海拉尔盆地乌尔逊—贝尔凹陷构造特征与油气成藏过程分析

Analysis on structural features and reservoir-forming process of Wuerxun and Beier sags in Hailaer Basin

海拉尔盆地是一个多期叠合和多期改造的中生代断陷盆地,由下而上可以划分为下部伸展断陷盆地(T5-T22)、中部走滑拉分盆地(T22-T04)和上部坳陷盆地(T04-地表)3个原型盆地。每一期盆地建造之后都经历了相应的盆地改造过程,分别对应着南屯期末、伊敏期末和晚期(青元岗期后)。盆地的这种多期建造和多期改造特征控制了盆地内油气成藏要素的时空配置关系,因而决定了油气勘探的方向。伸展断陷发育时期,乌尔逊-贝尔凹陷为受NEE和近SN向断裂控制的箕状断陷盆地群,控制了盆地内有效烃源岩的空间分布。中部走滑拉分时期,受近SN向和NEE向断裂控制,形成了乌北、乌南、贝西、贝中4个次洼,控制了乌尔逊-贝尔凹陷区域盖层的空间分布,并与上部坳陷盆地一起控制了下部烃源岩的成熟范围。3期改造作用提供了油气运移的输导体系,并决定了有利构造圈闭的空间分布特征。伊敏期是油气的主要成藏期,在此之前定型并受晚期构造微弱改造的构造圈闭是有利的油气勘探目标。

Hailaer Basin is a Mesozoic faulted-depression basin which undergoes several periods of superimposition and reformation. From the bottom to the top, it can be divided into three prototype basins： extending faulted-depression basin in the bottom （T5- T22 ）, strike-slip pull-apart basin in the middle （T22-T04 ）, and depression basin in the top （T04- surface）. In every period, after the basin was formed, it underwent reformation, separately corresponding to the end of Nantun Age and Yimin Age and after Qingyuangang Age. The characteristics with several superimpositions and reformations controlled the relationship of space-time allocation of oil-gas formation and accumulation, so it determined the direction of oil-gas exploration activity. During the extending faulted-depression period, there were two half-graben faulted depression basins under the control of NNE trending and SN trending faults, and they controlled the distribution of available hydrocarbon source rocks. During the strike-slip pull-apart period, the basin was under the control of SN trending and NNE trending faults, and formed four sub sags, including Wubei sub-sag, Wunan subsag, Beixi sub-sag and Beizhong sub-sag, which controlled distribution of regional caprock of Wuerxun and Beier sags, and maturity area of hydrocarbon source rock in the bottom with depression basin in the top. The three reformations provided transforming system for oil-gas migration, and determined the distribution feature of favorable structural traps. Yimin Age is the main period of reservoir-forming of oil and gas. Structural traps formed before Yimin Age and slightly reformed by late structure are favorable exploration area.