断裂坡折带低位扇成因及成藏特征

断裂坡折带为同沉积构造长期活动引起的沉积斜坡明显突变的地带,其低位扇主要包括水下冲积扇、大型浊积扇、小型低位三角洲和低位扇三角洲。陡坡带低位浊积扇及扇三角洲远端部分分布于低台阶,扇三角洲主体、下切谷充填分布在高台阶,更高处为三角洲近端部分及冲积扇。缓坡带在沉积基准面快速下降时,其上产生陆上下切水道,其下形成水下冲积扇砂体;当基准面缓慢上升时,在断裂坡折带之上形成下切水道充填,之下发育水下冲积扇砂体和远岸浊积扇砂体。断裂坡折带低位扇易形成原生隐蔽岩性油气藏,此类油气藏具有离源岩近、次生裂隙发育和疏导条件优越的特征。

ORIGIN OF LOWSTAND FAN IN FAULTED-TOPOGRAPHIC BREAK & PLAY'S CHARACTERISTICS

The faulted-topographic break zone is a zone of depositional slope with apparently abrupt-change caused by longtime activities of homo-depositional tectonics.A lowstand fan of the zone mainly consists of underwater alluvial fans,larger turbidite fans, smaller lowstand delta and lowstand fan delta.Lowstand turbidite fans in the steep gradient zone and distant part of the fan-shaped delta are distributed in low terrace,and main body of the fan-shaped delta and the incised-valley fillings are distributed in high terrace while near shore part of the delta and the alluvial fans are developed in higher terrace.During quick subsidence of the depositional base-level,continental incised-channels were formed on the slow slope zone and underwater alluvial fan bodies were developed down below the slope.When the base level was slowly uplift,incised channel fillings were formed on the faulted topographic break zone and down below the zone underwater alluvial fan sandbodies and distant turbidite fan bodies were developed.It is believed that original subtle litho-reservoir could be easily formed in lowstant fans of the faulted-topographic break zone.This kind of reservoir is characterized by its closer distance to source rocks,by well-developed porosity and by superior dredging conditions.