缅甸若开海域生物气藏气源成因分析

Genesis analysis of gas source of biogenic gas reservoir in Rakhine Basin,Myanmar

缅甸若开盆地深水区域具备良好的生物气成藏条件和勘探前景,生物气藏作为一种非常规天然气藏,其成藏条件较为苛刻,研究生物气藏的气源成因,是成藏因素分析的首要问题。通过地震正演建模、地球化学分析等方法,证实缅甸若开海域中方勘探区含气地震响应特征明显,且为生物气藏。若开海域生物气烃源岩表现为"三低、一高、弱成岩"的地质特征,即较低的有机碳下限值、较低的热演化程度、低地温环境、较高的沉积速率、岩石固结程度处于弱成岩阶段中早期。峡谷泥岩、中新统泥岩、上新统泥岩均可以为甲烷菌提供食物—物质来源,上新统以上的泥岩不具备形成生物气的物质基础。推测研究区可能存在天然气水合物,且受到外界因素影响,使得压强和温度改变,导致天然气水合物的稳定状态改变,致使其排出大量生物甲烷气,进而在圈闭内聚集成藏,表现为气体不断形成、不断增压—突破封存压力界限—气体泄漏—再次压力平衡—气体再生成、再增压—再突破封存压力—气体再泄漏—再平衡的排驱模式。

There are good biogenic gas accumulation conditions and exploration prospects in deepwater area of Rakhine Basin in Myanmar. Biogenic gas reservoirs,as unconventional natural gas reservoirs,form under very strict conditions. It is the primary issue to study the genesis of biogenic gas-source for analysis of factors of gas accumulation. It is proved by geochemistry analysis and seismic forward modeling that the seismic response created by gas chimneys,also known as biogenic gas reservoir,is obvious in Chinese exploration area of Rakhine Basin in Myanmar. Hydrocarbon source rocks of biogas is characterized by geologic features of'three high and one low,weak diagenesis',indicating low lower organic carbon limit,low thermal maturation,low geothermal temperature environment and high deposition rate. The rock consolidation degree shows that it was in the earlier stage of weak diagenesis. Canyon shale,Miocene mudstones and Pliocene mudstones could provide food and material source for methane bacteria. The overlying mudstones on the Pliocene mudstones does not have the material conditions of biogas formation. It is speculated that natural gas hydrate might exist in the research area,and its stability changes with the variation of pressure and temperature caused by external factors. As a result,amounts of biogenic methane gas is expelled and entrapped to form biogenic gas reservoir. The process of gas expulsion and accumulation can be described as continuous gas generation and pressure increase-breaking pressure balance-gas leak-pressure rebalancing-gas regeneration and the second pressure increase-breaking pressure balance again-the second gas leak-pressure rebalancing.