Colette Lyle examines the Rakhine Basin using a series of 1D Basin Models to investigate both thermogenic and biogenic sources and also uses data, models, and regional geological knowledge to assess the likelihood of gas generation and investigate the relative timings of the key petroleum system elements.
Exploration history/introduction
Gas exploration in Southeast Asia is currently increasing, driven by both rising domestic energy needs and the global transition towards cleaner fuels. Gas is considered a transitional fuel between traditional energy sources such as coal, and renewables such as wind, solar, or biofuels, and as such it will remain a significant contributor to the energy mix well into the future. Southeast Asian nations are, therefore, looking to increase investment and exploration in the region and many are modifying foreign investment laws to support this renewed exploration drive.
The Ganges-Brahmaputra Delta and associated Bengal Fan has yielded several large gas fields (mutli-Tcf) in recent years (Figure 1) and, if compared to other Neogene deltas, could indicate the presence of hundreds of Tcf yet-to-find, making the offshore extensions of the Rakhine Basin an enticing future exploration prospect, should political stability return to the region.
The area is relatively underexplored and as such, detailed knowledge of the petroleum system elements is limited (Evans-Smith 2024). Source rock is a key element that requires investigation and while the presence of gas in the Shwe gas fields proves that a working source rock is present within the offshore area, questions remain about its heritage (thermogenic vs biogenic) and the timing of its generation. This uncertainty means that source rock maturity and understanding the timing of generation are key risks in the offshore areas of the Rakhine Basin, for the wider Bengal fan region as well as conceptualising potential extensions into the frontier Andaman Sea.
In this article, we examine the Rakhine Basin using a series of 1D Basin Models in Permedia®, a Halliburton basin modelling application, to investigate both thermogenic and biogenic sources. We also use data, models, and regional geological knowledge within the Neftex® solution from Halliburton, to assess the likelihood of gas generation and investigate the relative timings of the key petroleum system elements. Using 1D modelling methodologies for select locations within the Rakhine Basin (Figure 1), a potential thermogenic kitchen is identified and generation timing (if source is present) is potentially de-risked. The 1D modelling also reveals a positive story for biogenic gas timing, but this work has identified that specific subsurface conditions are required in order to stall migration of gas generated before trap formation. Therefore, understanding depositional history, heat flow, and timing of generation relative to trap formation will be critical for reducing geological risk and unlocking opportunities in this promising frontier and surrounding or analogous basins.
Source rocks of the Rakhine Basin
Within the Rakhine Basin, the gas discovered in Plio-Pleistocene reservoirs is predominantly biogenic in origin, although there is evidence of a thermogenic contribution in some instances, for example, well Phyi Thar 1 (Ridd and Racey 2015; Myint 2019).
Very little information regarding the stratigraphy of the offshore Rakhine Basin is known, owing to not only the limited number of wells drilled, but also the stratigraphic coverage of those wells. Most wells in the area only reach into Pliocene Stratigraphy, with a select few reaching the Miocene e.g. BINA_2 (Figure 2).
For the thermogenic source, three potential organic-rich horizons are identified – Early Eocene, Late Eocene, and mid-Cretaceous. These horizons broadly correlate with contemporaneous horizons onshore, although it is not known if these fully extend offshore (Basu et al., 2010; Ridd and Racey, 2015). The theoretical driving mechanism for offshore organic enrichment varies depending on the tectonic setting at the time of deposition. The Cretaceous organic enrichment is inferred to correspond to Oceanic Anoxic Event 2 (OAE2), due to a lack of restriction in the relatively open setting of the Indian Ocean at the time. Conversely, restriction is the inferred driving mechanism for Eocene organic-enrichment, owing to the development of silled basins and a deepening forearc during collision along the Indo-Burma plate boundary. All organic-rich horizons are likely to be clastic in nature.
The biogenic source rocks in the Rakhine Basin are well documented and known to source a number of significant gas discoveries, such as the Shwe Gas Field. The main biogenic source rock is mid-Late Miocene to Pliocene and is associated with deposition on the Bengal Fan (Chung et al., 2012; Myint, 2019).
Testing maturity of thermogenic source rocks
One of the key driving mechanisms of oil and gas generation from organic-rich rocks in the subsurface is heat flow. Heat flow and its variation through time in relation to the deposition of organic-rich rocks impacts not only the level of maturity the rock reaches, but also the timing of generation from that source rock.
The most impactful event affecting heat flow offshore in the Rakhine Basin is the development of the Ganges-Brahmaputra Delta and the associated Bengal Fan (Figure 3A). This massive influx of deltaic sediments significantly impacted heat flow in two main ways. Firstly, the rapid deposition of cool sediments significantly reduces surface heat flow, and secondly the associated subsidence and filling of the basin which thickens the sedimentary package (Figure 3B) thus increasing the distance between the sediment surface and basement (i.e. the heat source). In general, cooler heat flows are not conducive to thermogenic hydrocarbon generation.
However, the initiation and development of the delta isn’t all bad news. While the influx of sediments does cool the surface heat flow, it also provides a mechanism by which sediments are buried deeper and rapidly, potentially pushing organic-rich rocks through the suppressed oil and/or gas windows.
Through 1D basin modelling, these derived heat flows (Figure 3A) have been applied to wells and pseudo wells in the Rakhine Basin (Figure 1) revealing that maturity of the 3 hypothetical source rocks — Late Cretaceous, Early Eocene and Late Eocene — is directly correlated to the location of the well within and around the delta and fan. For example, wells located within the delta (e.g. BINA_2) and fan (e.g. pseudo well RAKHINE_3) have thicker stratigraphic sections and significantly higher burial depths. Modelling revealed that all three postulated source rocks are over-mature under these conditions. Conversely, with models built away from the delta and fan, where the heat flow is less impacted by the effects of deltaic sediments (well A7a and A7b, and pseudo well RAKHINA_4), all three hypothetical source rocks are immature in the present day. Therefore, it is unlikely that source rock maturity would be ubiquitous across the Rakhine Basin, and indeed, in other offshore regional extensions.
Investigation generation timing of thermogenic source rocks
Maturity isn’t the only consideration when investigating the viability of a potential source rock. The timing of generation relative to reservoir deposition and trap formation also plays a key role in the development of a working petroleum system.
Using 1D basin modelling on wells that experienced sufficient burial and temperatures to generate hydrocarbons (in this case BINA_2 and RAKHINE_3 located within the delta and upper fan respectively), we can investigate the timing of hydrocarbon generation and determine if this was positive or negative in relation to the development of Oligoene, Miocene, and Plio-Pleistocene reservoirs and traps.
1D basin modeling at BINA_2 reveals that 2 out of the 3 postulated source rocks were generated at a time conducive to trapping. These were the mid-Cretaceous and Early Eocene units, both beginning generation in the Eocene and continuing up to the Late Miocene (Figure 4). Much of the generation of hydrocarbons was therefore coincident with, or subsequent to reservoir deposition and trap formation.
Further out into the basin and into the upper Bengal Fan (pseudo well RAKHINE_3), the duration of gas generation was significantly shorter and occurred in the Miocene (mid-Cretaceous and Early Eocene source rock) and Pliocene-present (Late Eocene source rock). This timing suggests that the postulated Miocene reservoirs will have been in place prior to generation from all three source rocks, and for all but the Late Eocene source rock, generation occurred prior to deposition of the Plio-Pleistocene reservoirs.
Investigation generation timing of biogenic source rocks
The timing of generation from a biogenic source is a key geological risk in the Rakhine Basin. In shallow water areas, such as at the wells BINA_2, A7a and A7b, the Miocene-Pliocene biogenic source sits below the biogenic gas window in present day. This suggests that much of the generation is likely to have taken place before Plio-Pleistocene reservoirs were deposited. It is therefore postulated that there must have been temporary storage of that gas within the subsurface, before final migration into the reservoir. At the Shwe gas fields, this mechanism could have been gas hydrates (Myint 2019).
Gas hydrates can temporarily halt the migration of gas in two ways. The first is by trapping the gas within the crystal lattice of the hydrate itself. The second is by acting as an impermeable barrier that prevents upward migration of free gas from below. It is only when the unit is buried beyond the hydrate stability zone that the gas hydrate destabilises, and the gas is released.
In shallow water areas of the Rakhine Basin, water depths conducive to hydrate formation occurred throughout deposition of the mid-Late Miocene biogenic source and, therefore, it is possible that generated gas was temporarily stored as hydrate until after Plio-Pleistocene reservoir deposition.
Hydrate destabilisation — taken as the point at which the top of the biogenic source rock passes through temperatures above 16°C — typically occurs after deposition of Plio-Pleistocene reservoirs, although the difference between the two events varies depending on a number of factors including subsidence rates, timing of reservoir deposition, and water depth, among others (Figure 5).
Summary of findings
The conditions for thermogenic gas generation within the offshore Rakhine Basin are possible. However, they are unlikely to be ubiquitous across the basin due to the complex and varied interplay between tectonics and depositional environments through time – a risk that is common along this margin. These factors significantly impact heat flow and burial depths across the basin, which in turn could have a significant impact on both the maturity of any potential source rock and the timing of any gas generation.
1D basin modelling at sites across the Rakhine Basin have revealed that any potential source rock must be buried below 8-9 km in order to pass through the gas window, suggesting that any potential kitchen would need to be located within the delta and fan complex. This in itself presents a challenge as there are areas of the basin where the base of the stratigraphic pile does not reach 8 km. We must also consider the challenges of migration through such thicknesses of predominantly mud-rich, deltaic, and deep-water facies.
Key risks regarding the thermogenic source remain, such as presence, thickness, and quality of any source rock in the offshore area.
The story is much more positive for biogenic gas, which has been proven at a number of large fields across the Rakhine Basin. 1D basin modelling has demonstrated that this success could be replicated at other sites where conditions are right for hydrate formation, temporarily storing generated gas until after deposition of Plio-Pleistocene reservoirs.
1D basin modelling across the Rakhine Basin demonstrates that any future discoveries are likely to be dominated by biogenic gas with some potential for thermogenic contributions in limited circumstances. The basin potentially offers exciting opportunities for future exploration in shallow water, if the temporary storage of biogenic gas in hydrates safely contained significant portions of gas until after reservoir deposition and trap formation.
References
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