Rapid burial and high rates of sedimentation in the Malay Basin has lead to development of overpressure by disequilibrium compaction. Overpressure developed by this process can be quantified using industry-standard techniques that rely on porosity/effective stress relationships. However, where thermally-driven secondary processes create overpressure, porosity-based analysis that uses sonic (or seismically-derived velocity data) and resistivity data as a measure of porosity change underestimate overpressure. These processes will be active in relatively shallowly-buried shales in basins with high geothermal gradient such as the Malay Basin.
Using comparative datasets from several regions where secondary overpressure generation are present (Gulf of Mexico; Halten Terrace, Mid-Norway and the Malay Basin), we discuss such secondary mechanisms and their quantification by integrating velocity vs. density cross-plots with understanding of basin history. Analysis of velocity vs. density relationships is a powerful tool to help discriminate overpressure generating processes and using this technique and log data from the Malay Basin, we identify load transfer (where rock compressibility is affected) as present, in addition to unloading and cementation effects documented by previous authors. Overpressure generated by load transfer may only be partially detected by fluid expansion-based relationships such as Bowers (1994), leading to inaccurate pre-drill pressure predictions.
The identification of load transfer (and cementation) processes and their quantification is vital to accurate prediction of pore pressure in hydrocarbon-charged.