THE CAUSE OF WIRELINE LOG RESPONSE DISCREPANCY WITH OVERPRESSURE MAGNITUDE IN THE SOUTHERN PARTOF NORTH SUMATRA BASIN

• Main Author: GORGA SIAHAAN, DARIUS
• Format: Theses
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/15279
• Institution: Institut Teknologi Bandung

The North Sumatra Basin is known as one of the Tertiary sedimentary basin with overpressure character. Overpressured interval within the basin can be found in the post-rift sequence of Baong Formation which is dominated by low permeability fine-grained sediments. This thesis analyzed wireline logs, direct pressure measurements, drilling mudweight and drilling events in the Pertamina’s North Sumatra Basin onshore working area (NAD 1 and NAD 2 working areas). The Eaton’s method was used to estimate overpressure magnitude within the research area and it shows that Eaton’s exponent required to match overpressure magnitude is very low (1). The condition is reflected by strong sonic reversal, yet the overpressure magnitude is low. The research work is intended to investigate the cause of the discrepancy between strong sonic reversal with the magnitude of the overpressure within the shallower marine shale interval. Hypotheses that could be proposed to explain this phenomenon is the presence of carbonate cement in the shallower section that caused shifting the compaction line into more compacted region (low sonic velocity at shallower depth). Assumptions used in this research work are (1) drilling operations in the research area was conducted in underbalanced condition and (2) the presence of carbonate cement in the shallower section. Based on the petrophysical work using M-N Plot method, the occurrence of dolomite and calcite has identified within the shallower section of the shale which was deposited under marine environment.