Limestone Sedimentology of The Lengkong Formation at Neglasari District, Sukabumi-West Java, Based on Outcrop and Petrography

Research about Oligo-Miocene limestones in Java has always been interesting to study in order to understand the relationship between tectonic-volcanic, sedimentation, and petroleum potential. The Lengkong limestone formation is interesting to study especially its sedimentology aspects. There is no Scientific Publication about the Lengkong Formation which discusses detailed sedimentology aspects. Those aspects are important to modify the geological map, scale of 1: 50000 which was published by the ministry of energy and mineral resources. This research was conducted to determine the sedimentology process and development of a depositional environment of the Lengkong formation in the Cimapag River. The research area is located in Neglasari, Lengkong District, Sukabumi. The method used includes field surveys and laboratory analysis. A field survey embodies sedimentology data collection and rock sampling, which was then made into a detailed stratigraphic column. Rock samples were analyzed petrographically for 12 samples to determine microfacies. The results cover three limestone facies, namely wackestone, wackestone-packstone, and packstone facies. The sedimentation of the Lengkong Formation indicates a sea-level decrease of the Interbuild Basin Floor to Deep Marine Off reef

Ombilin Basin as Inverted Oblique Rift in Barisan Mountains, Sumatra: Considerations on Subsidence Mechanisms and Fault Development

Ombilin Basin is a NW-SE inverted oblique rift which is currently being part of Barisan Mountains in western Central Sumatra. Regarding its current position, Ombilin Basin can be one of the windows to see the evolution of Barisan Mountains since Paleogene. Two schools of thought, namely rift basin and pull-apart basin, have been established to explain the evolution of Ombilin Basin. This paper aims to present another perspective on the evolution of Ombilin Basin based on subsidence mechanisms and fault development. This study integrated remote sensing and subsurface interpretations. Remote sensing interpretation took the role to delineate surface fault lineaments using digital elevation model, while subsurface interpretation dealt with log and seismic interpretations, subsidence analysis, and palinspatic reconstruction. Fault lineaments derived both from remote sensing and seismic interpretations were combined to construct structural framework of the basin. Subsidence analysis generated geohistory and backstripped tectonic subsidence charts. Palinspatic reconstruction illustrated structural configurations through time. This study figured out that Ombilin Basin went through fault-controlled subsidence in Middle Eocene – Late Oligocene and thermal subsidence in Early Miocene – Late Pliocene. Each subsidence mechanism was terminated by an uplift. Subsidence mechanisms in Ombilin Basin represented the criteria of rift basin in terms of amount and rate of tectonic subsidence, duration of subsidence, and contribution of thermal subsidence. On the other hand, fault development captures extensional and strike-slip components during rifting and development of flower structures during inversion of the basin. Oblique rifting operates when dominant extensional component works together with strike-slip component. Therefore, subsidence mechanisms and fault development are in agreement to regard Ombilin Basin as inverted oblique rift.

The Massive Fluvial Channel System in The Balam Graben: New Insight and Future Expectation from Menggala Formation in The Northern Rokan Block, Central Sumatera Basin

Menggala Formation has been long recognized as high quality oil bearing sand and favorable reservoir in Central Sumatera Basin (CSB). Regionally, this formation is unique in characteristic, distribution and has not been thoroughly evaluated during early exploration and development. This paper aims to explain elaborately the new stratigraphic feature and regional paleogeography model of Menggala Formation throughout Balam grabens by integrating cores, biostratigraphy, water salinity, well log and 2D seismic image to better understand regional stratigraphic play concept. Lower Menggala Formation in the northern Rokan Block has been clearly identified as the large scale of fluvial channel system that deposited parallel with Paleogene border fault remnants during early post-rift phase (22-25 million years ago). The fluvial channel belt is characterized by 4 to 5 km width, thick multi stacking fluvial sequence in the north and gradually change into river mouth sediment in the south. These sediments accumulated in depression area and also proven as huge productive reservoir in many fields. The north-south trending channel axis and geobodies confirmed by paleo bathymetric zonation, water salinity and 2D seismic image. The new paleogeography model helps better understanding of the regional stratigraphic concept of Menggala Formation across Balam grabens. In addition, this conceptual model also leads to strengthen the future exploration and development play concept within study area in Rokan Block.

ntegration of Petrophysical Analysis and Elastic Log Properties as an Input to Optimize the Development Wells Target in Unique Globigerina Limestone Gas Reservoir in Madura Strait

The Madura Strait PSC is located in the southern part of North East Java Basin with biogenic gas from Globigerina limestone Pliocene Mundu and Selorejo sequence as main target. At early stage of field development,understanding and knowledge about petrophysical and elastic properties of reservoir rock quality is required and very important. The petrophysical analysis provide properties such as clay volume, porosity, permeability, water saturation and mineral volume to separate reservoir and non-reservoir zone. The elastic rock properties such as acoustic impedance (AI), shear impedance (SI), velocity ratio (Vp/Vs), and Poisson’s ratio (σ) were generated to identify clay zone, gas and non-gas also focused to distinguish reservoir rock quality inside gas zone as an effective reservoir characterization. This research is done by utilize core data, quad combo logs from eleven wells and shear velocity from eight wells. The purpose of this research is to optimize development well target in Globigerina limestone gas reservoir, which have good to best reservoir rock quality shown with high porosity and permeability,low clay volume and water saturation. Results from this research indicate that lime mud matrix have significant impact in the reservoir rock quality. Meanwhile, gas saturation can affect the elastic properties due to this high gas saturation can decrease compressional velocity (Vp) value. Finally, the integration of petrophysical result and combination of elastic properties implementation can help in distinguishing the best reservoir rock quality, which contains gas that should be penetrated by the development wells

Contrasting Genesis of Lateritic Bauxite on Granodioritic and Andesitic Rocks of Mempawah Area, West Kalimantan

The lateritic bauxite deposits in the Mempawah area, West Kalimantan, were formed by the chemical weathering of Cretaceous granodioritic and andesitic rocks. They occurred locally on the low hills surrounded by swampy areas. Detailed surface geological mapping, test pits, mineralogical and geochemical analyses were performed to determine the characteristics and genesis of bauxite from different parent rocks. From bottom upward, the deposits are composed of fresh parent rocks, clay or pallid zone, bauxite zone with a few sparse ferricrete at the top of the bauxite zone, and soil. Bauxite derived from granodiorite exhibits brownish-red, massive, boulder to gravel-sized concretion in clay matrix and is composed of predominant gibbsite with subordinate kaolinite, quartz, goethite, and a minor amount of magnetite and hematite. In contrast, bauxite derived from andesitic rocks exhibits reddish-brown and is composed of predominant goethite. During the leaching process, SiO2 as a mobile compound decreased significantly in neutral pH, while Al2O3 and Fe2O3 precipitated as residual materials to form bauxite concretion. The enrichment anomaly of bauxite derived from andesitic rocks is caused by physio-chemical changes from hydrothermal alteration. Bauxite was formed by indirect bauxitization through the leaching of primary minerals under a tropical-humid climate.

Groundwater Recharge Estimation Using Water Budget and Water Table Fluctuation Method in the Jakarta Groundwater Basin

The Jakarta Groundwater Basin is one of the groundwater basins with the highest development, economic, and business activities in Indonesia. Groundwater damage has become a major growing issue in the Jakarta groundwater basin. Intensive development has led to the overuse of groundwater in this basin. Efforts are needed to manage, protect, and conserve groundwater in this basin to support the development and economic activities sustainably. Jakarta, as the capital city of Indonesia, is located in the groundwater basin. Groundwater sustainability is determined by the amount of groundwater recharge in those basins, so knowledge of groundwater recharge is important. Groundwater is an important part of a hydrological cycle, and groundwater recharge ensures groundwater sustainability in some areas. This study aims to estimate groundwater recharge in the Jakarta groundwater basin using the water budget and water table fluctuation method. The water budget method used is Thornthwaite, Dingman, and Edijatno-Michel. The Water Table Fluctuation methods used are Dellin and Delottier. Analysis of the amount of groundwater recharge estimation is carried out using the ESPERE Version 2 software. Output data is then further analyzed using descriptive and inferential statistical approaches to determine whether there is a difference in groundwater recharge amount based on the water budget and water table fluctuation. The results show that groundwater recharge based on water budget methods is 209–885 mm/year. The estimation of the largest amount of recharge was obtained using the Edijatno-Michel approach. The smallest amount of recharge was estimated using the Dingman-Hamon method. The average recharge of groundwater in Tanjung Priok is 305 mm/year, Kemayoran is 209 mm/year, and Bogor is 885 mm/year. Only 8–15 % of the annual rainfall that converted into groundwater recharge at the study area. Based on the analysis using the water table fluctuation method, groundwater recharge in this basin has a value of 240 mm/year. The variation of the amount of groundwater recharge is caused by the pros and cons of each method. Apart from that, geological factors, land use/land cover factors, and climatic variations in this basin can affect the research results. By considering the amount of groundwater recharge, groundwater management in the Jakarta groundwater basin needs to be carried out for harmonious development and groundwater conservation.

Geological, Geochemical, and Radiometric Study of Sandstone-Type Uranium Deposit Exploration in Menukung Area, West Borneo

BATAN has been carried out uranium exploration in West Borneo since 1969. So far, the exploration is focused on metamorphite-type uranium deposits in Kalan Area. The previous study concluded that mineralized uranium is originated from Sepauk Tonalite consisted of felsic-intermediate igneous rocks, and is hosted in medium-grade foliated and non-foliated metamorphic rocks of Pinoh Metamorphite. As uranium exploration develops, the International Atomic Energy Agency (IAEA) introduces the sandstone-type uranium mineralization concept that offers a more cost-effective mining process. The Melawi Basin becomes an attractive probable location for sandstone-type uranium deposit exploration since it is situated downstream of Schwaner Mountain's Sepauk Tonalite. The sandstone-dominated Tebidah Formation of Melawi Basin can be the host rock for sandstone-type uranium deposit if there is a reduction zone to trap the mobile uranium in the groundwater. The geological mapping, geochemical sampling, and radiometric survey were conducted in Menukung Area to prove the hypothesis. It is located in the eastern part of the Tebidah Formation, which contains abundant carbonaceous mudstones associated with coal seams. Mobile uranium content analysis showed the anomaly of 36–60 ppm at the central of Tebidah Formation at the study area, while radiometric data denoted the anomaly of 6.5–11.3 ppm eU. At those locations, coal and carbonaceous sandstone were observed. Those data indicate the presence of a reductive environment that gives the advantage to uranium trapping. It can be concluded that there is a possibility of the occurrence of sandstone-type uranium mineralization in the Menukung Area.

etermination of Geothermal Reservoir Zone of Sorik Marapi, Mandailing Natal, North Sumatra

The Sorik Marapi Geothermal Power (SMGP) Field is located in Mandailing Natal (350 km to the southeast from Medan), North Sumatra. This study needed alteration mineral data validation with fluid geochemical characteristics from surface manifestations, and production wells were determined the geothermal reservoir zone. Therefore geothermometer and geoindicator calculations were used. Laboratory tests were carried out on 38 surface manifestations and 11 cutting samples of exploration well. Manifestations in this area include hot springs, fumaroles, and steaming ground. Alteration minerals found in the study area are anhydrite, epidote, calcite,chlorite, muscovite, pyrophyllite, and zeolite. The alteration zone is located in the zeolite zone, the epidote zone,and the anhydrite-sericite zone. Geochemical manifestations of surface and production wells containing chloride water, sulfate water, sulfate-chloride water, and bicarbonate water. The estimated reservoir temperature of SH-01 well shows temperatures ranging from 240–280˚C, SH-02 well shows temperatures of 249.89–302.92˚C, SH-03 well show temperatures of 231.09–280˚C. The mineral geothermometer calculation, which is correlated with the geochemical fluid geothermometer, shows valid results. This is evident from the estimated reservoir temperatures of the two geothermometers.

Integrated Remote Sensing and GIS Analysis for Landslide Susceptibility Assessment along the Trenggalek–Ponorogo Road, East Java Province, Indonesia

Landslide, the second most common hazard in Indonesia, after an earthquake, is causing enormous losses of public infrastructures with subsequent economic disruptions. Roads are the most frequent public property which is affected by landslides. Due to the geomorphological condition of Indonesia, the construction of roads often intersects the mountainous topography. The Trenggalek–Ponorogo Road is one of the roads passing through mountainous terrains that are very susceptible affected by landslides. The road has an important role as the main transportation connector of some regencies in East Java Province. Landslide mitigation strategies along the Trenggalek–Ponorogo Road are needed to prevent enormous losses. This research was aimed to conduct a remote sensing-based assessment of landslide susceptibility areas along the Trenggalek–Ponorogo Road. The landslide susceptibility areas were assessed by considering landslide triggering parameters; those were topographic slope, distance to geological structure, distance to stream, lithology, and land use/land cover. The landslide triggering parameters were presented in spatial data and processed using Geographic Information System (GIS) technology. The Analytical Hierarchy Process (AHP) method was applied to integrate the landslide triggering parameters which have the degree of effect to determine Landslide Potential Index (LPI). The resulting LPI delineated the area into four susceptibility zones: very high, high, moderate, and low, which were presented as landslide susceptibility map. The susceptibility map was then validated by landslide occurrences inventory in the study area. The very high susceptibility zones, which are strongly predicted affecting the Trenggalek–Ponorogo Road, are located in Nglinggis and Grogol Village.

The Magnificent of Geosites as Geoheritage Potential in Djuanda Grand Forest Park Area, Bandung, Indonesia

Djuanda Grand Forest Park (Tahura Djuanda) at Bandung, West Java Province, built-in 1985, has functioned as a conservation area concerning Indonesia’s biodiversity. It is the first Grand Forest Park among 27 of them, which was built in Indonesia. In addition, Djuanda Grand Forest Park potential for geodiversity and geoheritage concerning the geological history of the Tangkuban Parahu volcano formation since ancient times, so that conservation is needed. The Indonesian Geological Association (IAGI), in collaboration with the Djuanda Grand Forest Park Institution and the Geological Agency, researched the geoheritage potential of the Djuanda Grand Forest Park for accelerating the Tahura Djuanda to be a geoheritage area. The method used in this study consist of inventorying, identifying, analyzing, and mapping each geodiversity/geoheritage. The finding is that the Djuanda Grand Forest Park area has seven geoheritage potentials: the Dago Waterfall Lava, Pahoehoe Lava, Lalay Waterfall Lava, Omas Waterfall Lava, Ignimbrite at “Gua Belanda” and “Gua Jepang” and the Kraton Cliff Fault Scarp. By establishing the Djuanda Grand Forest Park area as a geoheritage area will expose more information about the geological history of Tangkuban Parahu Volcano’s formation through some interpretation boards (signboards) installed at each geoheritage location for conservation and education purposes through tourists visit points. In addition, West Java will be the second province to have a geoheritage after the Geoheritage of Pandeglang Regency, located in Banten Province.