Journal of Applied Geology
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Cooling history (from magma ascent to lava extrusion) of the Watuadeg pillow lava, Berbah, Yogyakarta, Indonesia
No full textThe Watuadeg pillow lava (WPL) is known as one of the most famous pillow lava outcrops in Yogyakarta, Indonesia, and its origin has been attributed to rapid-cooling process of subaqueous lava extrusion. However, there is no quantitative evidence that implies such hypotheses. Therefore, this study aims to reduce such a gap by revealing the cooling times (t) of WPL on the basis of a quantitative approach. In particular, we measured the size distribution (CSD) and number density (MND) of plagioclase microlites from the core, medial, and marginal (rim) domains of WPL. We found that the CSD slope significantly increases towards marginal zones, namely 30.4ᵒ for the core, 53.4ᵒ for the medial, and 228.1ᵒ for the rim. Because CSD slope is inversely proportional to cooling time ( ), by assuming a typical plagioclase microlite growth rate (G) of 1×107 mm/s, it is therefore inferred that the rim experienced the fastest cooling time (±12.1 hours), followed by the medial and core ((±52.0 and 91.4 hours, respectively). The fact that MNDs value increases toward the marginal zones also does not deny this idea (0.3×1016 m-3 for the core, 1.4×1016 m-3 for the medial, and 2.4×1016 m-3 for the rim), as higher MND with the domination of acicular-spherulitic habit represents a higher degree of undercooling. Because microlite is syn-eruptive product, our estimation represents the cooling time of magma since it migrated from the reservoir to the surface
Source Rock Quality and 1D Maturity Model in Pendalian Sub-basin, Central Sumatra Basin
Get PDFPematang Formation is well known as the main source rock in the Central Sumatra Basin (Mazied et al., 2008). The formation which acts as source rock has been studied thoroughly in the central and eastern parts of the basin, but research regarding source rock in the western part of the basin, which is the study area, is very limited. This paper aims to define the quality of source rock from a geochemical view through the determination of organic content quantity based on the Total Organic Carbon (TOC) data, determining the type of kerogen, and thermal maturity of the source rock according to the values of Tmax and Vitrinite Reflectance (%Ro). Burial history and thermal maturity models (1D basin modeling) were also constructed to understand the timing of hydrocarbon generation. The results show that Pematang Formation among Sihapas and Telisa Formation has good organic content with TOC ranging from 0.2 to 42.48 wt%, and the maturity parameters indicate that the Pematang Formation has reached the mature stage. Both formations are dominated by Type II kerogen. 1D modeling of the SHT-1 Well indicates that the Pematang Formation is currently in the oil maturity window starting from early oil to main oil at 20.61 Ma, but the model of the SMB-1 Well has not reached the oil maturity window. Tectonic activity is estimated to have a significant effect on this difference when the uplift activity of Bukit Barisan in the Middle Miocene increased the maturity in the northwest, but the inversion that occurred in the Late Miocene resulted in uplift and erosion of young sediments, thus lowering the temperature in some area
Determination of Overpressure Zone and Its Mechanism in Baong Formation of the "Y" Field in North Sumatera Basin
No full textThe “Y” Field is one of the offshore areas in the North Sumatra Basin which is believed to have an important role in hydrocarbon exploration in the future. The North Sumatra Basin is known as an area that has high overpressure conditions and sometimes overpredicts in determining the amount of overpressure, especially in the Baong Formation. The purpose of this research is to determine the top and bottom overpressure zones, to know the vertical distribution of overpressure, and to find the main factors causing the overpressure in the Baong Formation.The data used in this study were 5 wells which have wireline log data, formation pressure data, leak of test, final well reports, mud logs, as well as 29 lines of 2D seismic data and 1 3D seismic data. The method used in this study was the Eaton method to determine pore pressure whereas the cross-plot wireline log method, the AI (acoustic impedance) inversion method and the stacking velocity were used to determine pore pressure.The study indicated that the overpressure zone is located in the Baong Formation at 1650 - 2108 m depth with a pore pressure of around 2891.70 - 3580 psi. The overpressure is caused by loading mechanism, namely disequilibrium compaction. This is influenced by the thickness of the formation above Baong Formation
Petrophysical Study and Rock Type Determination of Siliciclastic Reservoir: Case Study Sand of Bekasap Formation, AF Field, Central Sumatra Basin, Indonesia
Get PDFAn integrated subsurface study has been performed for a large and mature field at the Bekasap Formation in Central Sumatra Basin. The Bekasap Formation sand represents an undeveloped reservoir because of its heterogeneity. There are five sand reservoirs (BK1, BK2, BK3, BK4, and BK5) from Bekasap Formation, which each zone or layer bounded by a flooding surface. Each sand reservoir has particular characterization based on petrophysical properties that represent geological process. The petrophysical properties consist of shale volume, porosity, and water saturation obtained by wireline log calculation. This study uses conventional core data to validate the log calculation to achieve an accurate interpretation. Bekasap reservoir is a sandstone reservoir deposited in an estuarine with tide-dominated. Formation evaluation was done to determine the interest zone by petrophysical properties. The result well-log calculation and reservoir cut-offs showed the thickest reservoir in the BK 3 with the best average values of petrophysical properties with an average shale volume 0.32; porosity of 0.245. Otherwise, in rock type determination, four lithofacies are divided in the reservoir based on flow units. The sample RT 1 and RT2 provided the best reservoir zones with HFU1 and HFU2. The RT 3 and RT 4 dominated in HFU3 and HFU4 had the lowest potential zones of reservoir. The final findings showed a good correlation between sedimentologic analysis and petrophysical properties in the rock type determination. As a result, the best reservoir quality development is controlled by the depositional environment (texture and structure) rather than the diagenetic process in this reservoir. It is proven by petrophysical properties in BK1, and BK2 is coastal barrier sand (tidal sand bar) has more clean sand rather than in BK3 and BK4 deposited in the offshore bar
Engineering Geology and Slope Stability of West Pit Coal Mine of PT. Tawabu Mineral Resource, East Kalimantan, Indonesia
Get PDFThe research area was located in the west pit of the open pit coal mine of PT. Tawabu Mineral Resource (TMR) which is located in Bengalon District, East Kutai Regency, East Kalimantan Province, Indonesia. The research was driven by several landslides that occurred in the research area, but the engineering geological conditions and stability of the remaining slopes have not been evaluated. The objectives of this study were to better understand the engineering geological conditions and stability of the research area. The engineering geological conditions (i.e., geomorphology, rock and soil, geological structure, and groundwater conditions) were evaluated by photogrametric analyses, field observations, and analyses of borehole logs and laboratory test results. The slope stability analyses were firstly carried out by conducting back stability analyses of failed slope on the northern lowwall slope segment. The shear strength parameters obtained from the back analyses were then used for forward stability analyses of the remaining 10 lowwall and highwall slopes. The slope stability analyses involved deterministic and probabilistic analyses, under static and dynamic using the limit equilibrium method (LEM). The results showed that the research area and the surrounding consisted of two geomorphological units, namely the alluvial plain and structural hills. Rocks in the study area consisted of claystone, sandstone, and coal with a general layer strike direction of N59°E – N63°E with a dip of 19°-26°. These rocks were grouped into two lithological units, namely the alternating of claystone and sandstone unit and alternating of sandstone and claystone unit. The geological structures were identified on the highwall, from west to east namely major sinistral shear fault with a relative direction of NNE-SSW, two minor sinistral shear faults with a relative direction of NE-SW, and a major dextral shear fault with a relative direction of NW-SE. These geological structures were interpreted as being formed by the folding process. The groundwater level was estimated at a level of -45 m to 20 m. The slope stability analyses showed that only the East HW-4 slope, which was located on the east highwall, was unstable. It is recommended to optimize the slope by either lowering the groundwater elevation by 4 m from the actual level or by reducing the angle the overall slope to 31°
Stability Evaluation of Diversion Tunnel Portal Slopes at Lau Simeme Dam Site, Indonesia, using Limit Equilibrium Method
Get PDFConstruction of the Lau Simeme Dam used a tunnel as a diversion channel. Slopes at the diversion tunnel portals were prone to failure due to the tunnel excavation and earthquake. Earthquake load was not considered in the designs of the inlet and outlet portal slopes. This research evaluated stability of the tunnel portal slopes under static and earthquake loads using limit equilibrium methods of the Bishop Simplified and Morgenstern-Price. Input material properties for the slope stability analyses were obtained from evaluations of soil and rock cores, including determination of lithology type and rock mass quality based on Geology Strength Index (GSI), and laboratory testing. Evaluations of soil and rock cores indicated that the inlet portal slope consisted of residual soil, good quality tuff breccia, and good quality sandstone. The outlet portal slope consisted of residual soil, poor quality sandstone, poor quality tuff breccia, fair quality sandstone, fair quality tuff breccia, and good quality tuff breccia. The earthquake load coefficient determined by the seismic analyses based on the peak ground acceleration map for 10% probability exceedance in 50 year was 0.125 g. The slope stability analyses showed that the designed inlet and outlet portal slopes were stable under static and earthquake loads. The Bishop Simplified and Morgenstern-Price resulted in relatively similar Fs values. The Fs values of the inlet and outlet portal slopes decreased with the earthquake load application. Although Fs values of the outlet slope under static and earthquake loads met the requirements specified by SNI 8460:2017, the rock mass conditions, particularly the poor rock masses of layers 2 and 3, required special attention. Application of slope reinforcement methods, such as shotcrete, is suggested to further increase the slope stability
Assessment of Groundwater Vulnerability and Total Organic Carbon in the Shallow Groundwater of Wonosari City, Gunung Kidul, Yogyakarta, Indonesia
Get PDFWonosari city is the capital of Gunung Kidul regency of Yogyakarta Special Province. The city is located in the karst plateau overlaying a succesion of limestone lithologies. The groundwater on this city is shallow and people used this groundwater as their daily water supply. This study aims to determine the intrinsic groundwater vulnerability and to identify the TOC level in groundwater also to investigate the relationship between groundwater vulnerability and TOC levels in the groundwater. Field observation was conducted on 40 groundwater occurrences (dug wells, bore wells and springs) to investigate the aquifer types, overlying lithology and depth to groundwater. At the same time, 16 groundwater samples were taken from the dug wells and analyse for the TOC content. Secondary data was also collected from previous researches especially the information of sub-surface lithology. Groundwater vulnerability was evaluate by using GOD method and correlation between groundwater vulnerability and TOC in groundwater was analyzed spatially and statistically by using map pverlaying, linear regression and Spearman’s rho test, respectively. The results showed that the groundwater vulnerability in the city is dominated by extreme and high vulnerability, covers about 84 % of study area. The TOC level in the groundwater ranges between 4.532 mg/L to 6.849 mg/L which showing a process of pollution loading may occur. Linear regression and Spearman’s rho value show that the TOC levels and groundwater vulnerability has a moderate positive correlation with r2 value of about 0.324 for linear regression. Despite the moderate correlation, the research prove that the groundwater vulnerability map reveals on this study is good enough to represent the ability of natural condition in protecting the groundwater quality
Opak Fault Strand Delineation Using Merapi Slope Shifted Indicator
Get PDFAbstract After the Yogya earthquake occurred on May 27, 2006, the opinions of experts split into two groups regarding location of responsible fault to earthquake shacking; (1), First group argues that Opak Fault displacement caused the earthquake. Where fault line is commonly known runs along Opak River, striking from Parangtritis Beach to Prambanan. (2) Second group of experts stated at deferent opinion, that another fault displacement triggered the earthquake shock. Where is located at the East side of the Opak River about 10 Km. On the other-hand, this paper proposes an idea to unravel Opak Fault position by understanding, that main active fault movement underlying Bantul region has been influencing continuum mechanical process, onto Merapi Sediments surface since the early periods of continuing Merapi materials sedimentation. It should has been reflected to the surface landform at above the fault strand. Further, indicates to morphotectonic feature as an en echelon slope shifted alignment. Thus, it can be the way to assist in defining attribute of main fault (PDZ) areas by using its en echelon indicator of shifting slope alignment. This paper also presents the results of determining Opak fault line location by using Digital Elevation Model (DEM-NAS) to generate custom shading in approaching landform features. With a further doing specific landscapes observations over the entire of Young Merapi deposits. There are other challenges to alternate identification of buried basin faults fill, by doing carefully identifications in morphotectonic aspects over the entire South Slope Merapi flank. It traces from Kepurun village at about elevation 400 m towards Parangtritis at 25 m of elevation. Independent field morphotectonic data sets of such scarp, terraces, water springs alignment and active cracks are encountered in the vicinity of Merapi sediments slope shifted, particularly in Tirtomartani Jetis village Kalasan. Identification of such structures obtained from morphotectonic analysis results are regarded as reliable indicator of faults, which is efficiently can be found in field.Key word: Opak Fault, Morphotectonic, Slope Shifted, Delineation
Landslide Susceptibility Mapping of Menoreh Mountain Using Logistic Regression
Get PDFMenoreh mountain is one of the priority areas developed for tourism and to support sustainable development, it must pay attention to disaster aspects, one of which is landslides. The map published by Center for Volcanology and Geological Hazard Mitigation of Indonesia (PVMBG) has a regional scale, so it is necessary to have a more detailed landslide susceptibility map in the Menoreh Mountains. Identification and evaluation of the landslide conditioning factor were done using logistic regression so that the zonation of the probability of landslide susceptibility can be made. The data was used from field observation conducted at 372 locations including 129 locations where landslides occurred and from a local disaster management agency (BPBD) of 200 landslide locations. Significant landslide conditioning factors include slope, lithology, distance to lineaments, distance to river, and distance to road. The research area is divided into three susceptibility zones classified into low landslide susceptibility zone (0-0.33) covering 39.82%, moderate landslide susceptibility zone (0.34-0.66) covering 25.86%, and high landslide susceptibility zone (0.67-1.00) covering 34.31% of the whole area. Analysis using the logistic regression method has a model prediction accuracy rate of 90.5%, which means that it can predict landslide occurrence in the Menoreh Mountains accurately
Geochemistry of shield stage basalts from Baluran volcano, East Java, Sunda arc
Get PDFWe report petrography and geochemistry of basaltic lava flows from the shield stage of Baluran, a Quaternary volcanic center in the rear of East Java, Sunda Arc, Indonesia. These basalts contain abundant plagioclase, clinopyroxene, olivine, and minor magnetite. Geochemically, they resemble other medium-K calc alkaline basalts from eastern Java’s volcanoes, but they are less enriched in light ion lithophile elements (LILE) and Pb. The predicted primary basalt of Baluran lavas can be sourced to a more primitive primary melt composition which may also generate medium-K calc-alkaline magmas in the region. The fractionation trajectory of these primary magmas shows the importance of plagioclase, clinopyroxene, olivine, and magnetite phase removal from the melt. Regardless of the diverse composition of the derivatives, the calculated primary basalts from the eastern Java are all in the field of nepheline-normative. This finding suggests variably small degree of melting of clinopyroxene-rich mantle source is at play in the generation of these magmas. Our result further suggests that the clinopyroxene source rock is possibly present as veins in peridotite mantle which have experienced metasomatism by addition of slab-derived fluids at differing proportion