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Petroleum Geology of the Gulf of Thailand

Introduction

The Gulf of Thailand contains several structurally complex trans-tensional basins. These are made up of asymmetrical grabens filled with non-marine to marginal marine Tertiary sediments as old as Eocene. Underlying the graben sediments are a variety of Paleozoic marine carbonates, granitic intrusive rocks, and metasediments. Many of the basins contain thick sequences of oil-prone source rocks, but the limited lateral extent of these deposits, combined with vatiations in heat flow and depth of burial of the source rocks, causes the distribution of hydrocarbons to be complex and difficult to predict. Numerous exploration opportunities remain, but the outlook is for a large number of smaller discoveries. The Thai Department of Mineral Fuels maintains a current concession map of the Gulf of Thailand.

Gulf of Thailand Sedimentary Basins Map

Gulf of Thailand Basins

Regional Overview

The regional pattern of the grabens and related faults strongly suggests that the grabens in the Gulf of Thailand are the result of the collision of India with Central Asia that began in Eocene time. The collision forced the area to the west of the Gulf of Thailand to the north and west relative to the area to the east, causing grabens and strike-slip faults with right- lateral movements, as well as en-echelon normal faults trending generally north-south. A similar structural picture has been mapped onshore Thailand.

The only modern Southeast Asian analogue to the Gulf of Thailand basins during Tertiary time is the Tonle Sap area in Cambodia. This large lake is today being filled with lacustrine sands and shales and in places with fresh-water limestones. These lacustrine shales are sufficiently rich in organic matter to be excellent oil source rocks. The reason that most hydrocarbon production in the Gulf of Thailand is gas is the combination of deep burial and high thermal gradients. Because the source units are not laterally extensive, they are absent on the basin flanks where they would be in the oil window.

Not all basins in the Gulf of Thailand have adequate lacustrine source rocks. An important factor in the deposition of lacustrine shale source rocks is that the lake in question should have a limited sediment supply relative to the rate of subsidence, so an open lake can form. In the Gulf of Thailand, the sediment supply was probably controlled by the river systems in existence during the Tertiary. The thick source rock sequences of the Chumpon, Kra and North Pattani Basins indicate that the Paleo Chao Phraya River system probably bypassed them.

Migration of hydrocarbons in rift basins tends to be lateral in the central parts but nearly vertical along the basin margins. The major reason for this is that the basin-bounding faults are usually active over much of the basin's history, which causes many normal and strike-slip faults to form that serve as barriers for lateral migration of hydrocarbons. In the central parts of the basins, faults are scarcer and turbidite distributary fan lobes act as conduits for hydrocarbon migration toward the basin margins.

Carbon dioxide content is a common problem in gas reservoirs in the Gulf of Thailand. This problem occurs intermittently from the Malaysia-Thailand Joint Development Area in the south all the way to Jasmine Field in the north. The problem is not unique to the deepest parts of the basins, suggesting that it may be due to overmature source rocks in the Pre-Tertiary section.

Malay Basin

The Malay Basin is a major oil-producing basin offshore peninsular Malaysia, but yields mostly gas in Thailand. The Thai portion of the Malay Basin includes the Bongkot Gas Field, Thailand's largest, as well as recent major gas discoveries in the Arthit area that have added substantially to Thailand's gas reserve base. The Malay Basin has more marine influence than the Pattani Basin. Although large voumes of oil are produced from this basin in Malaysia, the only oil production on the Thai side is from oil rims in shallow gas reservoirs in the northern part of Bongkot Field. These have been developed with horizontal wells.

The deepest part of the Malay Basin is beneath the southern part of Bongkot Field and the Malaysia-Thailand Joint Development Area. In these areas, carbon dioxide content is a problem in most gas reservoirs. The carbon dioxide content is highly variable but, as a general rule, deeper reservoirs and those further south tend to have higher carbon dioxide content.

The northern part of the Bongkot Field is a north-south trending trans-tensional structure similar to the Erawan Field in the Pattani Basin, but the structural style changes to the south because of a component of north-south compression. Fields such as Muda in the Joint Development Area have multiple structural culminations along an east-west trend. Many of the oil fields offshore Malaysia display this same structural style.

Pattani Basin

The Pattani Basin is an important producer of both oil and gas, with nearly all of the oil production in the northern part of the basin and most of the gas production in the southern part. The basin contains marginal-marine and non-marine sediments up to a maximum thickness of about 10,000 meters. The depth of burial of the source rocks in this basin is sufficient to generate oil and gas. The southern part of the basin contains thicker sediments and produces mostly gas and condensate while the northern part produces much more oil. This difference could be explained by differences in burial depth and thermal gradient (the southern part of the basin is deeper and hotter) but it is more likely due to differences in the source section.

Southern Pattani Basin

The largest gas field in the Pattani Basin is Erawan, operated by Unocal. This field is a large structure in a basin-center location. The structure is antiformal but pervasively faulted. A story about this field illustrates the structural and stratigraphic complexity of this basin. After the field had been discovered and delineated, the first development well was a dry hole. A major 3-D seismic effort followed (one of the first in the world) and resulted in a very successful development.

The second largest gas field in the Pattani Basin is Pailin, also Unocal operated, which has only recently been developed due to the need for costly carbon dioxide removal. Other, smaller, Unocal-operated gas fields in the southern Pattani Basin are Moragot, Gomin, Funan, Jakrawan, Satun, Trat, Pladang, Pakarang, Platong, Surat and Kaphong. Surat Field also has significant oil production.

Unocal geochemists have published articles describing a dual-source model for natural gas generation in the southern Pattani Basin. The two sources cited are coals in the shallower part of the section and lacustrine shales near the base of the Tertiary section. It is likely that the source section in the southern part of the Pattani is more gas-prone, while the source section in the northern part of the Basin contains more oil-prone lacustrine shales, with their type-1 kerogens.

Northern Pattani Basin

The most important fields in the northern Pattani Basin are Benchamas, which is mostly an oil field and Tantawan, which produces gas and some oil. Recent oil discoveries at Pakakrong, Maliwan and Jarmjuree account for much of the recent increase in Thailand's oil production. All of these fields are operated by Chevron in partnership with Thaipo. The Yala Field is the southern extension of the Tantawan structure into Unocal's acreage. Jasmine Field sits at the very northern end of the Pattani Basin on the basement high separating the Pattani and Rayong Basins.

Seismic mapping shows a deeper section in the North Pattani Basin, below the level often mapped as base Tertiary. These sediments are generally seen as a seismically transparent asymmetrical wedge of more than one second thickness with a prominent basal reflector. This could represent Lower Tertiary or even Mesozoic sediments that may be the source of the oil in this part of the Pattani Basin. This older graben trends north as far as the Jasmine Field and dips eastward. More recent movement along the bounding faults of this older graben has produced two north-south striking structural trends. Benchamas Field is associated with the eastern of these two trends and is characterized by a higher proportion of gas on the east flank of the structure, with more liquids on the west flank. This distribution of hydrocarbons could be explained if the liquids are being generated in the older graben fill.

The northern Pattani Basin is as complex as the southern part, as evidenced by the 8-238-1X dry hole drilled by Sun Oil. Producing wells of Chevron's Benchamas Field now surround the 8-238-1X well. The area was evaluated and dropped by Sun, Gulf and Unocal before Thaipo drilled the Benchamas Field discovery well. Besides basin-centered antiformal structures such as Benchamas, the northern Pattani Basin also contains oil-productive, basin-edge structures such as Pakakrong.

Chumpon Basin

The Chumpon Basin has one oil field, Nang Nuan. The field produces light oil from vuggy carbonates along a pre-Tertiary ridge in the center of the basin. This basin is known to have at least 1,000 meters of source rock. The thermal maturity of this source rock is demonstrated by the light (40° API) oil produced from Nang Nuan.

Kra Basin

The Kra Basin is known to have over 1200 meters of excellent source rock, but it has yet to yield a commercial oil discovery. One well, the B5/27-2, has tested oil. The oil tested in that well may have come from lacustrine carbonates of Tertiary age, rather than karstified Paleozoic limestones. The reservoir interval in that well displays good sonic log porosity, which should not be the case for the Paleozoic. Lacustrine carbonates associated with the initial opening of the South Atlantic Ocean during the Cretaceous have produced hundreds of millions of barrels of oil in Angola (Malongo West and Limba Fields) and Brazil's Campos Basin (Badejo, Linguado and Pampo Fields). These carbonates are skeletal grainstones commonly described as coquina.

Songkla Basin

Oil has been tested in the Songkla Basin, but there is no history of oil production.

Western Basin

The Western Basin has one well that penetrated a source rock sequence and had oil shows, but the well was lost before reaching its objective.

Hua Hin Basin

The Hua Hin Basin is an elongate feature with more than one sub-basin. One well drilled in the northern sub-basin did not encounter source rock.

Rayong Basin

The one well drilled to date in the Rayong Basin did not encounter source rock.

Onshore Mae Sot Basin and other Analogs

The uplifted and eroded lacustrine basin at Mae Sot is a possible analog to the older Tertiary lacustrine basins in the Gulf of Thailand. The sequence there has been described in some detail by Thanomsap and Sitahirun (1992) and begins with alluvial fans of the Mae Ramat Formation. That formation is overlain by the limestones and clastics of the Mae Pa Formation, which are overlain in turn by the marls and oil shales of the Mae Sot Formation. The Mae Sot Formation represents an oil prone source rock. Similar sections of varying ages in the deep lacustrine basins of the Gulf of Thailand are the most likely source of the oil and much of the gas there.

The Songliao Basin of China is the most important oil producing region in China. Daqing is not a single oil field, but two ternds of numerous oil reservoirs along the flanks of a rifted lacustrine basin. The large production of this area is obtained from many thousands of wells. At its peak, the Songliao Basin produced more than one million barrles of oil per day.

The San Jorge Basin of Argentina has been an important oil producing area since 1907. The oil in this basin is produced from basin-edge structures. Faulting has created updip seals in turbidite channels and fan lobes.

The oil fields of southern Sudan and Chad are also analogs to parts of the Gulf of Thailand. In these basins, oil is produced from pervasively faulted trans-tensional structures.

Geology Meets Economics

The combination of structural and stratigraphic complexity means that individual oil or gas reservoirs are small, even in the larger fields. If you look at an oil field map of Argentina's San Jorge Basin, the two trends of oil fields that run eastward along the flanks of the basin stop at the shoreline. Conditions in the Gulf of Thailand are not as rough as in the South Atlantic, but the economics of developing these fields offshore remains a challenge. This problem was recognized early on by the Gulf of Thailand operators and they have used a strategy of aggressively reducing drilling costs along with utilizing 3-D seismic data to optimize the wells.

Thailand uses a tax and royalty regime for oil and gas production. Most of the production in the Gulf of Thailand is covered by Thai 1 terms which are a fixed royalty plus income tax, but the newer Thai 3 terms include a sliding-scale royalty and a secondary tax. Because of the complex geology of the Gulf of Thailand, geophysical and drilling costs consume a large proportion of revenues. For this reason, increasing the Thai content of these services is as important as fiscal terms in any strategy to maximize Thailand's benefit from the development of its resources.

If a competitive, low-cost oil service industry is developed in Thailand, it could also become a valuable service export. This is not a far-fetched idea; France is a major exporter of oil services even though it is not an important oil producing country. It is also worth noting that no member of the Organization of Petroleum Exporting Countries is a major exporter of oil services. A good start would be to translate open-source technical software into Thai so it could be used in Thai universities. One example is Seismic Unix, a seismic data processing system provided by the Colorado School of Mines. Its user agreement only prohibits selling the software at a profit, not translation nor commercial use. This is but one example of the numerous open-source programs now available.

In the future it may become politically possible to improve Thailand's fiscal terms if the majority of the benefits remain in Thailand rather than accruing to foreign companies. This is a worthwhile goal because Thailand has substantial oil and gas resources that lie at or close to the economic margin, and Thailand's economy is vulnerable to oil price shocks otherwise.

References

1. Jardine, Eric, Dual Petroleum Systems Governing the Prolific Pattani Basin Offshore Thailand, Proceedings of the International Conference on Stratigraphic and Tectonic Evolution of Southeast Asia and the South Pacific, Department of Mineral Resources, 1997

2. Nares Sattayarak, Petroleum Exploration Opportunity in Thailand, Proceedings of a National Conference on Geologic Resources of Thailand: Potential for Future Development, Department of Mineral Resources, 1992

3. Surapol Thanomsap and Suparuek Sitahiran, Mae Sot Oil Shale, Proceedings of a National Conference on Geologic Resources of Thailand: Potential for Future Development, Department of Mineral Resources, 1992

4. Surawit Pradidtan and Robert Dook, Petroleum Geology of the Northern Part of the Gulf of Thailand, Proceedings of a National Conference on Geologic Resources of Thailand: Potential for Future Development, Department of Mineral Resources, 1992

5. Surawit Pradidtan, Suwit Jaroonsitha and Yutthorn Gonecome, Petroleum Systems of the Petroliferous Basins of Thailand, Symposium on Mineral, Energy and Water Resources of Thailand: Towards the Year 2000, Chulalongkorn University, 1999

6. Supaporn Pisutha-Arnond, A Discussion on the Pre-Tertiary Rocks in the Gulf of Thailand, Symposium on Mineral, Energy and Water Resources of Thailand: Towards the Year 2000, Chulalongkorn University, 1999



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