Key controlling factors of biogas accumulation in foredeep basin of subduction zone: A case study of the Rakhine Basin in Myanmar

The Rakhine Basin is a Tertiary foredeep basin along the eastern fringe of the Bay of Bengal  covers an area of 165,000km², with Tertiary foredeep sediments up to 20km . Consecutive discoveries of biogas fields (Shwe, Thalin, Pyi Thit and Aung Siddhi) suggest the Rakhine Basin has great exploration potential for biogas. However, the accumulation of biogas fields are complex and the main controlling factors of accumulation are still uncertain, also led to many exploratory wells failure.

In paper, the anatomical and statistical analysis of the 36 major biogas fields in the world suggests that the biogas fields generally have eight mail geological conditions, including young sediments, shallow burial depth, high sedimentation rate, rich content of organic matter, tectonic setting or structural traps, cap rocks, in-situ generation & charge and low temperature gradient. The Rakhine Basin also has the 8 geological conditions, but the biogas reservoir is controlled by three key factors, according to research.

First, the relatively good source rocks (TOC ≥0.5%),  . The lower limit of TOC of biogas source rock is 0.46% by the chemical analysis and theoretical calculation of modern sediment samples. TOC = 0.46% is a critical value, when TOC＞0.46%, the biogas generated can be greater than biogas adsorbed in the formation and dissolved in the formation water, and extra biogas can be accumulation. At last, the TOC is identified minimum of 0.5%.

Second, the large-scale inter-bedded sandstones. Thin inter-bedded sandstones provide plenty of biochemical reaction interfaces, favoring methanogens to thrive and produce large amounts of biogas. The large-scale inter-bedded sandstones of the Shwe biogas field in the Rakhine Basin is 1500-2000 km², and the gas reservoir is 40-60 km². Preliminary established the scale of sandstone at least is more than 1000 km². At the same time, the large scale sandstone is also conducive to the capture of biogas in a large area.

Third, the structural traps. The large biogas fields in Rakhine basin and in other basins are always on the 4-way-dip anticlines. Biogas fields discovered in lithologic traps are always small and not commercial. Therefore, the positive structure controls the enrichment and accumulation of biogas in the Rakhine Basin.

Based on the research of the three key control factors, the risks of biogas exploration in the Rakhine Basin is reduced, and this understanding can be applied to the biogas exploration in deep-water sedimentary basins of continental margin in the world.