乌石凹陷古近系层序地层及沉积体系

PALEOGENE SEQUENCE STRATIGRAPHY AND SEDIMENTARY SYSTEM IN WUSHI SAG

乌石凹陷位于北部湾盆地南部坳陷的东北部。为指导该凹陷的油气勘探工作,以层序地层学、沉积学理论为指导,以地震剖面为基础,结合钻井、测井、古生物、岩心等资料,研究了该凹陷古近系的地层沉积特征。结果认为:乌石凹陷古近系从下至上可以划分为7个三级地震(测井)层序,整体上为一个水进到水退的序列,分别对应不同的构造演化阶段;不同构造演化阶段发育的层序具有不同的充填序列和沉积体系分布;构造演化的多期性使得该凹陷多个部位都有烃源岩和储、盖层发育;其中以强烈裂陷阶段沉积的层序2—层序4中发育的烃源岩和盖层条件为最好且发育多种有利储集体,是该凹陷寻找油气藏的最有利层段。

The Wushi sag is located in the northeastern part of the southern depression,the Beibuwan basin.Under the guidance of the theories of sequence stratigraphy and sedimentology,seismic sections are integrated with drilling,log,palaeontologic and core data to study the sedimentary characteristics of the Paleogene in this sag,so as to guide oil and gas exploration.The following understandings are obtained.Seven third-order seismic(logging)sequences can be recognized from bottom to top in the Paleogene in the Wushi sag,which consist of a depositional sequence from transgression to regression as a whole and correspond to different stages of structural evolution.Sequences developed in different stages of structural evolution are different in filling sequence and distribution of sedimentary system.Multiphase structural evolutions result in development of source rocks and reservoir and cap rocks in various parts of the sag.The sequence 2-4 deposited in strong chasmic stage have the best source rocks and sealing conditions as well as various favorable reservoirs,thus are the most favorable targets for obtaining discoveries in this sag.