Quantitative Estimation of Loading and Unloading to Total Overpressure: The Analytical Results from the Lower Kutai Basin

We apply a modified Bowers Method to analyze the contribution of loading and unloading to total overpressure magnitude in the shelfal area of the Lower Kutai Basin. The method combines Bowers Method with density-sonic cross plots to differentiate contributors to overpressure. This method has a strong physical background, and has proven to give satisfactory overpressure estimation in the study area.

Several processes related to overpressure and compaction in the study area have also been deduced:

1. Top of overpressure is caused by unloading due to gas generation
2. Gas generation on its own can produce overburden-scale overpressure magnitude
3. Loading starts to contribute to overpressure at depth, where the lithology is dominated by mudrocks
4. Effective stress history experienced by mudrocks before unloading
5. The presence of two compaction lines and therefore two associated velocity-effective stress relations.

Our analysis resolves a long debate (for more than 25 years) related to generating mechanism of overpressure in the study area.

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