--> Hypothesis: Significant Erosion During the Middle Miocene Unconformity Age and Its Effect from Hydrocarbon Generation in the Gulf of Thailand, Fujiwara, Masashi, #90100 (2009)

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Hypothesis: Significant Erosion During the Middle Miocene Unconformity Age and Its Effect from Hydrocarbon Generation in the Gulf of Thailand

Fujiwara, Masashi1

1Exploration & Production Division, Mitsui Oil Exploration Co., Ltd., Tokyo, Japan.

More than 20 oil and gas fields are located in the Pattani Trough in the Gulf of Thailand. The Pattani Trough is rift type sedimentary basin of Tertiary age and is approximately 200 kilometers in length and 50 kilometers in width. Maximum thickness is more than 7,500 meters divided into five sedimentary units from Unit 1 to Unit 5. Main reservoirs are deltaic to fluvial sandstones (Units 3 and 4) and the source rocks are lacustrine shales of Oligocene age and coaly shales of Middle Miocene age (Units 1 and 3, respectively). Structural style is graben-anticline or collapsed anticline.

Although the presence of the Middle Miocene Unconformity (MMU) of 10.5 Ma is well understood in the Gulf of Thailand, and is commonly observed in other areas of offshore South East Asia, its eroded thickness is not well documented because its erosional surface is located in Unit 4 in which key marker beds are poorly developed due to the predominant continental sedimentation. There has been little attention paid to the eroded thickness of sediment as an input to basin modeling. The eroded thickness in the Erawan gas field was estimated to be approximately 4,500 feet based on the shale compaction trend analysis using sonic log data. Interval velocities at the Erawan-1 well changes from 8,400 feet/sec in Unit 5 to 10,800 feet/sec in Unit 4 below the MMU with an increase of more than 20% but only within a couple hundred feet apart in actual separation. Another supporting evidence for erosion at the MMU is the “Paleo-Anticlinal structure” that was present at a time of the MMU.

There is no report on reverse faults in the
Gulf of Thailand although more than 2,000 wells have been drilled so far. This observation contradicts the above hypothes. However, there are many large anticlines of Mesozoic and Paleozoic sediments onshore Thailand (Khorat) formed by the Himalayan orogeny during Tertiary age. Therefore, it is most reasonable to assume that this kind of deformation may have also occurred in the Gulf of Thailand.

This hypothesis suggests that this significant erosion at the MMU age may have an impact on the pattern of hydrocarbon generation and accumulation in the
Gulf of Thailand.


AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil