西藏特提斯构造域海相油气前景

MARINE PETROLEUM POTENTIAL IN XIZANG(HBET)TETHYS TECTONIC DOMAIN

伊朗扎格罗斯(Zagros)前陆盆地是世界上石油最富集的地区之一.西藏高原与伊朗高原同属特提斯构造域的一部分,在构造演化与成油地质条件等方面具一定的可比性.西藏地区经历了古、中、新3个特提斯大洋的扩展、俯冲与消亡,以及羌塘、冈底斯、喜马拉雅3个微板块张裂、增生与碰撞的复杂过程.羌塘地区在古生代稳定被动大陆边缘的碳酸盐岩台地上,叠置了中生代前陆盆地的沉积,为油气聚集提供了有利的生储盖条件.冈底斯地区白垩纪的弧后盆地叠置在侏罗纪活动大陆边缘的沉积之上,以及喜马拉雅山北坡晚白垩世—第三纪的残余洋盆叠置在被动陆缘盆地之上,均为油气聚集提供了良好的生油物质基础.在叠置的前陆盆地中,寻找下凹上凸的反转构造区,在侏罗、白垩或第三系膏盐层作封盖的地区,可望在海相油气勘探方面有所突破.

The Zagros foreland basin in Iran is one of the most abundant petroleum areas in the world.Xizang Plateau and Iran Plateau belong to the same Tethys tectonic domain,therefore,it is com- parable in tectonic evolution and petroleum geology.Following a brief account of Middle-East tectonic domain and the petroleum geology of Zagros foreland basin,a further discussion is given on the evolution of Xizang Tethys tectonics and its sedimentary formations for marine oil potential which is regarded favourable in the areas under consideration.Tethys oceans of Xizang area has experienced three successive cycles of expansion,subduction and extinction processes,which involved rifting,aggregation and collision of three microplates.The subduction and extinction of a formal Tethys oceanic crust would lead to pro- duce and/or expand a new Tethys ocean.Each microplate could be separated from the north margin of the Gondwana and drifted with its aggregated margin to collide and finally welded to the south margin of Eurasia continent to form a brand new tectonic system.The superimposition of Mesozoic sediments over a carbonate plane of stable Paleozoic passive margin in Qiangtang area created favoumble conditions for the generation,conservation and entrapment of oil accumulation.In Gangdise area,sediments of Jurassic active continental margin were overlain by Cretaceous back-arc basins.On the north slope of the Himalayas,Late Cretaceous-Tertiary oceanic basin was superimposed on the passive margin.All these geological settings are beneficial for oil and gas accumulation.The key to the formation of traps in Xizang area is good conservation conditions in the post stages.It seems to be promising that the stable landmasses in the active region are widely covered with evaporates.Based on a careful petroleum geological analysis of the satellite gravity anomalies,5 first order zones and 12 second subzones of possible marine petroliferous areas have been located.The 5 first order zones are north Qiangtang,south Qiangtang, north Gangdise,south Gangdise and north Himalayas.Among them,the most promising is north Qiangtang;south Qiangtang and north Gangdise,the second.In the superimposed foreland basins, exploration breakthrough for marine oils could be made where reverse structures,characterized by concave at depth and convex at shallow part,developed salts or gypsum layers of Jurassic,Cretaceous and Tertiary exist.