珠江口盆地L凹陷裂陷期始新统文昌组沉积充填模式

Depositional filling model of the Eocene Wenchang Formation in rift stage of L sag,Pearl River Mouth Basin

珠江口盆地L凹陷始新统文昌组处于勘探初期,钻井资料较少且分布不均,基于多井多资料的常规沉积相研究方法受到限制,对于层序地层和沉积体系没有系统研究,本次研究充分利用地震、测井以及岩心资料,在全区开展地震相研究工作,井震结合对沉积相展布进行研究,建立裂陷期文昌组沉积演化模式。结果表明:L凹陷裂陷期文昌组划分为3个三级层序,8个体系域;根据三级层序体系域内部地震反射外部形态和内部反射特征差异,识别出充填、席状、楔形、块状、丘形5种地震相类型;通过分析岩心相、测井相以及各种地震相特征,井震结合将地震相转为沉积相,识别出辫状河三角洲、扇三角洲、湖相等沉积相;初始裂陷期为小盆小湖模式,凹陷分割性较强,主要为近源扇三角洲粗碎屑沉积,强烈裂陷早期为大盆浅湖模式,洼陷连通,主要为辫状河三角洲沉积,强烈裂陷中晚期为大盆深湖模式,主要对应半深湖泥岩沉积,弱裂陷期主要为大盆浅湖模式,主要对应大规模长轴辫状河三角洲充填沉积体系。

The exploration of the Eocene Wenchang Formation in L sag,Pearl River Mouth Basin,is in the initial-exploration stage. Due to limited drilling data and their uneven distribution,the conventional multiple-data research methods of sedimentary facies is restricted in this area,resulting in the lack of systematic analysis on sequence stratigraphy and depositional system. Based on cores,well logs and seismic data,the integrated seismic facies analysis was carried out in this area. The integration of well and seismic data was applied to predict the distribution of the Paleogene rift sedimentary system,and the sedimentary model of the Wenchang Formation in rift period has been established. The results show that the Wenchang Formation developed three 3rd-order sequences,consisting of 8 system tracts. According to the differences between externel form and internel reflection characteristics in the sequence system tracts,five seismic facies types were identified in the study area,which are filling type,mat type,wedge type,massive type and hummocky type. Seismic facies were converted to sedimentary facies based on the combined analysis of core facies,well-log facies and seismic facies. Braided-river delta,fan delta and lacustrine facies were identified. The study area developed the model of small basin with small lake in the initial syn-rift stage,when several small isolated sags were mostly filled with near-source fan delta clastic deposits. Model of large basin and shallow lake was developed in the early rift-climax stage,when the sags became connected with each other,filled with braided river delta originated from the southwest. Large basin with deep lake model was formed when the tectonic stage entered middle-late rift-climax,therefore deep lake mudstone was widespread. The model of large basin and shallow lake in the late syn-rift stage was developed,when the accommodation was gradually filled by a large-scale braided-river delta originated from the northeast along the long axis of L sag.