珠一坳陷小型洼陷文昌组烃源岩发育模式与主控因素

Lacustrine Source Rock Deposition Model and Main Controlling Factors of the Wenchang Formation in Asmall-scale Sag,Zhui Depression,Pearl River Mouth Basin

小型洼陷烃源岩的形成条件区别于大型洼陷,近年逐渐得到勘探重视。以珠一坳陷小型洼陷始新统文昌组烃源岩为研究对象,利用地球化学资料,结合洼陷构造演化与沉积背景,分析总结其烃源岩的发育模式和主控因素。珠一坳陷小型洼陷文昌组烃源岩形成于温暖潮湿的淡水湖盆,沉积模式包括洼陷带单源型和边缘带陆源贡献型两种:前者烃源岩有机质来源单一,基本为水生生物,陆源高等植物贡献较少;后者有机质来源除水生生物外,存在明显的陆源有机质的贡献;前者烃源岩品质优于后者。小型洼陷对沉积物供给敏感、易过补偿,高沉降速率及低沉积物供给是保障烃源岩发育环境的关键,较一般大型盆地要求更严苛。欠补偿、富营养水体形成的高生产力控制小型洼陷优质烃源岩发育的规模。小型洼陷湖盆烃源岩的发育环境不具备空间优势,烃源岩品质相对于规模更重要,以洼陷带单源型为优势的洼陷易成为富烃洼陷。

The geological forming condition of source rocks in a small-scale sag is different from that in a large basin.In this paper,the source rocks of the Wenchang Formation for small sags in the ZHUⅠdepression are studied.Based on the organic geochemistry and element geochemistry of the source rocks and crude oil,combined with the tectonic and sedimentary analysis of small-scale sags in the rifting stage,the formation patterns and main controlling factors of the formation of source rocks are analyzed and summarized.The Eocene source rocks of the small-scale sags in the ZHUⅠdepression were formed in a warm and humid freshwater lake.The formation patterns include the single source type in the deep depression zone and the terrigenous contribution type in the marginal zone.The source rocks of the former have a single source of organic matter,which is mainly from aquatic organisms,while the terrigenous plants have less contribution;the organic matter of the latter has a significant contribution from terrigenous organic matter besides aquatic organisms.The quality of the source rock formed in a single source pattern is better than that of the latter,and most of the discovered hydrocarbons come from the source rock formed in the former pattern.Due to the limited spatial scale,small depressions are sensitive to sediment supply and prone to overcompensation.High tec⁃tonic subsidence rate and low sediment supply are the keys that ensure the development environment of source rocks.A warm and humid climate background and high productivity of eutrophic water bodies control the scale of high-quali⁃ty source rocks in small depressions.