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.
The Tembungo field located offshore Sabah is a highly faulted anticlinal structure where oil and gas accumulations occur in different fault blocks. This paper discusses source rock potential, characteristics of Tembungo oils, and oil-oil and oil-source rock correlations between oils and source rocks from Tembungo and adjacent fields. The shales of the Tembungo wells have poor to fair source rock potentials mainly of Type III gas-prone organic matter and minor contributions from Type II oil-prone organic matter. Maturity data show that the organic matter in the Tembungo well sections are immature. The Tembungo crude oils from the different fault blocks are genetically similar, paraffinic, contain low sulphur and wax, and have moderate API gravity. The presence of C 24-tetracyclic terpanes, oleananes, C 80-resin triterpanes and predominance ofC 29-steranes in all the samples suggest that the oils are derived from source rocks of terrigenous origin containing different mixtures of land-plant organic matter including resins. GC and GeMS analyses indicate that the crude oils produced from the same fault block have similar biomarker distributions but some variations occur in oils from different fault blocks. These variations are interpreted as due to the effect of migration and biodegradation whereby each fault block has a separate fluid system and there is no or very little intermixing between them. Oil-oil correlation indicates that the oils in the study area have similar biomarker fingerprints and could have been generated from the same source rock type containing abundant terrigenous organic matter.
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.
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