Geology Of Brunei Deltas, Exploration Status Updated

Note the wave dominated Baram delta just west of the Brunei-Sarawak border and the very different Brunei Bay system in the Northeast. Steep ridges clearly outline the large Belait syncline, which is largely covered by rainforest, and the Berakas and Limbang (Sarawak) synclines respectively north and east of the capital, Bandar Seri Begawan. To the south and southeast of Brunei are the highs of the Crocker Rajang range, with Mount Mulu (near the well known Mulu caves) just south of the Belait syncline. The Baram delta plain consists of large raised peat bogs (the dark colored patches in the lower left of the image). The oldest peat deposits in the south are dated at approximately 5,000 years. The youngest peat deposits are near the coast, which illustrates the rapid progradation of the Baram delta. Landsat composite image, bands 5 (red), 4 (green), and 2 (blue) + PC (intensity, to enhance water image). This article summarizes the petroleum geology of Negara Brunei Darussalam, the smallest but oil and gas richest country in Northwest Borneo.

Exploration history

Brunei is one of the most ancient sovereign states in Southeast Asia, which once ruled large parts of the island of Borneo.

Most of its former territories were ceded to the Brooke Rajahs of Sarawak and to British North Borneo Co. (now part of Sabah) in the previous century. It is in that time that the first signs of oil were noted by Motley¹ in his geological description of Labuan, a small island off the Brunei coast.

Oil seeps were soon also reported from eastern Brunei. The first exploration well in Brunei was drilled in 1899 near Bandar Seri Begawan, the capital, at the site of such an oil seep. The 198 m deep hole did not find commercial hydrocarbons, but the hunt for oil was on.

Early geological work recognized complex anticlinal structures near the coast (Jerudong), but also in the rainforest covered interior (Belait). The first part of the century saw many "dry" shallow holes drilled in these areas.

Anglo Saxon Petroleum Co. (from which later Brunei Shell Petroleum Co. evolved) acquired all concession rights in the area. Its well Belait-2 struck the first oil in Brunei, albeit noncommercial, in 1914.

Anglo Saxon continued with its substantial exploration efforts, encouraged by its discovery of Miri field in 1910 across the border in Sarawak, the first commercial oil field in Northwest Borneo. Its efforts paid off with the discovery of large Seria field in 1929.²

Despite intensive exploration, no new hydrocarbon accumulations were discovered until the early 1960s. At that time exploration moved offshore following the extension of Brunei's boundaries to include the continental shelf in June 1954.

Brunei Shell Petroleum drilled the first offshore well in 1957 from a fixed platform. Disappointing results, high costs, and technical problems temporarily halted further offshore drilling despite the promising exploration opportunities.

The introduction of mobile offshore drilling units and rapid improvement in marine seismic technology in the late 1950s and early 1960s provided new impetus for offshore exploration.

Brunei Shell in rapid succession discovered a whole series of oil and gas fields, of which the most important are Southwest Ampa (1963) and Champion (1970), each with close to a billion barrels (oil equivalent) of hydrocarbons. Together with the 166 million cu m (1+ billion bbl) of oil produced by Seria field, these accumulations form the backbone of Brunei's hydrocarbon industry.

From 1913-95 Brunei Shell and its predecessors drilled 154 exploration wells and more than 1,500 development wells. Other companies drilled 29 exploration wells, half of these in the beginning of this century. Brunei currently counts 12 commercial oil and gas fields. All producing fields are operated by Brunei Shell.

In 1989 Jasra Elf discovered Maharaja Lela-Jamalul Alam field, straddling the Sabah border. The field is presently being developed.

Besides Brunei Shell two groups hold concessions in Brunei. They are joint ventures of three companies: Jasra, Elf Aquitaine, and Fletcher Challenge.

During 1995, Brunei Shell's daily production averaged some 27,800 cu m of liquids and 30 million cu m of gas. Oil and gas from the developed fields is transported via pipelines to Brunei Shell's Seria oil and Lumut liquefied natural gas (Brunei LNG) terminals. A total of 186 cargoes of 75,000 cu m of LNG each were exported in 1995 to Japan and Korea. These are substantial quantities, certainly if one looks at the relative small size of the sultanate.

Fig. 1 [65,273 bytes] shows the current concession map and hydrocarbon discoveries of Brunei. The recently published Geology and Hydrocarbon Resources of Negara Brunei Darussalam, edited by Sandal,³ provides more detailed information.

Brunei geology

Brunei is situated within the Neogene Baram Delta Province that flanks Northwest Borneo along the South China Sea. The delta province evolved over a broad deformation zone, the Rajang-Crocker accretionary belt. Along strike the delta province extends from Sarawak across Brunei into Sabah (Fig. 2 [57,899 bytes]).

A detailed discussion Brunei in terms of regional plate tectonic models is beyond the scope of this article. Most models explain the development of fore-arc type basins and their subsequent deformation and accretion by active southward subduction of the proto-South China Sea under Borneo.^4-6

The main prospective sequence for Brunei was deposited from the Middle Miocene onwards in one overall regressive phase with superimposed a number of major transgressions.

Three main delta systems can be distinguished (Fig. 3 [33,411 bytes]):

• The Meligan Delta System (the Proto-Champion Delta, or early Baram Delta) is the oldest, pre-Middle Miocene, delta system, only preserved as erosional remnants in the uplifted and deeply eroded hinterland of onshore Sabah and Brunei.

• The Champion Delta System (the East Baram Delta), with its main axis of outbuilding generally along the border between Brunei and Sabah. Its development is mainly in the Late-Middle Miocene to Early Pliocene (14.2-5.2 Ma), in Brunei's present day onshore and eastern offshore areas.

The structural style is complex, with syn- and post-depositional multidirectional deformation and uplift. Mainly N-S trending, narrow anticlines/ridges, separating wide synclines and major SW-NE trending syn-sedimentary counter-regional faults, are the most conspicuous elements in addition to syn-sedimentary down-to-basin growth faults.

• The Baram Delta System, with its main axis of outbuilding situated in Brunei's western offshore area. This is the youngest (Early Pliocene, 5.2 Ma to present day) phase in the Baram Delta Province. The structural style is dominated by E-W to NE-SW trending syn-sedimentary down-to-basin growth faults.

The successive delta systems resulted from a process of deposition, deformation, erosion, and subsequent re-deposition in younger depocenters farther seawards. The sediments in the Baram Delta Province have been through various phases of reworking, which explains their generally high mineralogical maturity and a fine grained texture.

Structural geology

The structural geology of Brunei is best described as the complex interaction between deltaic growth faulting ('thin-skinned'), and basement-rooted tectonics ('thick-skinned').

Distributed, right lateral simple shear along the Northwest Borneo accretionary margin controlled a clockwise rotation of crustal-scale fault block domains associated with sinistral, antithetic (north-south) fault activity.

Early rotation resulted in upper crustal stretching and transtension across the block faulted domains with collapse of the basin floor and short-lived, pull-apart rates of subsidence. Sediment loading and block rotation caused diapirism of the basinal shales (Setap shale Formation) underlying the deltaics. Early diapirism resulted in syndepositional highs in the rapidly subsiding areas, which affected delta progradation.

Advanced stages of rotation caused compression/ transpression across the rotating fault blocks. Locking ultimately resulted in reverse faulting, uplift, and inversion, mainly concentrated over the edges of the original fault block domains. The deltaic overburden overlying the relatively undeformed fault block domains attained the shape of very wide synclines, with relatively steep and occasionally upthrusted/imbricated edges (ridges). These are clearly visible on satellite image (Fig. 4 [12,085 bytes]).

The complicated deformation history explains the structural complexities found in the anticlinal ridges of Brunei and adjacent parts of offshore Sabah. They contain the main hydrocarbon accumulations in the area.

Hydrocarbon habitat

Reservoir

The main hydrocarbon reservoirs are in coastal, fluviomarine, and lower coastal plain sediments ('topsets'). Within the overall outbuilding delta system, the main reservoir-belt progressively moved basinwards through time. Topset reservoir potential decreases both downwards and basinwards (Fig. 5 [21,836 bytes]).

The search for deep turbidite reservoirs has only started recently. Foreset and bottomset sediments are deeply buried and highly overpressured in nearshore areas and in deep water (200 m) farther offshore. Most penetrations of deep reservoirs are in channelized slope turbidites with limited lateral reservoir continuity.

Seal

Within the overall outbuilding delta system, the sand/shale ratio decreases with depth, and seal quality and sealing potential (seal capacity) therefore increase with depth (Fig. 5).

The increase of seal potential with depth results in a gradual change of leak prone traps, shallow in the succession, to spilling traps at deeper levels. In overpressured successions (see below) the risk of seal breaching is very high. Seal development is the main exploration risk in the proximal deltaic sediments. Seal breaching is the main risk in overpressured traps.

Hydrocarbon charge

Hydrocarbon charge is one of the lesser exploration risks in Brunei. Oil seeps are abundant onshore, and most offshore wells have at least hydrocarbon shows. Late structuration onshore implies a charge-timing risk in these areas.

As with reservoir and seal development there is a similar geological motif behind delta outbuilding and hydrocarbon composition/distribution trends (Fig. 5). These trends can be explained by the combined effects of spilling, leakage, and shallow biodegradation.

Good source rocks have never been found in Brunei despite intensive searches throughout the years.

Geochemical oil and gas typing indicates that land plant organic material is the source of most, if not all, of Brunei's hydrocarbons. Published geochemical studies indicate that this also holds for most of Northwest Borneo's Tertiary delta hydrocarbons.^7-9

This has led to a model that assumes dispersed organic matter of land plant origin, distributed throughout the proximal topset deltaic sediments, as the main hydrocarbon source. However, the mature hydrocarbon kitchen areas of offshore fields are for a large part within mainly deep marine sediments. This strongly suggests a significant charge contribution from deep sediments as well.

Recent studies provide evidence for "deep" outer neritic/bathyal source rocks which contain land plant derived material and indications that there is oil potential in, at first sight, gas prone organic matter.

Formation pressures

The available well data indicate that substantial overpressures (pressures close to lithostatic) always occur within or close to outer shelf to bathyal sediments.

The main cause of overpressures in these sediments is undercompaction. Overpressures are clearly related to shaliness of the sediments and high sedimentation rates. In an overall outbuilding system the sediments become more shaly with depth.

The top of overpressures is encountered at subsequently younger stratigraphic levels from the onshore to the distal offshore. Undercompaction related overpressures can be successfully predicted from seismic interval velocities, fine-tuned with geological models, as is routinely done in Brunei Shell.

Exploration now

From 1965 to end-1995, over 47,500 km of 2D seismic has been shot in Brunei. Since the late 1980s Brunei Shell has acquired mainly 3D seismic surveys with more than 8,600 sq km shot to end 1995.

Most of Brunei's offshore continental shelf area is now covered with 3D seismic data. After the substantial exploration successes in the 1960s and early 1970s it was the introduction of 3D seismic that opened up new opportunities.

Recent discoveries such as Peragam gas field below Champion oil field (1990) and Bugan and Selangkir gas fields (1993 and 1995, respectively), are based on 3D seismic exploration, supported by quantitative interpretation techniques (seismic attribute maps, amplitude versus offset, optical stacking, and volume interpretation).

In mature Champion oil field, high-resolution seismic has helped define the small fault blocks in the complex faulted crestal area. Long cable (6 km) 3D seismic helps the quantitative evaluation of seismic data (AVO). Sequence stratigraphic analysis, based on seismic and high-resolution biostratigraphy from wells, is used to identify prospective sequences. Such an integrated seismo/sequence stratigraphic approach is essential to further evaluate Brunei's turbidite potential.

In Brunei's young geology "the present is the key to the past." Despite its small size the country boasts two very different sediment distribution systems only 150 km separate: the Baram Delta and Brunei Bay (Fig. 4). Only 5,000 years ago the Baram system was very comparable to the present day Brunei Bay.¹⁰

The lower delta plain of the Baram Delta is now filled with huge raised peat bogs. In this short time the system prograded more than 50 km and changed from estuarine embayment to a classical wave-dominated delta. When interpreting the subsurface we should be aware of such rapid changes. Tectonics, affecting topography, sediment supply, as well as subsidence, strongly control deltaic sedimentation patterns, irrespective of global sea level changes.

Acknowledgements

This article is submitted with permission of the Petroleum Unit of the Government of Negara Brunei Darussalam and Brunei Shell Petroleum Co. Most of the material is taken from the recently published "Geology and Hydrocarbon Resources of Negara Brunei Darussalam," edited by Sandal.3 I thank the editor and authors of this book as well as all Brunei Shell exploration staff involved in its compilation.

References

1. Motley, J., Report on the geological phenomena of the island of Labuan and neighbourhood, Journal of the Indian Archipelago, 1852.
2. Harper, G.C., The discovery and development of the Seria oilfield, Penerbitan Khas Bil. 10, Muzium Brunei, 1975, pp 99.
3. Sandal, S. T. (ed.), The Geology and Hydrocarbon Resources of Negara Brunei Darussalam, 1996 Revision, 243 p., ISBN 99917-900-0-4.
4. James, D.M.D. (ed.), The Geology and Hydrocarbon Resources of Negara Brunei Darussalam, 1984, 164 p.
5. Rangin, C., Bellon, H., Benard, F., Letouzey, J., Muller, C., and Sanudin, T., Neogene arc-continent collision in Sabah, northern Borneo (Malaysia), Tectonophysics, Vol. 183, 1990, pp. 305-319.
6. Benard, F., Muller, C., Letouzey, J., Rangin, C., and Tahir, S., Evidence of multiphase deformation in the Rajang-Crocker Range (northern Borneo) from Landsat imagery interpretation: geodynamic implications, Tectonophysics, Vol. 183, 1990, pp. 321-339.
7. Awang Sapawi Awang Jamil, Eric Seah Peng Kiang, & Mona Liza Anwar, Geochemistry of selected crude oils from Sabah and Sarawak, Geol. Soc. Malaysia, Bull. 28, 1991, pp. 123-149.
8. Van de Weerd, A., and Armin, R.A., Origin and evolution of the Tertiary hydrocarbon-bearing basins in Kalimantan (Borneo), Indonesia, AAPG Bull., Vol. 76, 1992, pp. 1,778-1,803.
9. Woodroof, F.B., and Carr, A.D., Source rock and hydrocarbon geochemistry, offshore northwest Sabah, Malaysia (abs.), AAPG Int'l. Conf., Sydney, Australia, AAPG Bull., Vol. 76, 1992, pp. 1,133-34.
10. Caline, B. and Huong, J., New insights into the recent development of the Baram Delta from satellite imagery, Geol. Soc. Malaysia, Bull. 32, 1992, pp. 1-13.