缅甸若开海域深水生物气藏形成条件与富集规律

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.