BASEMENT PETROLOGY OF RENGASDENGKLOK AREA, WEST JAVA, AND ITS IMPLICATION TO TECTONIC AND HYDROCARBON RESERVOIR POTENTIAL

• Main Author: SETIAWAN (NIM: 32007001); Tim Pembimbing : Prof. Dr. Emmy Suparka; Prof. Dr. Eddy Ariyono Su, TAVIP
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/19538
• Institution: Institut Teknologi Bandung

Exploration drilling campaign on basement reservoirs in West Java, Indonesia, has resulted in some discovery of significant hydrocarbon resources. This success story led to further basement exploration activity in West Java, including Rengasdengklok Area, which is the area of interest of this study. The area is located in the North West Java Basin, which consists of Ciputat and Kepuh Sub-basins and Rengasdengklok High. These sub-basins and high are part of north – south trending horst and grabens formed in the early Tertiary extensional tectonic regime. <br /> <br /> Basement of the study area, which is assigned to pre-rift sequence in tectonostratigraphy terminology, can be classified lithologicaly into four categories, those are plutonic igneous rocks (granodiorite, monzonite, diorite and microdiorite), altered pyroclastic rocks (crystal and lithic tuffs), recrystallized sedimentary rocks (dolomitic limestone and shale) and metamorphic rocks (actinolite-biotite schist) <br /> <br /> The basement was defined by using seismic data, wireline logs and petrograpic analysis. Basement reflector was easily traced in a regional seismic section, which was tied to basement depth of RDK-2 well. Onlap features were common especially in the Ciputat Subbasin. <br /> <br /> Interpretation of formation micro imager (FMI), indicated the difference between basement character and the overlying sediments. FMI data of PDM-1 well shows a distinct contact of dolomitic limestone basement with the overlying sandstone and shale of syn-rift sequence (Talang Akar Formation equivalent). The basement is characterized by great quantity of open as well as closed fractures indicating that the rock had been deformed. Vuggy porosity is also common in this limestone representing leaching process that involved meteoric water in aerial diagenesis. <br /> <br /> Deformation feature can also be seen in thin section petrographic observation. A number of thin section of dolomitic limestone and shale basement samples taken from some wells in Pondok Makmur field, exhibit an intensive fracturing. <br /> <br /> Undulating extinctions and bended veinletts were observed in some limestone and shale samples respectively. These features of fracturing, undulating extinction and bended veinletts are strong indication of deformation on the basement. Meanwhile, the overlying sediments of the Talang Akar equivalent does not exhibit such features. <br /> <br /> Biostratigraphic analysis of cutting samples from several wells in Pondok Makmur Field resulted the age of basement of dolomitic limestone and shale ranged from Eocene to Late Oligocene. Based on this analysis result, the timing of the rifting, that responsible in formation of rift basins in Northwest Java Basin, could be defined. <br /> <br /> Geochemical analysis on several igneous core samples obtained from four wells in Rengasdengklok high was carried out in order to define the tectonic environment and magmatic affinity of those igneous rocks. Major elements analysis indicated that the parent magma was of calc-alkaline affinity that has undergone co-magmatic differentiation. Whilst the trace and rare earth elements analysis revealed that the magma was classified as orogenic type which related to subduction zone. <br /> <br /> Hydrocarbon occurence in basement rocks as found in Pondok Makmur Field, was controlled by petroleum systems that have been proven in this area. The dolomitic limestone, which is the main basement reservoir in Pondok Makmur field, has a quiet good porosity. Formation and development of vuggy and fracture porosity in the limestone is suggested to have occured coincident with the aerial exposure of the rock. Tectonic processes have triggered the fracturing and the presence of meteoric water into the exposed limestone have caused the diagenetic processes, including dissolution, which created vuggy porosity, and cementation, to take place effectively. <br /> <br /> Recent position of dolomitic limestone reservoir is on the northeastern flank of Ciputat subbasin, covered by a proven source rock of the lower Talang Akar Formation. Hydrocarbon migration could occur horizontally as well as vertically from the source rock with a same depth or deeper position to the limestone as basement reservoirs.