缅甸安达曼海弧后坳陷天然气成藏要素及成藏模式

Main controlling factors and models of gas accumulation in Back-arc Depression of Andaman Sea,Burma

缅甸安达曼海已成为中国石油企业的重要海外探区之一。受印度板块向欧亚板块和西缅甸微地块多期斜向俯冲的影响,该区先后经历了被动陆缘、过渡型陆缘及主动陆缘的演化阶段,盆地属性与油气成藏特征极为复杂。通过系统梳理安达曼海弧后坳陷的烃源岩、储盖层、圈闭、输导体系与油气生-运-聚匹配史等成藏要素,总结出该区油气成藏主控因素。结果表明:本区发育渐新统—下中新统、中中新统与上新统3套泥质烃源岩,前二者为成熟烃源岩,后者为生物气源岩;岛弧带上碳酸盐岩礁体与斜坡带上砂体的储集条件良好;斜坡带继承性古隆起之上的断背斜圈闭容积大;高角度张性断裂与连通性砂体分别构成以垂向、侧向为主的输导体系。结合油气成因、运移方式与圈闭类型的主要特征,建立了2种成藏模式:①中央凹陷带浅层生物气或混源气垂向运移断块圈闭成藏模式;②东部斜坡带、西部斜坡和岛弧带热解气侧向运移断背斜-岩性圈闭成藏模式。西部斜坡及岛弧带上的碳酸盐岩礁体和东部斜坡带继承性古隆起之上的大型断背斜是有利勘探目标。

Andaman Sea in Bruma is one of the major foreign exploration regions for oil companies of China. Controlled by multiple episodes of oblique subduction of Indian Plate to Eurasian Plate and western Burma micro block, the tectonic attribution of this block evolved from passive continental margin, then transitional continental margin and last to active continental margin, with extremely complex basin structure and hydrocarbon accumulation. The main controlling factors of hydrocarbon accumulation were summarized by analyzing the source rock, reservoir, trap, migration pathway and the histories of hydrocarbon generation, migration and accumulation in Back-arc Depression of Andaman Sea. There were three sets of hydrocarbon source rocks in this depression, including mature rocks in Oligocene-Lower Miocene and Middle Miocene and biogas rocks in Pliocene. The reservoir conditions of carbonate reefs on island arc and sandbodies on slope were favorable. The capacities of faulted anticline traps developed on inheritable palaeohigh in the eastern and western slopes were large. High-angle extensional faults and connective sandbodies were the main pathways of hydrocarbon migration. Considering the features of hydrocarbon generation, migration pathway and trap types, two types of accumulation models were established： one was biogas or mixed gas migrated vertically and accumulated mainly within fault block traps in shallow layer of central sag zone; another was thermogenic gas migrated laterally and accumulated mainly within anticlinal and lithological traps in the eastern slope and western slope and island arc zone. The results suggested that carbonate reefs on the western slope and island arc and large faulted anticline trap on inheritable palaeohigh in the eastern slope could be the favorable exploration targets.