西藏岗巴盆地烃源岩评价

ASSESSMENT OF HYDROCARBON SOURCE ROCKS IN XIZANG GANGBA BASIN

西藏岗巴盆地是青藏高原南部重要的中新生代海相残留盆地 ,在大地构造上属特提斯喜马拉雅地块。盆地自三叠纪以来经历了裂谷拉张、被动大陆边缘到前陆造山的全过程 ,由于逆冲叠覆构造作用及挤压褶皱作用的影响 ,原型特征遭到破坏。为了落实岗巴盆地烃源问题 ,针对岗巴盆地特殊的大地构造背景及其属于改造型盆地的特点 ,根据烃源岩地球化学特征结合青藏高原地球物理研究成果分析 ,开展了烃源岩评价 :岗巴盆地烃源岩为侏罗系海相黑色泥岩、页岩和白垩系黑色、灰色泥岩、页岩、泥灰岩 ,烃源岩总体上呈现东厚西薄的展布特点 ,其中侏罗系黑色泥岩、页岩为好的烃源岩 ,下白垩统下部泥岩、页岩为中等烃源岩 ,下白垩统上部—上白垩统下部泥岩、页岩、泥灰岩为差烃源岩 ,有机质类型以过渡型 (Ⅱ型 )为主 ;干酪根镜质体反射率Ro 分析表明烃源岩处于凝析油—湿气演化阶段 ;

Gangba Basin in Xizang is an important residual marine basin in the Mesozoic - Cenozoic in the south part of Qingzhang Plateau, tectonically belonging to the Tethgan Himalayan Block. The Basin had experienced the whole process of rift extension, passive continental margin to foreland orogenesis since the Triassic, with its destroyed prototype characteristics caused by thrust nappe tectonics and extrude folding. In order to define the hydrocarbon sources in Gangba Basin, hydrocarbon source assessment is carried out aimed at the special tectonic background and its features as a transformed basin and on the basis of combining the geochemical charateristics of the hydrocarbon source rocks with the study results of Qingzang Plateau Geophysical Institute. The hydrocarbon source rocks in the Basin are composed of the dark marine mudstone, shale from Jurassic and the dark gray mudstone, shale and marl from Cretaceous. The extension of these source rocks generally appears thickening in east and thinning in west. However, the dark mudstone and shale from the Jurassic are good hydrocarbon source rocks; the mudstone and shale from the lower part of the Lower Cretaceous are moderate one; the mudstone, shale and marl from the upper part of the Lower Cretaceous - the lower part of the Upper Cretaceous are poor one. The type of organic material is mainly the No. II. The analysis of vitrinite reflectance Ro of kerogen indicates that the hydrocarbon source rocks is at the evolution stage of condensate - wet gas. Radiately heating in the crust of Gangba area was intensifying with the crust-thickening and the bottom of the rock layer was becoming more and more heating with the melting into thinner of it. The special crust formed by bilateral heating process and multilayer thermal texture model in the upper mantle controlled the evolution of the hydrocarbon source rocks.