-The characteristic of Cimandiri Fault Zone has not been completely defined despite plenty of studies had already been accomplished. Therefore, an audio-magnetotelluric modeling was carried out. An audiomagnetotelluric survey was conducted at two parallel lines (N166 o E) that intersected Cimandiri Fault Zone in Cibeber area, Cianjur. The distance between those two lines was 4.5 km and each line consisted of twenty-one stations with the distance between stations was 500 m. From the acquired forty-two apparent resistivity curves, inversion was executed to obtain two models. The models indicate layers with resistivity value of > 1000 ohm.m at about 500 m depth at both lines, which are associated to the basement layer. Columns of low resistivity zones in about the middle of each model represent fault zones as the weak zones of the area, and both models displayed them slightly dip southward as thrust faults.
Garut Basin are is part of Bandung-Garut Greater Basin (Bandung Zone) characterized by a large basin surrounded by mountain ranges. Active volcanoes had distributed their material as pyroclastic deposits around the outer border of the zone and as lava flow deposit separating the two basins. Bouguer gravity anomaly data had also indicated the presence of several low anomaly closures at about the area of Bandung and Garut Basins that were surrounded by high gravity anomaly zones. Two magnetotelluric surveys were completed to acquire the subsurface model that might explain the tectonic evolution of studied area. The first stage was characterized sby the presence of horst-graben structures that might imply an extensional regime of the area. The next stage of evolutionwas indicated by the horizontal layering correlated to the relative non-active tectonic. In addition, a most recent structure that appeared near the surface might suggest a possible extension force as the current stage.
Two important aspects in the exploration of oil and gas are technology and exploration concepts, but the use of technology is not always suitable for areas with geological conditions covered by young volcanic sediments or limestone. The land of the Northwest Java Basin is mostly covered by young volcanic products, so exploration using seismic methods will produce less clear image resolution. To identify and interpret the subsurface structure and the possibility of hydrocarbon trap, gravity measurements have been carried out. Delineation of nose structures of a Bouguer anomaly map was used to interpret the probability of hydrocarbon traps. The result of the study shows that the gravity anomalies could be categorized into three groups : low anomaly (< 34 mgal), middle anomaly (34 -50 mgal), and high anomaly (> 50 mgal). The analysis of Bouguer anomaly indicates that the low anomaly is concentrated in Cibarusa area as a southern part of Ciputat Subbasin, and in Cikampek area. The result of delineation of the Bouguer anomaly map shows the nose structures existing on Cibinong-Cileungsi and Pangkalan-Bekasi Highs, while delineation of residual anomaly map shows the nose structures occurs on Cilamaya-Karawang high. Locally, the gas fields of Jatirangon and Cicauh areas exist on the flank of the nose structure of Pangkalan-Bekasi High, while the oil/gas field of Northern Cilamaya is situated on the flank of the nose structure of Cilamaya-Karawang High. The concept of fluid/gas migration concentrated on nose structures which are delineated from gravity data can be applied in the studied area. This concept needs to be tested in other oil and gas field areas.
Penelitan batuan induk hidrokarbon yang dilakukan pada Formasi Cibulakan didaerah Palimanan, Cirebon bertujuan untuk memperoleh data permukaan endapan klastik berbutir halus serta karakteristik litofasies yang diduga sebagai batuan induk hidrokarbon. Metode yang dipakai adalah penelitian lapangan dan laboratorium. Penelitian lapangan meliputi pengamatan stratigrafi detil dan pengambilan conto batuan, sedang analisis laboratorium terdiri dari analisis kandungan Total Organic Carbon dan analisis Pirolisis Rock Eval. Hasil analisis Total Organic Carbon terhadap 17 conto batulempung dan batulanau yang diambil dari Formasi Cibulakan menunjukkan nilai berkisar 0,27-4,43%, dan umumnya mempunyai potensi membentuk hidrokarbon dalam katagori rendah hingga sangat baik. Pirolisis Rock Eval dilakukan terhadap 9 conto yang berpotensi membentuk hidrokarbon dan memperlihatkan nilai T sebesar 331-557°C. Nilai yang menunjukkan tingkat kematangan termal sebanyak 6 conto dinyatakan matang hingga pasca matang dengan nilai T max berkisar 445-557°C, sedang 3 conto lainnya belum matang dengan nilai T sebesar 331-362°C. Nilai Hydrogen Index berkisar 3-338 mg Hydrocarbon/g Total Organic Carbon, dan berada dalam fasies BC, CD dan D. Batuan induk tersebut dapat menghasilkan minyak dalam kuantitas sedang dan gas dalam kuantitas kecil. Potensi hidrokarbon di daerah penelitian menunjukkan kategori kekayaan material organik rendah hingga sangat baik, dengan kerogen termasuk tipe II dan III. Kualitas batuan sumber berdasarkan nilai Hydrogen Index termasuk dalam kategori gas prone dan oil prone.
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