东非凯瑞巴斯盆地多期构造变形及对油气聚集的控制作用

Multi-stage Tectonic Deformation and its Control on Hydrocarbon Accumulation in the Kerimbas Basin, East Africa

利用最新的钻井、地震资料,对东非凯瑞巴斯盆地(Kerimbas Basin)进行构造–地层解释和构造演化过程恢复。结果显示,凯瑞巴斯盆地共发育了4期构造变形:(1)二叠纪–早侏罗世晚期的冈瓦纳陆内裂谷活动,全区发生伸展构造变形;(2)早侏罗世晚期–早白垩世晚期马达加斯加向南漂移,全区发生右行走滑变形;(3)新发现晚白垩世局部伸展构造变形;(4)中新世–第四纪的东非裂谷海域分支活动,导致研究区发生第三期伸展构造变形,形成凯瑞巴斯盆地现今地堑形态。三期伸展构造变形的应力方向均为近E-W向,断层展布方向均为近S-N向。每一期构造变形的范围,强度及对沉积地层的控制作用差异显著。凯瑞巴斯盆地控坳断层活动具有继承性。基于研究结果,建立凯瑞巴斯盆地构造成因模式。冈瓦纳陆内裂谷活动有利于二叠系–下侏罗统构造圈闭的形成,并沟通了烃源岩和储层,有利深层油气的聚集;东非裂谷海域分支裂谷活动沟通了前新生界烃源岩和西部陆坡古近系储层,但同时也破坏了盆地内及东部的圈闭。断层不发育的西部陆坡为主要油气聚集区。

On the basis of the interpretation of new seismic and well data, balanced cross-sections were constructed to demonstrate the deformation characteristics and evolution of the Kerimbas Basin, offshore Northern Mozambique. The results indicate that the tectonic deformation of the Kerimbas Basin can be divided into four stages：（1） Gondwana intra-continental rifting from Permian to late Early Jurassic, and extensive half-grabens, grabens and horsts were formed;（2） Madagascar southward drift from late Early Jurassic to late Early Cretaceous, and transtensional structures developed;（3） Newly discovered local half-grabens formed in Late Cretaceous;（4） The rifting of East African Rift System offshore branch from Miocene to Quaternary, which is the third extensional tectonic deformation and caused the formation of the Kerimbas graben. Excluding the Madagascar southward drift stage, the orientations of extensional faults in the three extensional stages were mainly N-S-trending, which indicate that the three stages extensional deformation formed under regional N-S extensional stress field. However, the differences of distribution and intensity of tectonic deformation are significant. The main boundary faults of the Kerimbas Basin show apparent multi-stage inherited features, they formed in the early extensional deformation and reactived in the third stage of extensional deformation and still active recently, which is evidenced by large-scale fault scarps on the seafloor. Based on the results of our study, the genetic model of the Kerimbas Basin was built. The Gondwana intra-continental rifting promoted the trap formation in the strata from Permian to Lower Jurassic, and connected the source rocks and reservoirs, which promoted the oil and gas accumulation of the deep layers. The faults formed duding rifting of East African Rift System offshore branch connected the pre-Cenozoic source rocks and Paleogene reservoir in the western slope, whereas it also destroyed traps in the Kerimbas Basin and the eastern Davie Fracture Zone. Oil and gas mainly accumulated in the western slope where faults less developed.