Geological structure of Zhusi Depression in ultra-deep water area on the continental margin of the northern South China Sea and its control on the development of source rocks
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摘要:
珠江口盆地珠四坳陷整体水深超过1 500 m，属极低勘探程度区域，钻井发现的油气显示证实了该区存在烃源岩。地震资料揭示古近系是主力烃源岩发育层系，沉积厚度及面积均较大。珠四坳陷作为南海北部向超深水迈进的油气勘探新区，其各凹陷的石油地质条件亟待剖析。在珠江口盆地6种断陷形成机制认识的基础上，通过对新采集地震资料的系统分析，将珠四坳陷划分大西洋贫岩浆型大陆边缘的壳间韧性流变型、大西洋富岩浆型大陆边缘的壳上岩浆底侵型、大西洋贫岩浆型大陆边缘的壳上先存拼合型3类断陷结构。运用2Dmove软件及去压实技术恢复各个洼陷沉降速率和伸展速率，结合大量地震剖面深入研究了珠四坳陷地质结构及其对烃源岩发育的影响，明确了不同断陷结构下烃源岩发育背景的差异，以此认为荔湾、鹤山凹陷烃源岩发育条件优越，可作为南海北部超深水地区的重要勘探突破点。
Abstract:The overall water depth of Zhusi Depression in the Pearl River Mouth Basin is more than 1500 m with very low degree of the exploration. The oil and gas revealed by drilling has shown the existence of source rocks in this area, and seismic data present a vast and thick source rocks in the Paleogene strata. As a new oil and gas exploration area marching towards ultra-deep water in the north of the South China Sea, we analyzed the advantages and disadvantages of petroleum geological conditions in each depression. Based on the knowledge of the formation mechanism of six types of fault depressions in the Pearl River Mouth Basin, Zhusi Depression were divided into three representative fault depression structural styles: inter crustal ductile rheological type of Atlantic magma-poor continental margin, supracrustal magmatic underplating type of Atlantic magma-rich continental margin, and pre-existing splicing type of supracrustal magmatic on Atlantic magma-poor continental margin. Using the 2Dmove software and decompaction technology, the subsidence rate and extension rate of each depression were restored. Combining a large number of seismic profiles, the geological structure of Zhusi Depression and its impact on the development of source rocks were studied in depth, and the differences of source rock development background under different fault depression structures were clarified. Results show that the development conditions of source rocks in Liwan and Heshan depressions are superior, which can be regarded as an important exploration breakthrough point in the ultra-deep water area in the north of the South China Sea.
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表 1 珠四坳陷不同凹陷伸展沉降速率统计
Table 1. Statistics of extensional subsidence rates in different sags of Zhusi Depression
凹陷名称 凹陷结构 伸展速率/（m/Ma） 沉降速率/（m/Ma） 荔湾凹陷 壳间韧性流变型 425 371 鹤山凹陷 壳间韧性流变型 529 420 靖海凹陷 壳上岩浆底侵型 330 322 兴宁凹陷 壳上先存拼合型 193 218 揭阳凹陷 壳上先存拼合型 150 197 
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