In the Gulf of Thailand more than 4,000 wells have been completed since the first exploratory well, Surat-1 was drilled in 1971 and more than 30 oil and gas fields have been discovered starting with the first discovery of the Erawan structure in 1972 with first production in 1981. Most discoveries are located in the Pattani Trough and are of Cenozoic age. The Pattani Trough is rift type sedimentary basin approximately 200 kilometers length and 50 kilometers in width. Maximum basin thickness is more than 8,000 meters divided into five sedimentary units from Sequence 1 to Sequence 5. Two major unconformities have been identified, one of which is called Middle Tertiary Unconformity (MTU) and the other is Middle Miocene Unconformity (MMU). MTU is located in the deeper part of the basin and is deeper than can be penetrated by drilling whereas the MMU is located at a shallow depth. In addition to the Gulf of Thailand the MMU is commonly observed elsewhere in offshore South East Asia, but its erosional thickness and tectonic study are not well documented because the erosion surface is located in Sequence 4 where key marker beds are poorly developed due to the predominant continental sedimentation. Author estimated the erosional thickness of MMU for the 6 major gas fields located in the southern Pattani Trough based on “shale compaction trend” method proposed by Magara (1978) using sonic logs. Data from over 70 wells were used for this study. A result was obtained indicating more than 1,000 meters of erosional section for all fields in the study area. However, based on other geologic phenomena, such as tectonic movement, field structure, reconstructed structure, lithology, etc., this amount of erosion was considered to be too large. The causes of this erroneous value (high velocity on shale compaction trend of Sequence 4) is not clear at this moment but some possible causes could include chemical compaction due to high temperature as proposed by Bjorlykke (1999) and Puttiwongrak et al. (2011) and/or hydrothermal fluid entry into Sequence 4 precipitated in calcite veins through highly developed faults. Further study on causes of the high velocity of Sequence 4 and additional study in the northern part of the Pattani Trough will be needed to confirm whether high velocity shale in Sequence 4 is common and whether it is an indicator of the amount of erosion.