Analysis of Overpressure Mechanisms in a Field of Southwestern Malay Basin

This study demonstrate the utilization of wireline logs for pore pressure mechanism analysis in a field of southeastern Malay basin. Development of overpressures means that fluid movement in the pores is retarded both vertically and laterally. Detailed knowledge of lithology, rock history, and pressure mechanism is important for overpressure prediction. In many tertiary basins overpressure is mainly generated by compaction disequilibrium due to high deposition rate and low permeability in shales. In Malay basin, temperature also plays an important role in generating overpressure at shallow depth as geothermal gradient is very high 40-60 oC. Pore pressure data from 10 wells is used to for mechanism analysis. Overburden pressure is calculated for all the wells by using density log and sonic velocity-vertical effective stress, vertical effective stress-density and pore pressure cross plots are used for mechanism analysis. The results obtained from the cross plots reveal that in the study area overpressure is generated by both primary (undercompaction) and secondary mechanism such as chemical compaction and fluid expansion/hydrocarbon generation in different wells. Overpressure magnitude analysis suggests that overpressure generated due to secondary mechanism is very high as compare to primary mechanism. Eaton's method with exponent 3 for sonic velocity under-predicted the pore pressure in the wells where both primary and secondary mechanism are present but in such wells pore pressures are reasonably predicted by using higher Eaton exponent. Overpressure behavior is very complex in the study area, in some wells it start at shallow depth and increase gradually with depth whereas in some wells it start at greater depth and ramp up quickly, this complex behavior also shows the presence of different mechanisms. For the safe drilling and completing the well economically, knowledge of overpressure is essential. Overpressure mechanism analysis is very important for overpressure prediction and it will help in understanding the behavior of pore pressure and applying the appropriate prediction method .?Accurate overpressure prediction is very important for construction of deep water seafloor facilities in HPHT environments.