珠江口盆地白云凹陷古近系文昌组源-汇系统特征及演化

Characteristics and evolution of the source-to-sink system of the Paleogene Wenchang Formation in Baiyun Sag,Pearl River Mouth Basin

珠江口盆地是南海东部典型的边缘海盆地,白云凹陷为珠江口盆地珠二坳陷中、新生代地层发育最全的深大凹陷,其古近系是深水区已证实的主力层系及近期深层油气勘探的重点目标。文昌组是白云凹陷已经获得勘探证实的主要烃源岩层系,在凹陷周边物源控制区也是有利储层发育的重要场所。通过定量表征其源-汇系统和分析源-汇系统特征及演化规律为白云凹陷主力烃源岩及规模砂体分布预测提供依据。基于锆石定年及重矿物分析确定物源体系及其演化,利用PSDM地震资料及古地貌刻画了搬运通道类型及分布,井-震结合厘定了沉积体系及沉积相类型特征;结合古地貌起伏、剥蚀量及沉积汇聚量计算结果对源-渠-汇系统进行了定量耦合。研究表明,古近纪文昌期发育番禺低隆起、云荔低隆起、云东低凸起以及云开低凸起4个近物源体系,以断沟和沟谷为渠,形成了4套完整的源-渠-汇系统。不同沉积时期的源-汇体系发生明显转变,受各时期物源差异的影响,文昌期白云凹陷发育古沟槽及大型构造转换带控制下的近物源小型湖相、辫状河三角洲及扇三角洲沉积。白云凹陷源-汇系统控制下的文昌组规模滨浅湖在文昌组五段(文五段)、文四段和文三段分布范围最广;半深湖-深湖相沉积分布局限,在文五段仅零星分布,在文四段、文三段及文二段的湖盆中央局部范围分布,这些认识为主力烃源岩的分布预测提供了基础。在宽缓裂陷结构及大物源控制下,白云凹陷北坡发育的辫状河三角洲富砂沉积体系控制了规模砂体的平面展布。白云凹陷东部辫状河三角洲及扇三角洲规模小,是有利相带砂体的主要分布位置。

The Pearl River Mouth Basin(PRMB)is a typical marginal sea basin to the east of the South China Sea.The Paleogene strata of the Baiyun Sag,a deep and large sag with well-developed Mesozoic-to-Cenozoic strata in the ZhuⅡDepression of the PRMB,are the major targets for recent hydrocarbon exploration in the deep-water area.The Wenchang Formation is the hydrocarbon source rock sequence verified in exploration in the Baiyun Sag,and the provenances around the sag are also favorable for reservoir development.The quantitative characterization of the sourceto-sink system,and the analysis of its characteristics and evolution of the formation provide a basis for predicting the distribution of the main hydrocarbon source rocks and large-scale sand bodies in the Baiyun Sag.A combination of zircon dating and heavy mineral analysis serves to determine the provenance system and its evolution.The types and distribution of transport channels are characterized by PSDM seismic data and palaeo-geomorphology.The characteristics of sedimentary system and sedimentary facies are depicted by well-tied seismic interpretation,while the source-channel-sink system is quantitatively coupled with the calculation results of palaeo-geomorphic relief,denudation thickness and sedimentary accumulation.The research results show that during the deposition of the Paleogene Wenchang Formation,there are four proximal provenance systems developed,that is,the Panyu low uplift,Yunli low uplift,Yundong low salient and Yunkai low salient,resulting in four complete suites of“source-channel-sink”systems with fault-related ditches and ravines as channels.The source-to-sink system tends to undergo significant changes in different sedimentary periods.Under the effect of differential provenances in different periods,near-provenance braided river delta and fan delta deposits of small-scale lacustrine facies are developed in the Wenchang Formation of the Baiyun Sag as controlled by paleo-grooves and large-scale tectonic transition zones.The deposits of shore-shallow lake facies are widely developed in the 5th,4th and 3rd members of the Wenchang Formation under the control of the source-to-sink system of the Baiyun Sag.While the occurrence of semi-deep to deep lake facies is sparsely seen in the 5th member of the Wenchang Formation,and seen in the central of the 4th,3rd,and 2nd members,which provide a basis for predicting the distribution of the main hydrocarbon source rocks.Under the control of wide and gentle rift structure and large provenances,the braided river delta of sand-rich sedimentary system is developed on the north slope of Baiyun Sag that controls the planar distribution of large-scale sand bodies;the braided river deltas and fan deltas in the east of the Baiyun Sag are small in scale with sand bodies of favorable facies widely seen.