珠江口盆地阳江东凹始新统的源汇过程:碎屑锆石定年及物源示踪

Tracing source-to-sink process of the Eocene in the Eastern Yangjiang Sag,Pearl River Mouth Basin:Evidence from detrital zircon spectrum

阳江东凹为近年来珠江口盆地取得重要勘探突破的凹陷。为阐明阳江东凹始新统文昌组-恩平组物源区、物源转换等源汇过程,以锆石U-Pb定年为主要技术手段,对研究区文昌组和恩平组7件砂岩的碎屑锆石形态和年龄进行了分析。结果表明,(1)在文昌组时期,阳江东凹以周缘凸起的中生代岩浆岩为物源区,为珠江口盆地内部(简称“盆内”)近物源输入,且物源输入量小,利于半-深湖相烃源岩的发育。文昌组的优质烃源岩为本地区油气勘探提供了物质基础。(2)在恩平组下段沉积期,凹陷主体的物源来自阳江-一统暗沙断裂带西侧较远区域出露的加里东晚期岩浆岩或其再循环沉积物,局部仍由周缘凸起供源,洼陷由盆内近物源转变为盆内远物源为主;恩平组上段沉积期,洼陷兼具盆内物源和珠江口盆地外部(简称“盆外”)物源的贡献,其中盆外物源来自华南板块,并且从该时期开始,华南板块物源供给增强,并逐渐控制了整个凹陷的沉积充填。整个恩平组时期,物源供给持续增强及控洼断裂活动性减弱造成洼陷被浅水辫状三角洲所主导。

Important breakthroughs have been achieved in hydrocarbon exploration recently in the Eastern Yangjiang Sag of the Pearl River Mouth Basin.In order to clarify the provenance of sediments and the process of provenance transition of the Eocene Wenchang-Enping Formation in the Eastern Yangjiang Sag,the detrital zircon morphology and ages of 7 sandstones from the Wenchang Formation and the Enping Formation are analyzed,with zircon U-Pb dating method.The results show that:(1)the sediments of the Wenchang Formation are sourced from some peripheral uplifts consisting of Mesozoic magmatic rocks.They are called in-basin provenances in this paper,which,with limited input,is beneficial to the formation of semi-deep lacustrine source rocks.The sediment source as such founded the material basis for petroleum generation in this area.(2)During the time of Early Enping Period,the provenance of the sag is dominated by Caledonian magmatic rocks or their recycled sediments exposed in the area far to the west of the Yangjiang-Yitong Fault Zone.Even the sediments for some small areas are still supplied by the peripheral uplifts,the basin provenances had changed from near-source to far-source ones.While the upper member of the Enping Formation was deposited,the sag is filled by the materials from both South China plate and peripheral uplifts with the gradually increase of material supply from the South China Plate.Both the enhancement of material supply and the weakening of sag-controlling faults lead to the sag dominated by shallow braided delta during the whole Period of Enping Formation.