| Since 2004, several gas fields (Shwe, Shwe Phyu, May, Thalin etc.) have been discovered from shelf edge to
deep-water (more than 1 000 m) in the Rakhine Basin, northeast Bay of Bengal. The gas is likely to be biogenic based on
the methane content more than 97% and δ13C ranging from -70‰ to -60‰. The biogenic gas was trapped in turbidity
sandstones (lobes, channels, levees and sandy MTCs) of NW-SE anticlines from Lower Pleistocene to Pliocene. The
reservoir quality is moderate to good with the porosity of 20%-30% and the permeability from tens of millidarcy to
hundreds of millidarcy. This paper uses high-resolution seismic data covering from shelf edge to the slope and basin, well
data and gas play data to analyze the biogas generation and accumulation in deep-water turbidites, in the hope of
providing some experiences for the biogas exploration in offshore Rakhine and other areas with the similar depositional
environment. The research reveals the following results：(1) The gas bearing sandstones are buried at depth of 1 200-3 000 m
below the sea level, where the reservoir temperature ranges from 30 ℃ to 90 ℃. This is the suitable environment for
methanobacteria. Low geothermal gradient, high sediment accumulation rate and rapid temperature rise provide good
conditions for biogas generation. (2) The discovered gas fields are located at the Pliocene lower slope. The thick
mudstones in the lower slope can generate a large amount biogas which can migrate in short distance and accumulate, and
the high sedimentation rate in the slope is also good for preservation of porosity and suitable for methanobacteria to live.
(3) The stacked large-scale channel-levee systems transported rich terrestrial organic materials. The biogas generated
from these organic material can migrate into adjacent channel sandstones and lobe sandstones, forming the favorable
source-reservoir-cap assemblages. (4) The anticlines in the gentle folded belt can trap a large quantity of biogas. (5) The
organic matter in offshore Rakhine Basin mainly derived from the terrestrial plants. The experimental analysis of drilling
in this area proved that the source rocks of the Pliocene-Lower Pleistocene is much better than the Miocene. In addition,
the isotopic data shows that organic matters in Pliocene-Lower Pleistocene are dominated by C4 biomass which is more
favorable for biogas generation than C3 biomass based on a series of experimental analysis. Thus, the Lower Pleistocene
and Pliocene turbidite sandstones are the main targets for biogas exploration in the offshore Rakhine Basin. |
