箕状断陷湖盆裂陷期层序地层及沉积演化——以松辽盆地西南缘奈曼凹陷九佛堂组下段为例

Sequence stratigraphy and sedimentary evolution during the rifting period of a half-graben basin: a case study of lower section of Jiufotang Formation in Naiman Depression, southwestern margin of the Songliao Basin

基于T-R(湖进—湖退)的层序划分方法,综合运用元素录井、三维地震、钻测井、岩心等资料,研究松辽盆地西南缘奈曼凹陷白垩系九佛堂组下段层序地层特征、沉积相和岩石学特征,建立层序地层格架及沉积演化模式。结果表明:奈曼凹陷九佛堂组下段为1个T-R三级旋回,可划分为湖进、湖退两个体系域。构造上为西陡东缓的单断箕状凹陷,沉积体系受控盆断裂影响,具有明显的东西分带特征,西部陡坡带兼具牵引流及重力流特征,发育扇三角洲沉积;东部斜坡带以牵引流特征为主,发育辫状河三角洲沉积。层序演化过程中,构造沉降、沉积物供给、古地貌和可容空间等因素不断变化,造成层序格架内沉积相类型及发育规模的差异。湖进体系域(T旋回)沉积时期,物源供给充足,西斜坡控盆断裂运动不断产生新的可容空间,扇三角洲发育规模大,东斜坡可容空间受构造影响小,辫状河三角洲发育规模较小;湖退体系域(R旋回)沉积时期,西陡坡物源供给减弱,扇三角洲发育规模变小,东斜坡沉积物快速充填可容空间减小,辫状河三角洲砂体向湖盆进积,发育规模大。该结果为岩性油气藏勘探及裂陷期优势储层预测提供指导。

Based on the T-R(transgression-regression)sequence stratigraphy method and comprehensive analysis of data from elemental logging,3D seismic,drilling,and core samples,this study examines the sequence stratigraphy,sedimentary facies,and petrological characteristics of the lower section of the Jiufotang Formation in the Cretaceous Naiman Depression of the southwest margin of the Songliao Basin.The study establishes a sequence stratigraphic framework and sedimentary evolution model.Results show that the lower section of the Jiufotang Formation in Naiman Depression consists of one T-R third-order cycle,which can be divided into two systems tracts:transgressive and regressive.The structural setting is a single fault-controlled,asymmetric half-graben with a steep western slope and gentle eastern slope,resulting in distinct east-west zonation of sedimentary facies.The western slope zone exhibits both traction and gravity flows,and is dominated by fan-delta sedimentation,while the eastern slope zone is mainly characterized by traction currents and braided river-delta sedimentation.The evolution of the sequence is influenced by changing factors such as tectonic subsidence,sediment supply,paleotopography,and accommodation space,resulting in variations in sedimentary facies types and development scale within the sequence framework.During the transgressive system tract(T cycle),abundant sediment supply on the western slope,controlled by fault movement,created new accommodation space resulting in large fan-delta development.In contrast,the eastern slope was less affected by tectonic activity and had limited accommodation space,resulting in smaller braided river-delta development.During the regressive system tract(R cycle),the sediment supply on the western slope decreased,resulting in smaller fan-delta development,while rapid sediment accumulation in the eastern slope reduced accommodation space,resulting in a larger braided river-delta development.The results provide guidance for the exploration of lithological oil and gas reservoirs and the prediction of favorable reservoirs during the rift basin stage.