MALAYSIAS GAS RESOURCES*
By Khalid Ngah
PETRONAS Research and Scientific Services
Kuala Lumpur, Malaysia
*Text revised from expanded abstract, entitled "Gas Resources of Malaysia," by the author printed in Impacts of Technology on the Global Gas Resource Base: Report of Investigations No. 223, Bureau of Economic Geology, The University of Texas at Austin, 1994, p. 201-202, and presented by the author, Dr. Khalid Ngah, at the Global Gas Resources Workshop, September 19-21, 1994, Vail, Colorado, sponsored by the IEA International Centre for Gas Technology Information (Gas Technology Information, Inc., Operating Agent), Gas Research Institute, and Bureau of Economic Geology, The University of Texas at Austin, under GTI Contract No. 940003.
VISION 2020 envisages doubling of the Malaysian economy every 10 years for the next 30 years and Second Outline Perspective Plan sets the pace for Malaysia to become a fully industrialized nation by year 2020. This industrialization program requires increasing demand for energy, and this includes oil and gas. Statistics for 1992 show that Malaysia earned US$3.6 billion from oil and only US$1.0 billion from gas, based on 610,000 bpd and 2300 mmscfpd, but this scenario will change before thc turn of the century. Although oil production will remain stable for sometime, gas production will increase very rapidly, from the current rate of 3000 mmscfpd to more than 5000 mmscfpd in year 2000. Already US$18 billion has been spent on oil and gas development, and another US$3.S billion will be spent within the next 5 years for the same purpose. Much of the gas produced has been used for power generation, petrochemical products including urea, methanol and synthetic fuels (middle distillate) and in steel and aluminum smelting mills. Natural gas vehicles are now being introduced and will be a major gas user in the near future.
Malaysia's commitment to the vision poses one very basic issue: Will oil and gas production be maintained at a critical level beyond year 2020, in order to maintain current development momentum? This paper is concerned with the future supply of gas beyond year 2020, and it will examine the remaining gas potential of the Malaysian sedimentary basins and deepwater areas in terms of tectonics, stratigraphy and geothermal history and illustrate them with seismic examples.
Six major Tertiary sedimentary basins are present in Malaysia: the Malay, Penyu, Sarawak, Sabah, Sandakan and a portion of Tarakan basins (Fig. 1). Of these basins, only the Malay, Sarawak; and Sabah basins have been proven to contain significant oil and gas accumulations, with a total in-place of 20 billion barrels of oil end 130 tcf of gas (Fig.2). Marginal oil and gas accumulations have been found in the Penyu and Sandakan basins. Very limited exploration has been conducted in the Malayian sector of the Tarakan basin, and consequently its hydrocarbon potential has not been established.
and Penyu Basins (Fig
3 and Fig 4)
The Malay, Penyu, and West Natuna (Indonesia) basins are believed to have been formed as a result of upward buckling of the Sunda landmass in Late Cretaceous. Subsequent collapse of this landmass along the northwest and east-northeast and westerly directions, resulted in formation three depocenters: the Malay, Penyu and West Natuna basins. High heat flow observed in both the Malay and Penyu basins provides strong evidence that the basins were formed by crustal splitting and rifting. The rate during Tertiary of rifting and basin development, which varied with different depocenters, appears to have been higher in the Malay basin than in the Penyu and West Natuna basins; correspondingly, the Malay basin received more sediments than the others. A basement ref1ector has not been recognized in the seismic data acquired in the Malay basin, and its absence has led to the belief that the basin contains sediments at least 15 km thick (Fig. 5). In contrast, a basement reflector is easily traceable throughout the Penyu basin, with sediment thickness of about 7 km (4-second TWT) in the deepest part of the basin (Fig. 6).
The sediments penetrated by exploration drilling in the basins indicate that the oldest sequence encountered is Oligocene. These Oligocene and lower Miocone sequences were deposited in relatively enclosed lacustrine environments, and only toward middle early Miocene were marine conditions dominant. Much of the sediment input was primarily terrigenous. During Oligocene, the environments were markedly seasonal, and non-seasonal (overwet) with swamp conditions predominated only during late early Miocene to late middle Miocene.
Both the basins experienced one major tectonic episode in the Miocene-Pliocene, and most of the known hydrocarbons were found trapped within the structures formed during this period.
Sarawak and Sabah Basins (Fig 7, Fig 8, Fig 9)
The Sarawak and Sabah basins contain several tectono-stratigraphic provinces: Luconia Province, Baram Delta Province, Balingian Province, Tatau Province, Tinjar Province and northern Sarawak Province (in the Sarawak basin), and Inboard Belt, Outboard Belt, and northwest Sabah Trench and Trough (in the Sabah basin). Each of these provinces, with its own peculiar characteristics, is differentiated on the basic of tectonic styles. For instance, the Luconia Province is believed to represent a micro-continent that is tectonically stable, supporting major carbonate build-ups in the area (Fig. 10); in the northern Sarawak Province, "North Sea" type block faulting typifies the region (Fig 11 & Fig 12); in the outboard area, offshore Sabah, turbidite sediments predominate in "trench-associated" forearc basin (Fig.13 & Fig14).
Like the Malay basin, it is difficult to determine the thickness of Tertiary sediments in thee Sarawak and Sabah basins due to generally poor seismic resolution at depths below 5-second TWT; however, the sediment thicknesses inferred from gravity/magnetic data in the two basins, vary from 2 km in the Inboard Belt and northwest Trough to more than 6 km in the northern Sarawak Province.
The sediments deposited during late Eocone to early Miocene in the Sarawak basin consist of coastal to less common marine sequences, with coal development in the former sequences; however, toward the northeast and into the Sabah basin, a fairly wide distribution of deep marine shales is evidence of a more visible influence of marine conditions. From early Miocene to Recent, the marine environments in the Sarawak basin became more established, with predominant deposition of mixed clastic facies and carbonates. In most areas of the Sabah basin, deep marine conditions prevailed except toward the Inboard Belt, while shallow marine with coastal conditions existed.
More than 800,000 line-km of seismio data have been acquired, and 1700 exploration wells have been drilled in the offshore areas of Malaysia. These activities have led to the discovery of 20 billion barrels of oil and 120 tcf of gas in-place (Fig. 2). Many of the gas discoveries were made before 1985, when exploration effort was focused on oil. By year 2020, major gas reserves will be developed to support a maximum production of 5.2 bcf of gas per day for LNG, petrochemical products and domestic power generation. The importance of gas, because of its leading role in providing resource materials for petrochemicals and clean fuels, has resulted in an accelerated exploration focus in that direction, and major recent discoveries by Shell in the Sabah basin and by Occidental/Nippon in the Sarawak basin are two cases in point.
Many geological attributes support the contention that Malaysia has a great potential for oil and gas, and as much as 100 tcf of gas could still be found and developed through technologically advanced methods. This is in addition to the 85 tcf that have been discovered and remain to be produced by present methods. Some of the geological features that support this contention are:
1. The presence of very thick sediments within the Malay, Penyu, Sarawak and Sabah basins; of these the Malay and Sarawak basins have been proved to contain significant gas reserves. (Figure 1 demonstrates that at least 3 million cubic km of sediments are present in the basins; if one-third of these sediments represent potential source rocks, with TOC quality ranging from 5 to 30%, and if possible oil and gas escape and leakage are discounted, a gas resource estimate of 210 tcf is considered conservative, and drilling has shown that at good source rocks are present in the basin. Seismic data also suggest the presence of source rooks at depths that have not been reached by drilling.
2. The basins have appropriate geothermal regimes (Fig 3 and Fig 10), for providing excellent environments for gas generation.
4. Many of the reefal build-ups have not been tested to determine if they are hydrocarbon-bearing.
5. Only a few wells have been drilled in the Penyu and Sandakan basins; the extension of the prolific Tarakan basin into Malaysian territory has yet to be drilled.
6. Unknown potential of the Mesozoic sequences. Very little study has been conducted to determine the potential of the Triassic - Lower Jurassic sequences. Triassic reefs and Upper Triassic - Lower Jurassic clastics have been mapped by the Geological Survey of Malaysia, which reports of a total sediment thickness of 5 km. Analyses of selective surface samples show less than favorable reservoir quality, with porosity not exceeding 5%, and rather advanced maturity to overmaturity in potential source rocks, with vitrinite reflectance exceeding 1.5%, However, these strata have not been ruled out as potentially non-hydrocarbon-bearing, for the onshore gas production in Thailand is from Triassic carbonates.
Fig. 1 Sedimentary basins in Southeast Asia.
Fig. 2 Gas reserves of Malaysia.
Fig. 3 Location map of Malay and Penyu basins, with lines of sections for regional seismic lines in those basins (Figs. 5-6).
Fig. 4 Map of oil and gas fields in Malay basin.
Fig. 5 - Regional seismic section through Malay basin.
Fig. 6 - Regional seismic section through eastern part of Penyu basin.
Fig. 7 Location map of Sarawak and Sabah basins, with lines of sections for regional seismic lines (G-G, E-E, A-A, B-B) in those basins and line of geo-seismic section (D-D) in the Sarawak basin.
Fig. 8 Map of oil and gas fields in Sarawak basin.
Fig. 9 Map of oil and gas fields in Sabah basin.
Fig. 10 Geo-seismic cross-section (D-D), central Luconia Province, Sarawak. Location in Fig. 7.
Fig. 11 - Seismic section (F-F') in northern Sarawak Province (Sarawak deepwater), Sarawak basin. Line of section in Fig. 7.
Fig. 12 - Seismic section (E-E) in northern Sarawak Province (Sarawak deepwater), Sarawak basin. Line of section in Fig. 7.
Fig. 13 - Seismic section (A-A') in Sabah basin, through a well in Dudar area. Line of section in Fig. 7.
Fig. 14 - Seismic section (B-B) in Sabah basin, through two wells in Tembungo field. Line of section in Fig. 7.