被动大陆边缘大型峡谷供给型深水扇沉积与油气成藏——以西非刚果扇为例

Sedimentation and hydrocarbon accumulation of deep-water fan fed by large canyon in passive continental margin: a case of Congo fan in West Africa

被动大陆边缘大型峡谷供给型深水扇是一种典型且重要的深水扇类型,深入了解其沉积和油气成藏特征对于全球相同类型深水扇认识的深化具有很强指导意义。通过对第四纪刚果扇调查数据的梳理和解剖,认为第四纪刚果扇是由分割的北扇、南扇和轴扇叠置而成,各分割扇体内发育多套浊积体系。东西方向上各分割扇体以向海进积为主,扇体内水道体系沉积中心东西向迁移距离约100km;南北方向上扇体从北部—南部—中部逐渐迁移,分割扇体内部的浊积体系分别呈逆时针(北扇)、顺时针(南扇)和从SE向NW(轴扇)迁移。沉积构型要素以水道—天然堤和前端朵叶体为主,平面呈"树枝—树叶"网状。扇体发育主要受物源体系、气候和盐岩变形等异源因素控制。分析指出古代刚果扇与第四纪刚果扇具有相似沉积特征。统计分析表明,刚果扇具有较好的油气成藏条件,其烃源岩是赛诺曼阶—马斯特里赫特阶海相页岩,储层是渐新统—中新统水道或水道复合体砂岩,油气被层间页岩局部封盖,在地层—构造和地层等圈闭内成藏。现阶段油气勘探应以陆架边缘下切水道及峡谷边缘上扇部分网状决口水道为重点。

Deep-water fan fed by large canyon in passive continental margin is a typical type of deep-water fan. Deep understanding of the sedimentary and hydrocarbon accumulation characteristics is of significance for other deep-water fans of the same type in the world. Analysis of survey data indicates that the Quaternary Congo fan is superimposed by dissected northern, southern and axial fans, and each fan has a variety of turbidite systems. In the east-west direction, the fan is dominated by marine progradation and the depocenter of the channel systems in the fan migrated about 100km. In the south-north direction, the fan migrated from north and south to center, so the turbidite system migrated counterclockwise in the northern fan, clockwise in the southern fan, and from SE to NW in the axial fan. The architectural elements are mainly composed of channel levee and frontal lobe, which present a branch-leaf network. Allogenic factors such as source system, climate and salt rock deformation, control the fan development. Further analysis shows that the ancient Congo fan has similar sedimentary characteristics with the Quaternary Congo fan. The Congo fan has good hydrocarbon accumulation conditions based on statistical analysis. The source rock is the Cenomanian-Maastrichtian marine shale; the reservoir is composed of the Oligocene-Miocene channel or complex sandstone; and the hydrocarbon is blocked locally by the shale interlayer and accumulates in the stratigraphic-structural and stratigraphic traps. At present stage, exploration is suggested on the incised channels on the shelf margin and net crevasse channels in the upper fan on the canyon edge.