Natural Gas – Underexplored in Bangladesh?
Natural gas (NG) is the principal fuel for electrical generation in Bangladesh and the foundation for the nation’s rapid economic growth during the last two decades (World Bank, 2020). During this period NG supply grew through the further development of pre-2000 field discoveries. Abundant supply encouraged growth and was the parent of strong economic expansion (~4.5% GDP since 2004). Concurrently, Bangladesh has successfully managed economic development and climate change adaptation with its NG cornerstone (Roy, 2021).
Demand and Supply Mismatch
NG demand, presently at ~3,950 MMcfd (million cubic feet per day), will grow to an estimated ~4,850 MMcfd in 2030 (Figure 1). Supply from local gas fields declined from a peak of ~2,750 MMcfd in August 2018 to ~2,430 MMcfd in October 2021 (Petrobangla, 2021). This is a direct response to the small number of exploration wells drilled since 2000: only 22. A further drop in local NG supply is projected out to 2041 (Figure 1). Over the last two decades the country consumed about 13 Tcf (trillion cubic feet) but added only 2 Tcf from new discoveries (Imam, 2021).
Since late 2018, Bangladesh has imported liquefied natural gas (LNG) to supplement NG supply to meet increasing demand. The declining supply from presently operated local gas fields means that the share of expensive imported LNG is expected to increase in the future from the present level of 4 MMtpa (million metric tonnes per annum) to 21 MMtpa in 2030 (Wood Mackenzie, 2021). In-country gas fields with reserves of about 12 Tcf, and more expensive LNG imports should provide the energy for moderate to high growth until 2030; if demand remains below about 2.5–3.0 Tcf per year. However, the recent price hike in the LNG spot market from less than $10 to more than $35 per MMBtu (metric million British Thermal Unit) means that Bangladesh must bear added pressure on its economy and must survive on the price uncertainties of LNG in the international market. After 2030 the energy supply and dependent economic growth will become even more precarious. Preventing economic decline will require a combination of new NG discoveries and reserve growth from existing fields, or expansion of other reliable and resilient base-load energy sources such as carbon-intensive coal or development of carbon-free nuclear power.
As noted by in-country experts including Shetol et al. (2019), “The possibility of natural gas being exhausted within a decade and half worries us all…Bangladesh is likely to exhaust its gas reserve by about 2030.” Imam (2021) states, “…Bangladesh’s gas exploration has apparently been stalled because of a lack of dynamic vision.” NG exploration needs to dramatically increase over the next few years to avoid potentially severe economic contraction after 2030.
High NG Potential but Underexplored
Today, many onshore and offshore exploration gas opportunities are available in Bangladesh. Only a few blocks (Figure 2) are held by international oil companies (IOCs). Under its charter, Petrobangla has the right to enter into petroleum agreements with any IOC for the purpose of any petroleum operations and explorers must joint-venture with the Bangladesh Petroleum Exploration and Production Company Limited (BAPEX), a subsidiary of Petrobangla, to obtain licences.
A 2001 US Geological Survey estimate of mean-undiscovered, conventional-play NG in Bangladesh totals 32.1 Tcf and is divided into six assessment units (Figure 3). Another estimate of mean undiscovered NG resource in Bangladesh by the Norwegian Petroleum Directorate (NPD) indicated local Hydrocarbon Unit (HCU) totals of 41.6 Tcf (HCU-NPD, 2001). Today, there are 27 gas fields (Figure 3), all located within three of the USGS assessment units that forecasted 28.9 Tcf of undiscovered NG. As of the end of 2020 the proved plus probable NG reserves were 28.2 Tcf (Petrobangla, 2020) and nearly equal to the earlier USGS assessment. To date, only 95 exploration wells have been drilled and much of the country remains remarkably underexplored given its extensive and rich petroleum systems (Curiale et al., 2002). Much of Bangladesh is favourably located for NG accumulations as shown by the extent of the actively generating source-rock pod (Figure 3; USGS, 2001). Underexplored areas are the offshore, the western half of the onshore, and the two USGS assessment units with existing gas fields in the central and south-east. Geochemical modelling shows peak generation for NG occurs at depths of 18,000–20,000 feet (5,500–6,100 metres) or more, and subsurface geologic and geophysical mapping reveals that active NG generation pods should occur under all of central and eastern onshore Bangladesh and all of offshore Bangladesh (Figure 3; USGS, 2001).
Several high-potential gas plays are present across much of Bangladesh due to an active petroleum system, a thick sedimentary basin with rich, mature source rocks (Curiale et al., 2002), and various geologic settings developed across the basin provide a range of depositional and structural trap styles. Since the 1920s, only 10 exploration wells have been drilled in the extensive eastern fold belt that has surface hydrocarbon seeps and numerous large anticlines as shown by surface mapping (Figure 4) and cross-sections (Figure 5). Another very promising gas play is the numerous, untested stratigraphic traps developed in the complex depositional system that resulted from Bangladesh’s location along an evolving plate margin (Indian and Eurasian Plates collision), high-uplift and erosion rates (Himalayas) that resulted in high-depositional rates and complex depositional patterns in the Ganges-Brahmaputra Delta.
A Range of NG Plays
The low frequency of exploration drilling obscures the true potential of Bangladesh’s gas resources. As one of the world’s largest delta deposits, highly influenced by tectonic plate movements, its structural and stratigraphic gas plays are abundant and warrant more attention (Imam, in press).
The play types can be broadly categorised as follows:
Undrilled anticlinal plays: The geological map (Figure 4) and generalised cross-sections (Figure 5) across the eastern fold belt (EFB, aka Tripura-Chittagong Hill Tracts) show many untested and partially tested anticlinal and other convergent structural traps plus the potential for sub-thrust traps that are not obvious at the surface. Our structural work favours about 4–5 Tcf of undiscovered NG in the EFB that is similar to the high range of the 2001 USGS estimate for the two assessment units within the EFB.
Previously drilled anticlinal plays with inconclusive tests: Heavy mud used in the drilling of several exploration wells in the 1950s (Patya and Jaldi anticlines for example), may have deterred gas shows and retesting of these anticlines with conventional, lighter muds has yet to occur.
Synclinal plays – lessons from Tripura: Synclines are not normally considered as petroleum traps, but in the Indian Tripura fold belt, a northern continuation of the EFB fold belt, several discoveries have been made in broad synclinal valleys between the anticlines. Low-angle fault-detachments, angular unconformities in growth-strata, or relict structures (buried hills) probably explain the existence of these concealed traps. It is reasonable to expect that an analogous synclinal play exists in the adjacent EFB of Bangladesh.
Rollover anticline: Seismic evidence exists for the occurrence of listric faults in north-central Bangladesh and the development of hanging-wall, rollover anticlinal traps (Lohmann, 1995), and onshore blocks 8 and 11 contain prospective areas for such traps.
Deepwater fold-belt play: Geologic and geophysical studies indicate the deeper portions of the Bay of Bengal (Hatia Trough and Rakhine Sub-basin [Xu et al., 2015]) show the likelihood for well-developed, gravity-controlled, deepwater fold-belts and their associated hydrocarbon traps. World-wide deepwater fold-belts have become significant, commercial oil and gas provinces during the last three decades (e.g., Gulf of Mexico and Niger Delta).
Deepwater turbidite play: One of the most high-potential plays consists of stratigraphic traps in the Mio-Pliocene deepwater, turbidite-deposits that occur across a wide area of Bangladesh. A variety of turbidite sand facies are described as prospective reservoirs (Bowles et al., 2019). These are primarily channel-levee complexes and basin-floor fan-lobes that include sand facies in the feeder channel deposits, channel levees, crevasse splays, distributary channels, and fan lobes. Large gas finds with turbidite plays are known from the offshore Rakhain Basin (Myanmar), adjacent to south-eastern offshore Bangladesh in the Bay of Bengal i.e., Shwe, Shwe Phyu, Mya, Thalin and others (Figure 6). Offshore Bangladesh, adjacent to the maritime boundary with Myanmar is the most likely place where the next round of large gas fields are likely to be discovered.
Channel and incised valley plays: Seismic evidence depicts the Mio-Pliocene sections of Bangladesh as displaying erosioninduced intense channelling (Najman et al., 2012). Channels are either sand-filled or clay-plugged and in both cases stratigraphic traps are formed.
Drape over basement horst and fault: In the north-west stable shelf area of Bangladesh, deep-seated fault blocks or basement horsts are associated with overlying drape folds forming potential plays.
Onlap pinchout: The onlapping seismic facies are interpreted as high energy transgressive coastal sandstones pinching out against the slope (Salt et al., 1986) and such sediments should have excellent reservoir qualities.
Carbonate build up and shelf-margin reef prospects: A potential carbonate play is the shelf-margin carbonate reefs developed in the Eocene Sylhet Limestone Formation (Moore and Leyenburger, 1992).
Several unconventional NG plays exist but remain untested, including Change to tight sand reservoirs, thin-bedded sand sequences, and shale source bed accumulations. Over-pressuring is known to exist in the deep basinal areas and could assist production in these unconventional plays.