Origin of Overpressure in a Field in the Southwestern Malay Basin

Abstract

Summary This study demonstrates the use of wireline logs for the overpressure-mechanisms analysis in a field in the southwestern Malay basin. The development of overpressure means that the fluid movement in the pores is retarded, both vertically and laterally. In many Tertiary basins, overpressure is mainly generated by compaction disequilibrium caused by a high deposition rate and low permeability in shales. In the Malay basin, temperature and high-heat flow also play an important role in generating overpressure at a shallow depth, because the geothermal gradient is very high (40–60°C/km). Pore-pressure profiles and crossplots of sonic velocity/vertical effective stress and of velocity/density are used to derive the overpressure-generating mechanisms. The results obtained from the crossplots of 10 wells reveal that in the study area, overpressure is generated by both primary (compaction-disequilibrium) and secondary (fluid-expansion) mechanisms. The overpressure-magnitude analysis suggests that the overpressure generated by the secondary mechanism is very high compared with the primary mechanism. In all the wells, the Eaton (1972) method with an exponent of 3 gives good prediction when overpressure is the result of the compaction-disequilibrium mechanism, but it underpredicted the high pore pressure where the fluid-expansion mechanism is also present. However, by use of a higher Eaton exponent of 5 for the fluid-expansion mechanism, the overpressures are predicted quite well. The Bowers (1995) method, by use of the unloading parameter (U) of 6, is also used for pressure prediction and it gives a reasonably good prediction in the high-overpressure zone of all the wells.

Rock physics and geomechanical application in the interpretation of rock property trends for overpressure detection

“…greater than 0.445 psi/ft). The mechanisms which generate these abnormal pore pressures in subsurface reservoirs can be somewhat complex depending on the tectonic history, fluid expansion mechanism, and material constituents (Kukla et al 2011;Satti et al 2015).…”

Identification of stress regimes by using 1D geomechanical modeling of clastic and carbonate reservoirs

“…The Eaton's (1972) method is empirical, and its regionally defined exponent (the Eaton's exponent) can be easily varied to calibrate the trend to predict the pore pressure generated by different mechanisms [8]. In the sediments, where overpressure is generated by undercompaction mechanisms, an Eaton's exponent of 3.0 is typically used for pore pressure prediction.…”

Overpressures Induced by Compaction Disequilibrium Within Structural Compartments of Murree Formation, Eastern Potwar, Pakistan

“…The mechanisms causing overpressures include, compaction disequilibrium, gas cracking/hydrocarbon generation, aquathermal expansion, compression due to tectonics, transformation of clay minerals (e.g., illitization), hydraulic head, and hydrocarbon buoyancy (Yassir et al, 2002;Gutierrez et al, 2006;Zhang, 2013). The majority of abnormal pressure events involving compaction disequilibrium have been observed in geologically younger basins with rapid sedimentation (Satti et al, 2015). Examples for these cases include U.S. Gulf Coast, Beaufort Sea, Niger Delta, Mackenzie Delta, Adriatic Sea, North Sea, Mahakam delta, Malay Basin, Nile Delta, Eastern Venezuelan Basin (Trinidad), and the Potwar Basin of Pakistan (Heppard et al, 1998;Law and Spencer, 1998;Nelson and Bird, 2005;Morley et al, 2011).…”