, Papua Barat – Indonesia adalah pada kegiatan menekan pengulangan ganda permukaan sekaligus mempertahankan data primer. Beberapa metode yang dikembangkan untuk menghilangkan pengulangan ganda permukaan pada data seismik seperti least-square, filter prediksi kesalahan dan transformasi Radon.Dekonvolusi Prediktif dan Metode Surface Related Multiple Elimination (SRME) digunakan sebagai solusi yang baik pada struktur kompleks dimana metode-metode lain gagal untuk memisahkan perbedaan kecepatan interval data primer dan pengulangan ganda. Metode tersebut tidak membutuhkan informasi bawah permukaan selain parameter sumber dan reflektivitas permukaan. Metode SRME terdiri dari 3 tahapan utama : regularisasi SRME, pemodelan pengulangan ganda dan pengurangan adaktif least-square. Regularisasi near offset diperlukan untuk mengisi kekosongan pada near offset yang disebabkan oleh adanya sejumlah tras terdekat yang tidak terekam selama akuisisi. Pemodelan maju digunakan untuk memisahkan data primer dan pengulangan ganda kemudian inversi dengan pengurangan input data dengan model multiple.Hasil pada data seismik 2D laut dalam menunjukkan bahwa metode SRME layak diterapkan sebagai salah satu pengembangan metode atenuasi multiple permukaan serta untuk meningkatkan akurasi data seismik terutama untuk struktur geologi kompleks.Kata kunci : peredaman pengulangan ganda (multiple), seismik 2D laut dalam, transformasi Radon, Surface Related Multiple Attenuation (SRME).
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Mineable iron sand deposits in Cilacap – southern coastal area of Central Java have certain coastal characteristics that need to be studied in order to understand its depositional environment. With the knowledge of such environment, it can be applied to look for other places prospective of iron sand deposits that have the same characteristics especially recently when Cilacap’s deposits were almost depleted. Coastal characteristics of iron sand deposit in Cilacap is shown by successive sandy beach ridges separated by marshy valleys typical of prograded coasts and by dunes of sand elongated parallel to the shore line with elevation varies from 0 m to 15 m above sea level. The iron sand deposit was derived from denudation of andesite and “Old Andesite Formation†enriched in magnetite and ilmenite minerals in the steep elevated and deeply weathered rock hinterlands of Cilacap. High sediment loads of Serayu Basin in the hinterland (3,500-4,500 ton/km2/year; Citarum River basin only 800-1,200 ton/km2/year) was causing extensive deposition of iron sand in the coastal zone.
Key words: coast, characteristic, iron sand, Cilacap
Endapan pasir besi yang dapat ditambang di Cilacap – pesisir selatan Jawa Tengah memiliki karakteristik pantai tertentu yang perlu dikaji agar dapat dipahami lingkungan pengendapannya. Dengan pengetahuan tentang lingkungan pengendapan tersebut, dapat diterapkan untuk mencari daerah-daerah lain prospek endapan pasir besi yang memiliki karakteristik yang sama terutama pada akhir-akhir ini ketika endapan Cilacap akan habis. Karakteristik pantai endapan pasir besi di Cilacap dicirikan oleh urutan pematang pantai berpasir yang dipisahkan oleh lembah-lembah berawa khas pantai maju dan oleh gumuk-gumuk pasir memanjang sejajar dengan garis pantai dengan ketinggian bervariasi dari 0 m hingga 15 m dari muka laut. Endapan pasir besi di daerah ini berasal dari proses denudasi andesit dan “Formasi Andesit Tua†yang kaya akan mineral magnetit dan ilmenit pada pedalaman Cilacap dengan kondisi elevasi curam dan batuan sangat terlapukkan. Muatan sedimen yang tinggi dari Cekungan Serayu di pedalaman tersebut (3.500-4.500 ton/km2/tahun; cekungan Sungai Citarum hanya 800-1.200 ton/km2/tahun) menyebabkan pengendapan yang sangat luas pasir besi di wilayah pantai.
Kata kunci: pantai, karakteristik, pasir besi, Cilacap
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Secara geografis daerah penelitian terletak di muara Sungai Kapuas Kalimantan Tengah. Kawasan pantai ini sangat terbuka dipengaruhi oleh aksi gelombang dari arah tenggara, selatan, baratdaya dan barat. Di pesisir pantai muara Sungai Kapuas terdapat arus sejajar pantai yang bergerak masuk ke arah muara. Pergerakan arus sejajar pantai tersebut memasok sedimen dari arah tenggara dan barat ke muara. Sedimen tersebut cenderung diendapkan di sisi barat dari muara Kapuas. Hal ini menjadi kendala alur pelayaran di muara Sungai Kapuas.
Kata kunci : arus sejajar pantai, muara, Kapuas
Geographically, the study area is located on the coast of Kapuas River mouth, Central Kalimantan. This open beach area is affected by wave action from southeast, south, southwest and west direction. The longshore current drifts to the estuary on the coastal zone of the Kapuas River. It gives potential sediment supply from the southeast to the north. The sediment tends to be deposited in the western part estuary of Kapuas.This particular thing evokes the ship navigation in Kapuas estuary.
Keywords : longshore current, estuary, Kapuas
