四川盆地深层海相碳酸盐岩气藏成藏模式

Formation models of marine carbonate natural gas pools in the deep part of the Sichuan basin,China

四川盆地深层碳酸盐岩气藏主要指中三叠统雷口坡组及更老层位海相碳酸盐岩为储层形成的气藏,一般(曾)埋深在4500m之下,按类型可分为原生气藏(生气中心、储气中心和保气中心位于同一层位内)和次生气藏(生气中心、储气中心与保气中心位于不同层位)2类。原生气藏储层内含有大量沥青,天然气主要为原油裂解气。次生气藏储层内不含沥青,但天然气仍然主要为原油裂解气。原生气藏的成藏模式有三中心叠合模式、储气中心解体模式、三中心短距离移位模式和缺乏保气中心模式。次生气藏的成藏模式主要以天然气跨层运移为主要特征,其气源来自于先存的天然气藏。三中心叠合模式的原油裂解气成藏效率最高,储气中心解体模式的原油裂解气成藏效率中等,三中心短距离移位模式和次生气藏形成模式的原油裂解气成藏效率较低,保气中心缺乏模式的原油裂解气成藏效率为零。因此,在勘探策略上应重视三中心叠合和储气中心解体模式形成的原生气藏的勘探,同时应关注由震旦系灯影组古气藏(储气中心)破坏而形成的下古生界次生气藏。

A center in the present paper is referred to as an area or region which may include one or more hydrocarbon accumulations.A hydrocarbon generation center is referred to as an area containing high quality source rock which was subjected to thermal maturation.A gas generation center is an area in which an oil pool or accumulation was present,and oil was thermally cracked to generate gas.A gas accumulation center is referred to as an area in which natural gas generated from cracked oil accumulated.A gas preservation center is referred to as an area or region where the present natural gas pool/pools is/are located.The paleo oil pools（gas generation centers） were the main hydrocarbon sources of the paleo gas pools（gas accumulation centers）,which were in turn the main sources of hydrocarbons for today＇s（remaining） gas pools（gas preservation centers）.The deep marine carbonate natural gas pools in Sichuan basin principally mean that their reservoir rocks were deposited before Late Triassic and buried deeper than 4500m,which can be classified into two types according to the coupling relationships between the gas generation centers,gas accumulation centers and preservation gas centers.One type is called the primary natural gas pools,whose preservation centers were located at the same stratigraphical unit with the gas generation centers and gas accumulation centers.The other type is called the second natural gas pools,whose gas preservation centers were located at the different stratigraphical unit from the gas generation centers and gas accumulation centers.Most of gas both in the primary and the second gas pools is from oil crack.The reservoir rocks of the primary natural gas pools contain lots of bitumen generated by the crack of oil,but there is not bitumen in the reservoir rocks of the second natural gas pools.There are four formation models of the primary natural gas pools in the deep part of the Sichuan basin：（1） an accumulation mode with the ＇three centers＇ being superimposed;（2） an accumulation mode with ＇the preservation center＇ disintegrated;（3） an accumulation mode with the ＇three centers＇ migrated for a short distance;（4） a destruction mode with the preservation center lost.The accumulation ratio of gas from oil cracking gradually decreases to zero from accumulation mode（1） to destruction mode（4）.The key factors of gas accumulation and preservation were occurrence of rich hydrocarbon sources and good preservation conditions for the gas pools.The key factor forming the second natural gas pools is cross-flow of gas,which is from the destruction of previously underlain gas pools.Therefore the accumulation mode with the ＇three centers＇ being superimposed and the accumulation mode with ＇the preservation center＇ disintegrated should be paid great attention to in the future petroleum exploration.Besides the secondary natural gas pools in the Lower Paleozoic could not be ignored,which were formed in the process of destruction of Sinian paleo-gas pools（accumulation gas centers）.