南羌塘盆地构造演化及其油气形成与构造保存条件研究

羌塘盆地是我国陆域上面积最大的海相盆地,前人对该盆地构造演化过程及其油气远景存在截然不同的观点。以最近完成的1︰ 5万地质调查为基础,本文再次讨论了南羌塘盆地构造演化过程及其油气远景。羌塘盆地中央近东西向的羌中隆起山脉将羌塘盆地分为南、北两部分。最近的研究表明,在寒武-奥陶纪之交,南、北羌塘块体被古大洋分隔开。北羌塘盆地南缘形成的晚三叠-早侏罗世的那底岗日组火山岩,其上部为流纹岩,表明晚三叠世南羌塘块体北向俯冲于北羌塘块体之下,在南羌塘块体北部形成了富含有机质的前陆盆地。南羌塘盆地南缘发育一套代表成熟海盆的侏罗纪复理石建造,表明南羌塘南部地区在早侏罗世具有被动大陆边缘的特点,随着南部班公-怒江洋的扩张,在南、北羌塘块体内分别沉积了侏罗纪-早白垩世的浅海相地层,以富含有机质礁灰岩为特征。盆地内部孕育了巨厚的晚白垩-古新世陆源碎屑岩,不整合覆盖于早期海相沉积岩之上,表明在该时期南羌塘块体逐渐从被动大陆边缘海相盆地转变为陆相盆地。新生代时期,印度与亚洲大陆持续汇聚,南羌塘盆地南向逆冲于拉萨块体之上,盆地内发育了多条大型逆冲断裂带,再次将盆地内部的上三叠统、侏罗系、白垩系富含有机质的海相礁灰岩深埋,这有利于油气资源的生成与保存。横跨南羌塘盆地的构造剖面显示盆地内部主要大型逆冲断裂带之间,构造变形较弱,发育宽缓的向斜构造,向斜核部发育新生代陆相地层,推测该新生代陆相地层之下保存有深埋的富含有机质的海相地层,因此,南羌塘盆地逆冲断裂带下盘和宽缓向斜核部区域可能具有良好的油气资源前景。

Oil-gas formation,preservation and tectonic evolution of the southern Qiangtang Basin

The Qiangtang Basin is the largest marine basin of Chinese mainland. Tectonic evolution of the basin and oil-gas prospect has been disputed for a long time. Based on recently completed 1︰ 50000 geological survey, this article discusses the tectonic evolution and oil-gas prospects of southern Qiangtang Basin again. There is a nearly EW-trending mountain in the center of the basin, which divided it into southern and northern Qiangtang blocks. The latest geological survey and research show that there was an ancient ocean between the southern and northern Qiangtang blocks in the Late Cambrian-Early Ordovician. Late Triassic to Early Jurassic Nadi Kangri volcanic rocks, which its upper part is rhyolite, was found at the southern margin of the northern Qiangtang block, indicating that the southern Qiangtang block northward subducted beneath the northern Qiangtang in the Late Triassic, while led to the formation of organic-rich foreland basin at the northern margin of the southern Qiangtang block. Jurassic flysch which represent a mature basin is exposed in south of southern Qiangtang Basin, implying that the southern region of South Qiangtang has the characteristics of passive continental margin. Following the continuous expansion of Bangong-Nujiang tethyan ocean, Jurassic to Early Cretaceous shallow marine strata, which is characterized by organic-rich reef-limestone, was widely deposited in southern and northern Qiangtang blocks. Late Cretaceous to Paleocene terrigenous clastic rock is exposed in the area vary greatly inside the basin, which unconformably overlie the early marine sedimentary rocks, implying that the southern Qiangtang block transformed from marine basin situated on passive continental margin into a continental sedimentary basin from Late Cretaceous to Paleocene times. In Cenozoic, during the ongoing collision between India and Asian continents, the southern Qiangtang Basin was southward juxtaposed over the Lhasa terrane. Several large-scale thrust faults were found in southern Qiangtang Basin, which led to much Upper Triassic, Jurassic and Cretaceous organic-rich marine reef-limestone buried deeply in the basin again. Structural section across southern Qiangtang Basin shows that structural deformation is extremely weak between these large thrust faults, broad and gentle synclines are widely developed in this basin and Cenozoic continental strata is distributed in the syncline core, so we speculate that the organic-rich marine strata is still exist under the Cenozoic continental strata. Therefore, it indicates huge amounts of oil-gas potential resources in the southern Qiangtang Basin. We should look for oil-gas resources in the footwall of thrust faults and broad, gentle syncline core region.