Pattani Basin

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Figure 1. The Pattani Basin is located in the Gulf of Thailand. (Courtesy of Bustin and Chonchawalit, 2010)

The Pattani Basin is located offshore in The Gulf of Thailand. The size of this basin in modest, with an aerial extent of about 10,000 km2. The Gulf of Thailand region is comprised of numerous North-South trending rift basins. The Pattani basin is the most prolific Hydrocarbon basin in Thailand. Due to this, the Pattani Basin has been the focal point for petroleum exploration in Southeast Asia. Exploration started in the late 1960’s and since then, countless oil and gas explorations have brought about extensive exploration programs.[1]

Tectonic History

Figure 2. This image contains a geologic timescale. (Courtesy of Schwab and Gayes, 2009)

This Basin has a Tertiary section which occupies 8500 m created during the Oligocene period and younger sediments which would be the basement of The Basin occupying less than 300 m over the shallow sub cropping pre-Tertiary heights.[2] It is known that The Basin formed during the Tertiary period. The sedimentary succession of the Pattani can be divisible into syn-rift and post rift sequences. The syn-rift section tends to have relatively small displacement faults, which are characterized by rapid fault-controlled substances. While the post-rift phase is characterized by slow, thermal subsidence. One explanation of the formation is due to crustal extension caused by northward movement leading to the collision of the Indian plate and Eurasia plate. Another explanation is the Pattani Basin is a result of strike-slip plate tectonics. While the exact evolution is up for debate, there are several variations for the stratigraphic scheme of the Pattani Basin.

Five Depositional Sequences of Structural-Evolution
  1. Deposition starts from localized lacustrine and alluvial depositions in the Oligocene, described as syn-rift deposits.
  2. Fluvial and alluvial syn-rift settlements become deposited in early Miocene.
  3. Transgressive fluvial and marginal marine deposition occurred during middle Miocene.
  4. Overall regressive fluvial and alluvial deposition happened as of late Miocene.
  5. Predominantly transgressive marginal marine is specific of late Miocene.[3]

Petroleum Geology

Source Rocks

The Pattani basin is made up of two petroleum systems. The dominant system consists of Miocene gas-generating coals and shales which are matured in the deepest portions of the basin. The other system is made up of Oligocene oil-prone lacustrine shales, which are mature in basin flasks areas and overmatured in the area of the central trough.[4] The main three source rocks are shales, coaly

shale, and coal. To determine source rock richness, we use the percentage of total of organic carbon (TOC). The TOC will vary both vertically and laterally across the Pattani Basin. From wells drilled in the region TOC variance is pronounced.

  • Coal > 50% TOC
  • Coaly shale 10%-50% TOC
  • Non-coaly shale < 5% TOC

Mature source rocks are believe to be at the bottom of the basin, as they have yet to be reached. Most sediments in the basin can be described as nonsource to poor source rocks. Since the Pattani basin produces gas and condensates proves that the Tertiary strata have generated significant hydrocarbons, despite having low source quality. In that Pattani basin potential source rocks, despite having low oil generating capabilities, make up a large volume of source beds, which, together with interbedded sandstones, would function as effective carrier beds during migration. [5]

API gravity stands for American Petroleum Institute gravity. It is a measure of how heavy or light petroleum liquid is compared to water. The Pattani basins condensates consist of higher gravities ranging between 35-55 API, which is heavier than waxy oils.

Traps and Seals
This image includes the seismic lines of the Pattani basin. Present in the image are the grabens, half grabens, and horsts. (Courtesy of Bustin & Chonchawalit, 2010)

The three main ways oil and gas get trapped within this basin are north-south trending grabens, half grabens, and horsts. Areas of this Basin are bound by northwest and southwest changes in the main dip direction of groups of normal faults that form conjugate fault systems. Changes in the dip direction are commonplace in the Pattani basin and are called graben shifts. These graben shifts are complex transfer zones among arrays of downward-converging conjugate faults that make way for areas of anti-formal strata dips.[3] Structural traps in this Basin are unusual due to the rift-sag sequences. The large fields are linked to antiformal structures formed within the grabens of converging normal faults. Migration is limited and pathways are short to charge interbedded sandstone reservoirs. Countless amounts of crustal thinning and lithosphere stretching make up the tectonic history. There is no indication at active strike-slip tectonics formed the basin. Rather, strong NE–SW and NW–SE trends which effect normal faults account for the seals.[2]

Future Risks and Uncertainties

The discovery of oil and gas in the Pattani basin have resulted in extensive exploration programs. Thanks to these programs, extensive seismic and drill hole data has been acquired, allowing for stratigraphic succession to be well documented. Since 1981, more than 20 oil and gas fields and over 5000 wells have continued to produce hydrocarbons to the current date. The daily production rate is approximately 2 billion cubic feet of gas and 120,000 barrels of oil and condensate.[6] The main way oil is extracted through the Pattani Basin is through oil fields located in the post-rift structures which are seen to be open and gently dipping cut by converging conjugate normal faults. The risks of exploring are exemplified by poor source rock quality and thermal maturity. The nature of the pre-tertiary basement is not well documented or known. This is attributed to the high geothermal gradient.[5] In other words, the deeper you drill, the machinery is more prone to failure due to the increased pressure and heat. While high quality source rocks may be present in the basement, due to limitations in technology it is impossible to reach low enough depths to find out. While the source rocks have low oil generating potential, they represent a large amount of low-quality source beds, which, accompanied with interbedded sandstones throughout, function as highly effective carrier beds during migration. Reservoir sandstones of shallow hydrocarbon accumulations are expected to be incised valley-filled, meaning that there are no shortages of new reserves in the Pattani basin.

References

  1. Bustin, R. M., and A. Chonchawalit. “Formation and Tectonic Evolution of the Pattani Basin, Gulf of Thailand.” International Geology Review, vol. 37, no. 10, 1995, pp. 866–892., doi:10.1080/00206819509465431.
  2. 2.0 2.1 Watcharanantakul, R, and C.K Morley. “Syn-Rift and Post-Rift Modelling of the Pattani Basin, Thailand: Evidence for a Ramp-Flat Detachment.” Marine and Petroleum Geology, vol. 17, no. 8, 2000, pp. 937–958., doi:10.1016/s0264-8172(00)00034-9.
  3. 3.0 3.1 Kornsawan, A., and C.K. Morley. “The Origin and Evolution of Complex Transfer Zones (Graben Shifts) in Conjugate Fault Systems around the Funan Field, Pattani Basin, Gulf of Thailand.” Journal of Structural Geology, vol. 24, no. 3, 2002, pp. 435–449., doi:10.1016/s0191-8141(01)00080-3.
  4. Jardine, E. “Dual Petroleum Systems Governing the Prolific Pattani Basin, Offshore Thailand.” Proceedings of an International Conference on Petroleum Systems of SE Asia and Australasia, 1997, doi:10.29118/ipa.87.351.363.
  5. 5.0 5.1 Chonchawalit, Anun, and R. M. Bustin. “Basin Analysis of Tertiary Strata in the Pattani Basin, Gulf of Thailand.” AAPG Bulletin, vol. 78, 1994, pp. 1–24., doi:10.1306/a25fe53b-171b-11d7-8645000102c1865d.
  6. Fujiwara, Masashi, Takaoka, Shinichi, and Kayo Fukuda. "Shallow Hydrocarbons in the Pattani Trough, Gulf of Thailand" Paper presented at the SPWLA 21st Formation Evaluation Symposium of Japan, Chiba, Japan, October 2015.

External Links

https://www.nationalgeographic.org/encyclopedia/basin/

https://www.fossils-facts-and-finds.com/tertiary_period.html

http://www.geosci.usyd.edu.au/users/prey/Teaching/Geol-3101/EReport03/GroupB/Report1/structures.html