RELATIONSHIP BETWEEN MUDSTONES COMPACTION AND OVERPRESSURE MAGNITUDE: CASE STUDY OF NORTH SUMATRA BASIN

• Main Author: Syaiful, Mohammad
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/38672
• Institution: Institut Teknologi Bandung

In the Aru Field, North Sumatra Basin, there is an interesting phenomenon, i.e. wireline logs show strong reversals indicating high overpressure, yet the mudweight used during drilling was low indicating low overpressure. Shifting in compaction line due to carbonate cementation in shallower and hydrostatic zone is proposed as main hypothesis to explain this circumstance. The alternative hypothesis is that the mudweight data in the wells showing the above phenomenon were drilled in underbalance condition. The result of clay mineralogical analysis through wireline log and clay mineral petrography shows that the carbonate minerals are absent to occur in significant amount in the hydrostatic zone. Therefore, the shifting in compaction line hypothesis fails to pass the hypothesis testing. The result of clay mineralogical analysis, however, gives some new insights regarding the understanding of clay compaction and overpressure in the study area, i.e.: 1) the result of clay petrography confirms the result of wireline log analysis showing that there are two compaction lines in the study area, i.e. smectitic and illitic compaction lines, 2) grain to grain contact still can be observed at the temperature greater than 1000C meaning that mechanical compaction is still active at the above temperature. The alternative hypothesis, i.e. the drilling was performed in underbalance condition has successfully passed the hypothesis testing, and is therefore valid to explain the above phenomenon. However, this underbalance condition can be said as special underbalance, in the sense that the drilling was in underbalance in claystone dominated zone, while it was close to or slight overbalance with respect to sandstone pressure. The underbalance in the claystone dominated zone is proved by the occurrence of several drilling problems such as cavings. Meanwhile the close to or slight overbalance is proved by the absence of kicks and loss circulations in the sandstone section. This research results in new insights of the understanding of overpressure in the North Sumatra Basin and may also contribute to overpressure science in general. Three important points resulted from this study regarding these insights are: 1) the overpressure generating mechanism in the study area is disequilibrium compaction, and this happens in the illitic compaction zone, 2) compressional tectonic helps in maintaining overpressure condition through the present time, 3) lateral reservoir drainage has been occurring in the study area.