Overpressures in the Northern Malay Basin: Part 1 - Origin and Distribution

Abstract

Pore pressure data and sonic velocity-vertical effective stress plots from 31 wells reveal that overpressures in the northern Malay Basin are primarily generated by fluid expansion and located basin-wide within the 2A, 2B and 2C source rock formations. Overpressure magnitude increases towards the basin-centre, with maximum pore pressure gradients of >19.0 MPa/km observed in the southeast of the study area. The overpressures are predominately associated with gas, with gas sampled in over 83% of overpressure measurements. The association of overpressures with gas, combined with a regional geology that largely precludes other fluid expansion overpressure mechanisms, provides the first convincing in-situ evidence for basin-wide gas generation overpressure. Overpressure magnitude analysis suggests that gas generation accounts for approximately 70-50% of the measured excess pore pressure in the region, with the remaining 30-50% being generated by coincident disequilibrium compaction. Thus, the data herein indicates that gas generation, if acting in isolation, produces a maximum pressure gradient of 15.2 MPa/km (0.672 psi/ft), and not lithostatic magnitudes as is often hypothesized. The gas generation overpressures in this study are not associated with a significant porosity anomaly and thus represent a major drilling hazard, with traditional pore pressure prediction techniques underestimating pressure gradients by 2.3±1.5 MPa/km (0.102±0.066 psi/ft). However, pore pressure prediction may be attempted using a calibrated empirical model after careful picking and smoothing of the velocity data.