Indonesian Journal on Geoscience

5

Authors: Sri Yudawati Cahyarini

Abstract - Variation of δ13C content in coral skeletons shows the influence of metabolic fractionation in aragonite coral. Understanding coral δ13C variation can thus be useful to more understand e.g. past bleaching event which is further useful for coral health and conservation. In this study, δ13C content in Porites coral from Labuhan Bajau, Simeulue Islands was analyzed. To know the correlation between variation of coral δ13C and light intensity, the monthly varia- tion of coral δ13C is compared to solar radiation and cloud cover. The result shows that for the period of 2003 to 2008, coral δ13C shows it is well correlated (r=0.42 p=0.153) with cloud cover variation in annual mean scale. Meanwhile, in seasonal mean variation, coral δ13C is strongly influenced (r=0.85 p<0.0001) by cloud cover with 1 - 2 month time lag. Comparing to the solar radiation (cloud cover), SST influences dominantly the variation of coral δ13C from southern Simeulue Island waters (LB sample) in an annual mean scale than in a seasonal scale. Keywords: δ13C, coral, Porites, solar radiation, cloud cover, SST

10

Authors: Euis Tintin Yuningsih, Hiroharu Matsueda, Mega Fatimah Rosana

Abstract - The gold-silver ores of western Java reflect a major metallogenic event during the Miocene-Pliocene and Pliocene ages. Mineralogically, the deposits can be divided into two types i.e. Se- and Te-type deposits with some different characteristic features. The objective of the present research is to summarize the mineralogical and geochemical characteristics of Se- and Te-type epithermal mineralization in western Java. Ore and alteration mineral assemblage, fluid inclusions, and radiogenic isotope studies were undertaken in some deposits in western Java combined with literature studies from previous authors. Ore mineralogy of some deposits from western Java such as Pongkor, Cibaliung, Cikidang, Cisungsang, Cirotan, Arinem, and Cineam shows slightly different characteristics as those are divided into Se- and Te-types deposits. The ore mineralogy of the westernmost of west Java region such as Pongkor, Cibaliung, Cikidang, Cisungsang, and Cirotan is characterized by the dominance of silver-arsenic-antimony sulfosalt with silver selenides and rarely tellurides over the argentite, while to the eastern part of West Java such as Arinem and Cineam deposits are dominated by silver-gold tellurides. The average formation temperatures measured from fluid inclusions of quartz associated with ore are in the range of 170 – 220°C with average salinity of less than 1 wt% NaC equiv. for Se-type and 190 – 270°C with average salinity of ~2 wt% NaCl equiv for Te-type. Keywords: epithermal gold-silver deposit, fluid inclusions, selenides, Se-type, tellurides, Te-type, western Java

14

Authors: Erawati Fitriyani Adji, Febrian Asrul, M. Aidil Arham, Bayu Wisnubroto

Abstract - The carbonate on Fika Field has a special character, because it grew above a basement high with the thick- ness and internal character variation. To develop the field, a proper geological model which can be used in reservoir simulation was needed. This model has to represent the complexity of the rock type and the variety of oil types among the clusters. Creating this model was challenging due to the heterogeneity of the Baturaja Formation (BRF): Early Miocene reef, carbonate platform, and breccia conglomerate grew up above the basement with a variety of thickness and quality distributions. The reservoir thickness varies between 23 - 600 ft and 3D seismic frequency ranges from 1 - 80 Hz with 25 Hz dominant frequency. Structurally, the Fika Field has a high basement slope, which has an impact on the flow unit layering slope. Based on production data, each area shows different characteristics and performance: some areas have high water cut and low cumulative production. Oil properties from several clusters also vary in wax content. The wax content can potentially build up a deposit inside tubing and flow-line, resulted in a possible disturbance to the operation. Five well cores were analyzed, including thin section and XRD. Seven check-shot data and 3D seismic Pre-Stack Time Migration (PSTM) were available with limited seismic resolution. A seismic analysis was done after well seismic tie was completed. This analysis included paleogeography, depth structure map, and distribution of reservoir and basement. Core and log data generated facies carbonate distribution and rock typing, defining properties for log analysis and permeability prediction for each zone. An Sw prediction for each well was created by J-function analysis. This elaborates capillary pressure from core data, so it is very similar to the real conditions. Different stages of the initial model were done i.e. scale-up properties, data analysis, variogram modeling, and then the properties were distributed using the geostatistic method. Finally, after G&G collaborated with petrophysicists and reservoir engineers to complete their integrated analysis, a geological model was finally created. After that, material balance was needed to confirm reserve calculations. The result of OOIP (Original Oil in Place) and OGIP (Original Gas in Place) were confirmed, because it was similar to the production data and reservoir pressure. The model was then ready to be used in reservoir simulation. Keywords: reservoir modeling, carbonate, rock and oil types, simulation, Fika Field