再议四川盆地西部坳陷深盆气

RECONSIDERATION OF DEEP BASIN GAS IN THE WESTERN SICHUAN DEPRESSION OF THE SICHUAN BASIN

四川盆地西部坳陷(简称川西坳陷),在盆地类型、烃源岩性质、源—储组合关系、储层致密等方面具备深盆气成藏条件。但川西坳陷是一个多期构造作用形成的叠合盆地,上三叠统须家河组大量排烃作用发生在储层致密化之前,天然气主要在水动力和浮力作用下运聚成藏,在成藏机理上与深盆气有本质差异。上三叠统同一套储层在区域上不具气水倒置关系,天然气聚集带特征明显,主要受燕山期和印支期古隆起控制,气水分布规律与深盆气大面积饱含气不同,其最高单井日水产量可达1 000 m3以上。川西上三叠统须家河组天然气现今气藏类型主要以复合圈闭为主,含气丰度和地层水分布受圈闭内的裂缝及成岩储渗体控制,没有统一的气水界面。须4气藏可能是动态散失破坏的残余气藏,储量损失部分被地层水占据,含水丰度普遍较高。川西上三叠统须家河组是常规机理成藏,经过了多期构造运动作用的叠加—调整—改造后,成为具有非常规气水分布特征的天然气勘探领域,不属于深盆气范畴。

The Western Sichuan Depression of the Sichuan Basin is qualified for deep basin gas accumulation due to basin types, hydrocarbon source rock characteristics, reservoir-source combination and reservoir tightness. However, the Western Sichuan Depression is a combined basin influenced by stages of tectonic movements. Mass charges of hydrocarbon of the Xujiahe Formation of the Upper Triassic take place before reservoir tightening. Natural gas migrates under hydrodynamic and buoyancy effects, which is different from deep basin gas in accumulation mechanisms. Inversion of gas and water cannot be found in the same reservoir of the Upper Triassic. Natural gas is zoned obviously, mainly controlled by the Yanshanian and Indosinian uplifts. Distribution of gas and water is also different from deep basin gas. Water product of single well may reach 1 000 m^3/d. Present accumulation types of natural gas of the Xujiahe Formation are mainly combination traps. Gas content and water distribution are controlled by fractures in traps and diagenetic reservoir-leaking bodies. The Xu4 reservoir is probably a residual reservoir, partially filled with water. The Xujiahe Formation of the Upper Triassic in the Western Sichuan Depression is accumulated conventionally, yet deformed by stages of tectonic movements. Though it is characterized by abnormal gas and water distribution, it does not belong to deep basin gas.