Introduction

Thailand currently consumes approximately 1.5 million barrels of crude oil equivalence per day (excluding traditional renewable); and the demand growth for commercial energy is about 5% annually. Approximately 60% of this demand is imported. To ensure the sustainability of energy supply security, the Thai Government has been restructuring the energy sector by promoting the use of alternative energy and accelerating the investment of domestic petroleum exploration and production as shown in Figure 1. These include the use of biofuels, such as ethanol-based gasohol and palm-oil-based biodiesel, to partially reduce the consumption of gasoline and diesel, and the use of Natural Gas Vehicles (NGV) in the transportation sector.

Primary Energy Consumption Outlook
Figure 1: Primary Energy Consumption Outlook.

At the end of 2005 Thailand’s petroleum proven reserves were estimated as 2,300 million barrel oil equivalence. Approximately 80% of these reserves is natural gas. The Thai Government has achieved in promoting the development of petroleum exploration and production as clearly recognized from the past (Figure 2). Many oil and gas fields have been continually developed during the past decade with average daily production rate increased from 30,000 barrel and 1,100 million cubic feet in 1995 to 130,000 and 2,400 in 2006, respectively.

The promotion of gas utilization as fuel in power generation, industrial and transportation sectors as well as feedstock for petrochemical industry will subsequently initiate gas demand to sharply increase in the next decade. Predicted number of the gas demand is possibly over 7,000 million cubic feet per day by the end of year 2020 as shown in Figure 3. Approximately 4,000 million cubic feet per day is expected as the energy supply from domestic source.

Thailand Petroleum Production
Figure 2: Thailand Petroleum Production.

20 years gas supply plan
Figure 3: 20 years gas supply plan.

In response to the rising demand forecast, Thai government is accelerating the exploration and development of indigenous petroleum resources together with investment by Thai’s companies for hydroelectricity and gas supplies from neighboring countries. To enhance the domestic investment attractiveness, revising contractual and fiscal regimes, expanding of natural gas pipeline network, and opening of new bidding round of petroleum concession are impending.

On recently completion, the 19th bidding round was very successful with 16 awarded concessions covering 21 blocks. At this time, the 20th bidding round offering 65 exploration blocks located onshore and in the Gulf of Thailand, providing the great opportunity for E&P investments in the growing Thailand’s petroleum market.

Petroleum Province

For decades, hydrocarbons have been discovered and produced from Tertiary and Pre-Tertiary Basins.

ertiary Basins are widely distributed in various parts of the country, onshore (North, Central and South) and offshore (Gulf of Thailand and Andaman Sea). Pre-Tertiary Basins, are mainly located in the Northeastern Thailand, comprises Triassic and Permian Basin.

 

North – Central Plain

With the successful exploration in Fang, Phitsanulok, Suphan Buri, Kamphang Saen and Phetchabun basins, it shows that the fields in the northern and central parts of Thailand are very high potential. Basins are intermontain Tertiary basins with relatively narrow and small but deep and high in heat flow. They were formed as series of pull-apart basins of mainly N-S trending half-graben. Fang basin in the uppermost part of northern, operated by DED, is still producing oil with production rate approximately 1,000 bbl/d.

Stratigraphic Profile of Central Plains
Figure: 4 Stratigraphic Profile of Central Plains

For area in the central, all basins are N-S trending half-graben formed during Tertiary age. The main reservoirs in the area are Oligocene fluvio-lacustrine sandstone sealed and sourced by shale of similar age. Trap types are rollover anticlines with tilted fault blocks. Increasing oil production rate from volcanic reservoirs found in Wichian Buri and Si Thep oil field in Phetchabun basin have been operated by Pan Orient Energy (Thailand) Limited. CNPCHK discovered Nong Sa oil prospect in L21/43 block adjacent to Bung Ya/Bung Muang oil field.

Meanwhile, MOECO was also successful drilling of the first exploration well named Arunothai-1 in L10/43 block around Sukhothai depression area.

Five blocks in North–Central Plain have been awarded to oil companies during the 19th bidding round. Three blocks in the North covering Li, Pua and Nan basins are promoted to be interesting for 20th bidding round. These blocks are needed to prove the petroleum potential regarding they have similar tectonic setting to other successful basins. More seismic data is required to support the idea. In addition, new technology can help exploring new oil fields effectively.

Regional Basement Map and Top Basement TWT Structural Map in Sukhothai depression
Figure 5: Regional Basement Map and Top Basement TWT Structural Map in Sukhothai depression

Seismic section of western Sukhothai Depression (1985)
Figure: 6: Seismic section of Western Sukhothai Depression.

Stratigraphic and Schematic Cross-Section of Wichian Buri Sub-basin
Figure 7: Stratigraphic and Schematic Cross-Section of Wichian Buri Sub-basin.

Gulf of Thailand

In the past 24 years, Pattani Tertiary basin is one of the main offshore basins producing Petroleum mainly gas with condensate. Some areas produce oil e.g. Jasmine oil field in B5/27 block and Nang Nuan oil field in B6/27 block. The carbonate reservoirs in Chumphon, a half-graben Tertiary basin, were discovered oil reservoir in leached/porous zones of fault/fracture network at the top of karstification rocks overly Ratburi carbonate operated by PTTEP Siam Limited with maximum testing rate of 9,702 BOPD.

Stratigraphy of Gulf of Thailand compared with Noth Malay Basin
Figure 8: Stratigraphy of Gulf of Thailand compared with North Malay Basin.

The production from the whole fields within the Gulf is currently reported for gas about 2,000 mmscf/d, condensate 120,000 bbl/d and oil 46,700 bbl/d. Several identified prospects around Nang Nuan field show promising potential such as Hong Thong, Kra Tae, and Katiya, prospects.

We plan to attack our challenge in many marginal fields such as Bussabong area, a field that may not produce enough net income to make its worth development. This suggests that technical or economical conditions should have been changed and a field may become commercially in the future.

Stratigraphy of Nang Nuan Oil Field.
Figure 9: Stratigraphy of Nang Nuan Oil Field.

Seismic along well path and well location Namg Nuan B–02, B-01ST
Figure 10: Seismic along well path and well location of Nang Nuan B–02, B-01ST.

Schematic Cross – Section Chumphon Basin (after Hauck,1995)
Figure 11: Schematic Cross–Section Chumphon Basin (after Hauck, 1995).

Seismic section across Khun Thong prospect (Block G3/50)
Figure 12: Seismic section across Khun Thong prospect (Block G3/50).

Northeastern Region

Northeastern Thailand or E-Sarn or the Khorat Plateau, occupies 200,000 km2, or one third of the whole country. In general, the topography of the Khorat Plateau consists of low rolling hills bounded by mountains to the west, south and the middle of the region.

The surface data indicate the most area is covered by the Mesozoic sequence (the Khorat Group). Only the western rim, the outcrops, consists of Triassic (Huai Hin Lat & Kuchinarai Groups) and Permian rocks (Saraburi Group). The Khorat Group is a greater thickness unit, which is also found the continental sediments. The Huai Hin Lat Group is the fluvio-lacustrine clastics sediment, which seal the Triassic half-graben. The Kuchinarai Group is defined for the fluvio-lacustrine sediments, which deposited in the Triassic half-graben. The Saraburi Group comprises of the sediments of shallow to deep marine depositional environments during the Permian.

Several petroleum exploration wells and seismic data show the Huai Hin Lat, Kuchinarai and Saraburi Groups are occupied in the basin underneath the Khorat Group in the main area (not only in the western rim). There are two trends of the Saraburi Group, which deposited in the N-S (the Loei-Phetchabun Fold Belt) and NW-SE (underneath the Phu Phan mountain range) trends. For the Huai Hin Lat and Kuchinarai Group deposited in the half-graben or graben that occurred locally but not related to the present structure.

Geochemical analysis, source rocks are shale of the Triassic and Permian rocks. The main reservoir rocks are the Carbonate rocks of the Saraburi Group. More interested reservoirs rocks is some part of the Triassic rocks with some petroleum in the Mukdahan structure.

Seismic surveys have been conducted and petroleum exploration wells have been drilled since 1962. Seismic survey already covered the whole area. Largely, reservoir rocks in this vast region are Permian carbonates contained in anticlines with reactivated faults, creating fractures and adding porosity to the carbonates. More than 30 wells drilled here have confirmed this fact, but to date only the Nam Phong and the Phu Horm gas fields have been put on production. It is expected that more gas fields will be developed in the Dong Mun and Si That structures operated by Apico LLC. Moreover, the Chonnabot and Talat Sai structures are going to be discovered by PTTEP and Adani port.

However, many prospective structures are still available for challenging to explore and to develop in both carbonate reservoirs and other reservoirs. Examples of our prospective structures are following. (Please remind that other prospects, which we do not mention here, are also likely to be high petroleum potentials.)

Prospective Structures

That Phanom structure, anticlinal with a northwest-southeast trend, this structure lies at the eastern part of the Khorat Plateau and slightly north of the Kham Pa Lai Structure situated in Mukdahan Province. Seismic data have revealed that That Phanom was virtually a twin structure of Phu Horm, with the folded Khorat Group overlying a thin Huai Hin Lat Group. The reflector-free portion is most likely the Pha Nok Khao Formation of carbonate rocks, while the strong-reflector bottom portion is most likely the Si That Formation of lower clastics rocks. Beneath the Khorat Group, one finds many faults, which could have caused folding in this area. In addition, there are minor faults, which could have given the Saraburi Group, the reservoir rocks, more porosity. As regards source rocks, this could be the Huai Hin Lat Group lying on top along with the Kuchinarai Formation of the Mukdahan Structure, to the south. With Phu Horm now the largest gas deposit of the Northeast, one could speculate that That Phanom, with a very similar structure, could one day become a petroleum field.

Litho-stratigraphy and petroleum system of the Khorat Plateau
Figure 13: Litho-stratigraphy and petroleum system of the Khorat Plateau.
(modified from Sattayarak, 2005; DMF, 2005; Chantong, 2005 and Wanida, 2006: in Thai)

That Phanom structure
Figure 14: That Phanom structure (seismic line no. UKH91A103, Block L17/50 and L18/50).

Phu Horm structure.
Figure 15: Phu Horm structure.

Chaturat structure
Figure 16: Chaturat structure (seismic profile no. AHT98-207, Block L21/50, L29/50 and L30/50).

Chaturat structure, another anticlinal structure of virtually N-S trending, covers Amphoe Chaturat of Chaiyaphum province, southwest of the Chonnabot Structure. Seismic data showed that the structure had resulted from subsequent actions of thrust faults once the Khorat Group had been deposited. Beneath the Khorat Group is likely to be the Saraburi Group, cut across by faults. Reflector-free portions are likely to be the Pha Nok Khao Formation or carbonate rocks. The Saraburi Group of this structure should be highly suitable reservoir rock. The thin, overlying Huai Hin Lat Group, also to the east, is likely to be the upper clastics – also potentially good source rocks. Like Chonnabot and Nam Phong, Chaturat is a promising structure.

Kalasin structure
Figure 17: Kalasin structure (seismic profile no. K90A107, Block L16/50).

Kalasin is a structure beneath the horizontal Khorat Group. Below it is the Saraburi Group deposited in Permian basins of northwest-southeast trending. The shallower side is in the southwest, and the deeper side in the northeast. This structure is located around Amphoe Mueang of Kalasin province. Seismic data showed that Kalasin was an extension of the Dong Mun Structure to the northwest. Beneath the Khorat Group is likely to be the Saraburi Group consisting of reefs, although these reefs are not as distinct as in Dong Mun. Still, despite the appearance of reefs, the porosity of reservoir rocks may be low, calling for more investigation to define well locations.

Phu Khieo structure
Figure 18: Phu Khieo structure (seismic profile no. PK92-1, Block L15/50).

Phu Khieo structure, covering Amphoe Phu Khieo of Chaiyaphum province, is also anticlinal of NE-SW trending, resulting from inversion and reactivated faults involving Permian basins that contained the Saraburi Group; yet, the structure is not so apparent on the surface. Seismic data showed that beneath the Khorat Group were the Huai Hin Lat Group (or Triassic rocks) and the Saraburi Group. The Saraburi Group had formed anticlines before the Huai Hin Lat Group was deposited, as seen from the reflector portions of the Huai Hin Lat Group onlapping the Saraburi Group.

Texaco Exploration (Thailand) Co., Ltd., interpreted its seismic data as a paleo-high of the Permian Group of Triassic age or older. It was assumed that this structure resembled Nam Phong, which was a paleo-high of Permian carbonate reservoir rocks fractured by tectonics. The company drilled Dao Ruang-1 on 4 April 1993 to test structural-stratigraphic traps and dolomitic reservoir rocks, whose microfractures were produced by diagenetics. Drilled to 2,423 m, the well encountered Permian carbonate rocks that served as poor reservoir rocks because petroleum was prevented from accumulation by dolomitic microfractures. Still, the company tested for natural gas at the following depths: 1,478-1,546 m – 146,000 BCFD 1,752-1,176 m – 73,000 BCFD and 2,136-2,173 m – 132,000 BCFD

Despite the company’s acid injection to dissolve rock materials and improve porosity, the Permian Group’s dense texture allowed limited gas flow. With that result, the company put an end to the well.

Phu Khieo’s problem is its poor reservoirs, as samples from Dao Ruang-1 were very dense, almost without any fractures. Seismic data showed that the anticlines involving the Saraburi Group had very few faults, which probably made reservoir rocks tight. Further seismic surveys could locate good reservoir rocks and make the structure more promising.

Khemmarat structure
Figure 19: Khemmarat structure (seismic profile no. DMR96-120, Block L26/50).

Khemmarat structure, covering Amphoe Khemmarat of Ubon Ratchathani province, is another anticlinal structure of virtually west-east trending and possibly parallel to the Phu Phan mountain range, located southeast of the Non Sung Structure. Note that there was only one line of seismic record to date, showing a severely folded Khorat Group, beneath which was probably the Huai Hin Lat, Kuchinarai Saraburi Group, or Permo-carboniferous rocks. Because of very limited data concerning this structure, further work here is needed to reveal more about it.

Lam Phra Phloeng structure
Figure 20: Lam Phra Phloeng structure (seismic profile no. 93NR180, Block L37/50).

Lam Phra Phloeng structure is located at the southwestern edge of the Khorat Plateau of northwest-southeast trending. Covering Amphoe Pak Thong Chai of Nakhon Ratchasima province, the structure lies near the Lam Phra Phloeng River. Seismic data showed the uplifted Khorat Group, tapering to the southwest and thickening to the northeast. Beneath the Khorat Group is likely to be the Kuchinarai Group or the Huai Hin Lat Group cropping out at Ban Sup Phlu and consisting of shale, black sandstone, and conglomerates. Beneath the Kuchinarai Group and the Huai Hin Lat Group, reflector portions are most likely to be the Saraburi Group. NW-SE seismic lines indicated anticlines of the Saraburi Group, Kuchinarai Group, and the overlying Huai Hin Lat Group, with onlaps. It is therefore highly likely that the Saraburi Group had formed buried hills before petroleum originated in the Kuchinarai Group and Huai Hin Lat Group and then accumulated in the Lam Phra Phloeng structure, which was highly complex with intense faulting activities – which proved beneficial to reservoir rocks. Many buried hills are found here, so further investigation of the structure is recommended for possible petroleum development.

Si Kew structure
Figure 21: Si Kew structure (seismic profile no. 92NR115, Block L29/50 and L37/50).

Si Kew structure, located northwest of the edge of southwestern Khorat Plateau, covers Amphoe Si Kew of Nakhon Ratchasima province. Seismic data showed that the structure had NW-SE trending, as is Lam Phra Phloeng, and represented the buried-hill portion of the Saraburi Group at the deepest side of the Triassic graben and the thickest portion of the Khorat Group. A crucial question then arises whether the petroleum formed here would rise, and if so, how likely the petroleum is going to accumulate in this structure. A more thorough investigation of the buried history around the area is in order.

Dan Khun Tod structure
Figure 22: Dan Khun Tod structure (seismic profile no. 92NR55, Block L29/50).

Dan Khun Tod structure, located on a plain north of the Si Kew structure, covers Amphoe Dan Khun Tod of Nakhon Ratchasima province. Its trending is much the same as those of Lam Phra Phloeng and Si Kew. Seismic data showed that this structure was located near one edge of the Triassic graben-basin, and that beneath the Khorat Group is the generally thick Kuchinarai Group, which tapered to the northeast. Reflector portions indicated that buried hills underneath these mentioned rocks could be of the Saraburi Group, which formed prospective anticlines. Yet, since the structure is comparable in risks with Si Kew, additional investigation is called for.

Don Phrai structure
Figure 23: Don Phrai structure (seismic profile no. 92NR195, Block L38/50).

Don Phrai structure, north of the Khorat Plateau’s western half, lies southeast of the Lam Phra Phloeng structure and covers areas between Amphoe Khon Buri and Amphoe Chok Chai of Nakhon Ratchasima province. Seismic data showed that the structure was of NW-SE trending, with the thick Khorat Group overlying the Kuchinarai Group with its own basins toward the southwest. The thinner portions of Kuchinarai Group are likely to be the anticlinal Saraburi Group. This structure is also characterized by faults, but with currently limited data, additional investigation is required. Note that the structure could well be a continuation of the Lam Phra Phloeng structure.

Lam Nang Rong structure
Figure 24: Lam Nang Rong structure (DMR96-180, Block L55/50).

Lam Nang Rong structure, lying to the southernmost edge of the Khorat Plateau and to the southeast of Don Phrai, covers Amphoe Nang Rong of Buri Ram province. Only one seismic profile crossed this structure, and therefore it was surmised that its extent would be parallel to the southern portion of the Khorat Plateau, that is, of SW-NE trending. Seismic data also showed similarities to other structures mentioned, although this area contains thinner sections of the Khorat Group. Note that the Kuchinarai Group showed distinct erosional truncation with the Khorat Group; thrust faults could well have caused inversion of the Triassic graben before the Khorat Group was deposited. Some portions showed erosional truncation with the Nam Phong and the Phu Kradung Formations. Underlying the Kuchinarai Group is likely to be an anticlinal Saraburi Group, but because of unclear information, it remains uncertain whether reservoir rocks would be found in this structure.