Publications

Previous researchers were agreed that the cause of overpressure in the shelfal area of the Lower Kutai Basin is disequilibrium compaction. This conclusion was based on the clear evidence that the reservoirs of mid–late Miocene age have been buried moderately rapidly by a thick, mudstone-dominated succession. We have tested the disequilibrium compaction hypothesis through wireline log analysis, and determined that there is no compaction disequilibrium below the top of overpressure at the present day. The sonic and resistivity log trends display reversals at the top of the transition into hard overpressure, but the downward trend of increasing density continues through the top of overpressure. Hence unloading mechanisms of overpressure generation are implicated. Further analysis shows that the top of overpressure is at temperatures just over 130°C, indicating that gas generation is the principal process of overpressure generation. There is probably an additional contribution from clay diagenesis, especially illitization of kaolinite. At such high temperatures, mudrock compaction is dominantly chemical, and the rocks are overcompacted in the conventional sense of the term as applied to mechanical compaction. We also observe that where the pore pressure nears the lithostatic stress, at greater depths and temperatures, the density logs display reversals. These reversals are not compatible with rock fracturing, since the sonic log character is too consistent within them. We suggest that the density reversals are due to ‘chemical undercompaction’. During smectite-illite conversion, the escape of released water may have been inhibited by low permeability, so the pore pressure approached the lithostatic stress while the mudrocks became cemented with abnormally high porosity. An alternative possibility is lithology change, with pro-delta shales underlying delta-front deposits. Keywords: Lower Kutai Basin, unloading, gas generation, chemical undercompaction