OVERPRESSURE AND TECTONIC INFLUENCE IN MADURA SUB-BASIN, EAST JAVA BASIN

• Main Author: Warman Reksalegora, Sena
• Format: Dissertations
• Language: Indonesia
• Subjects: Teknik (Rekayasa, enjinering dan kegiatan berkaitan)
• Online Access: https://digilib.itb.ac.id/gdl/view/55154
• Institution: Institut Teknologi Bandung

The study area is located in Madura Sub-basin which is part of East Java Basin. The naming of Madura Sub-basin refers to the sub-basin which is geographically located south of Madura Island. East Java Basin is a Tertiary backarc basin and one of the targets for hydrocarbon exploration and development in Indonesia. The Madura sub-basin is well known for its overpressure as well as its complex tectonic and sedimentation histories. This study evaluates the relationship between the overpressure development and the influence of tectonic activities. The study in the role of tectonic in overpressure development and the regional distribution of pore pressure and overpressure in East Java Basin and specifically in Madura Sub-basin has never been conducted. The presence of overpressure in the study area is clearly seen from the integration of all well data. Extensive data from 30 exploratory wells and approximately 5,900 km of offshore 2 Dimension (2D) seismic lines which consist of SEGY and navigation data are used in this study. The use of higher mud weight during drilling than the hydrostatic pressure in some wells is the first indication of the presence of overpressure in the study area. The majority of wells experienced overpressure condition. Based on direct and indirect pressure measurements and other indication from drilling report, overpressure presents in both onshore and offshore locations. Despite of large variation in the top of overpressure, generally the top of overpressure is located at depth 300-1400 meters TVDSS at Pleistocene-Recent interval. Based on seismic data, structural pattern of a rift basin which experienced compressional tectonics and inversion can be seen clearly. Rifting is clearly observed in the eastern part at interval below Kujung. Folding and thrust faults as the result of compressional tectonics can be seen in the northern and southern part and the effect is decreasing toward the east. Generally, the structure of the study area consists of highs in the northern and southern areas and elongates in a westeast direction. The highs are separated by a west-east elongated lows and ridges in between. This structural configuration is clearly formed by north-south compressional tectonics. Isopach maps show the depocenter during Early Oligocene to Late Miocene was located in the northern part and moved to the center starting from the western side after Late Miocene to Recent. The central part was v originally a high and subsided to become a new depocenter with thick sediments especially in the western part due to inversion during Plio-Pleistocene to Recent. The quantitative analysis of vertical (Sv), minimum horizontal stress (Shmin) and maximum horizontal stress (SHmax) magnitude with their distribution in Pleistocene horizon shows west-east elongated pattern to follow the structure. In general, the magnitude of the three stresses increases toward the central area with the maximum magnitude at the eastern side. The stress regime is predicted based on the average stress ratio between minimum horizontal stress and vertical stress which is ? 1 (Shmin/Sv ? 1) and the average stress ratio between maximum horizontal stress and vertical stress which is ? 1 (SHmax/Sv ? 1). Both ratios indicates that the study area is in strike-slip fault stress regime in Pleistocene horizon (SHmax ? Sv ? Shmin). The top of overpressure in the study area does not relate to temperature, source rock maturity nor clay diagenesis. The source rock maturity and clay diagenesis are predicted to contribute to the development of hard overpressure. Overpressure is related to wells with rate of deposition of more than 1000 meter/million years in the Pleistocene-Recent interval which at the same time experienced compressional tectonics as shown by higher maximum horizontal stress than vertical stress. The active compressional tectonics during Pleistocene-Recent is concluded as the overpressure generating mechanism in the study area as demonstrated by the ratio between maximum horizontal stress and vertical stress ? 1 (SHmax/Sv ? 1). This study generates new method in pore pressure prediction for area with compressional tectonics which uses mean stress and the relationship between velocity and mean effective stress. The new method validates the influence of compressional tectonics to the overpressure generation. This new method gives prediction that closely matches direct and indirect pressure measurement. The result of the pore pressure prediction provides the pore pressure and overpressure distribution in Pleistocene horizon which follows the structure. It is elongated in west-east direction and increase in magnitude toward the basinal lows.