南海南部上白垩统—始新统Rajang群浊流沉积物源-汇对比分析*

南海南部新生代沉积中发育有大型三角洲、下切水道、深水浊积扇等油气有利勘探带,油气资源丰富,勘探前景广阔。其中,位于中央婆罗洲、形成于上白垩统—始新统的Rajang群由一套巨厚的深海浊积扇组成,时间跨度达30 Ma,属世界著名的古海底扇之一。由于缺乏系统的源-汇对比研究工作,对其沉积物来源及沉积充填过程认识不清,影响到该时期盆地油气资源的勘探研究。通过岩石学、重矿物和碎屑锆石U-Pb年龄谱系综合分析,对Rajang群沉积物及其潜在源区进行系统源-汇对比研究。结果发现,Rajang群主要受南海北部岩浆岩局部隆起区和中南半岛物源控制,沉积物来自南海北部地区。在始新世晚期,由于沙捞越俯冲造山运动,Rajang群沉积物发生强烈变形和隆升,主要出露于中央婆罗洲及近海曾母盆地。由于Rajang扇中的深海泥质夹层多、砂层相对较薄,无法形成有利储集体。但是,在曾母盆地北侧及其偏北的北康—南薇西盆地一带应该发育有同时代较好的浅水储集体,是该区油气勘探的潜在方向。

"Source-to-sink"analysis of turbidite deposits in the Upper Cretaceous-Eocene Rajang Group in southern South China Sea

The southern South China Sea(SCS)is widely developed with large-scale deltas,down-cutting channels and deep-water turbidite fans. These geological structures are enriched with oil and gas resources and have a good potential for exploration. The Rajang Group sediments in the central Borneo formed a very thick deep-water turbiditic sequence which was deposited between the Late Cretaceous and early-middle Eocene. The fan system was deposited in an interval of at least 30 Ma and became one of the world’s largest ancient submarine fans. Due to the lack of systematic source-to-sink analysis,the provenance evolution and sedimentary infilling processes of the Rajang Group sediments remains ambiguous,which strongly affect the petroleum resources assessment. In this study,we conducted petrography,heavy mineral analyses and detrital zircon U-Pb dating,to generate a comprehensive interpretation of sediment provenance evolution in the southern South China Sea during the Eocene. We find that during the late Mesozoic to early-middle Eocene,the Rajang group was mainly controlled by the local uplift area of magmatic rocks in the northern South China Sea and Indo-China Peninsula;Since the late Eocene,the Sarawak Orogeny induced intense uplift and deformation of the Rajang Group,resulting in the Rajang Group exposed in the central Borneo and extended into the oil-bearing Sarawak Basin now. There was no good reservoir in the Rajang Group,because the strata were dominated by mud with very thin sandstone deposited. However,there may be good shallow water reservoirs in the Beikang-Nanweixi basins on the north side of Zengmu basin,and these will be the potential target of hydrocarbon exploration.