Abstract

E-log provides not only reservoir parameters to evaluate hydrocarbon reserves but also geological information on erosion thickness in addition to sedimentary environments (Pirson, 1970 and Zimmerle, 1995). Erosion thickness is one of the key issues for hydrocarbon generation, expulsion, migration and trapping in oil and gas exploration.

Although there are several methods to estimate erosion thickness, two kinds of methods were applied in this study for reasons of practicality and accuracy, namely;

  • conventional reconstructed section method; and

  • Magara method (1978) with use of sonic logs.

Magara method is the only method which uses sonic logs to estimate erosion thickness based on shale compaction trend.

Shale compaction method was applied to the Pattani Trough of the Cenozoic basin in the Gulf of Thailand for 122 wells in 13 oil and gas fields. However, the method gave extraordinarily large values for erosion thickness of the Middle Miocene Unconformity (MMU) compared to the results of conventional reconstructed method because of the high velocity of Sequence 4 located immediately below the MMU. The main reasons for the high velocity of Sequence 4 are considered to be:

  • hardening by hydrothermal water;

  • high concentration of certain rocks or minerals such as calcite, iron, pyrites etc.; and

  • alternation of clay mineral by heat through "chemical compaction" (Pollastro, 1993 and Bjorlykke, 1999).

It is evident that a hydrothermal event occurred locally in the Pattani Trough (Sasaki, 1986 and Fujiwara & Sasaki, 1988). Mud logs obtained at 7 wells and the temperature gradient derived from bottom hole temperature (BHT) at more than 70 wells were reviewed. There appears to be no strong relationship between lithology and the high velocity of Sequence 4 and temperature gradient is high [2.74°F / 100 feet (4.99°C / 100 m)] on average which is almost identical to the island of Sumatra in Indonesia. Therefore, alternation of clay mineral from smectite to illite conversion under high temperature conditions is one of the main factors that explain the high velocity of Sequence 4.

In case of application of the Magara method in hot basins such as those in South East Asia, careful attention should be paid. Conventional reconstructed section method is also recommended together with the Magara method.

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