川东北天然气成藏特征

Characteristics of gas accumulation in northeastern Sichuan basin

川东北天然气藏分布于早寒武世—志留纪 (∈1—S)、早二叠世—早三叠世 (P1—T1)及晚三叠世—中侏罗世 (T3 —J2 )等 3个异常超压系统内 ,具有多层叠置的特点。成藏规模取决于超压条件下水溶相天然气的脱溶程度 ,以埋深 4 5 0 0m为界 ,界线之上产气 ,界线之下产水 ,且产气量随地层抬升幅度增大而增大。川东北天然气藏的形成与保存由燕山期的埋深、喜山期的抬升及异常压力系统的保持情况三者共同决定 ,压力封闭为成藏的关键因素。因此 ,川东北天然气藏勘探应着眼于异常压力系统此一整体 。

Gas reservoirs in northeastern Sichuan basin are distributed in three overpressured systems,i.e.Lower Cambrian-Silurian(∈ 1-S),Lower Permian-Lower Triassic(P 1-T 1)and Upper Triassic-Middle Jurassic(T 3-J 2),and are characterized by multilayer superimposition.The scale of gas accumulations depends on degasification of water-soluble gas under overpressured condition.A burial depth of 4500m can be taken as boundary,under which water is produced,while above which gas is produced,and gas output increases along with the increase of uplifting.Formation and preservation of gas accumulations in northeastern Sichuan basin are determined by burial depth in Yanshan period,elevation in Himalayan period and preservation of abnormal pressure system.Pressure sealing is the key factor to gas accumulation.Therefore,gas exploration in this area should be based on prospecting the superimposed multilayers,and aiming at the overall abnormal pressure system.