西藏它日错地区多巴组沉积环境及石油地质意义

OIL GEOLOGICAL SIGNIFICANCE AND SEDIMENTARY ENVIRONMENT OF THE DUOBA FORMATION OF LOWER-CRETACEOUS IN THE TARI LAKE AREA,TIBET

西藏它日错地区是中特提斯的一个重要深坳陷区 ,发育了巨厚的下白垩统海陆交互相和浅海相碳酸盐岩沉积 ,其中多巴组厚度超过了 15 0 0 m,主要沉积相类型包括陆源碎屑潮坪相、碳酸盐浅海、水下浅滩、混合沉积过渡带等。在它日错一带形成坳陷中心 ,主要物源从南、西南方向进积。多巴组中的烃源岩主要为浅海相灰岩 ,向它日错方向增多、加厚 ,有机质类型为混合型 ;可能的储集岩是浅滩相介壳灰岩、浅海相泥晶灰岩及潮坪相砂岩 ,裂隙是其主要的储集空间 ;而潮下带泥岩则可成为有利盖层。垂向上由于多旋回叠加 ,具有较好的生储盖组合 ,但所发现的烃源岩厚度较薄是其不利因素。

Taricuo area is an important deep depression of Middle Tethys.Deposition of carbonates of neritic facies and marine facies alternated with terrigenous facies developed with a great thickness,in which the Duoba Formation exceeded 1 500 m.Major types of sedimentary facies in Duoba Formation include terrigenous clastic tidal flat,neritic carbonate,subaqueous shoal facies and transitional zone of hybrid sediments.The centre of depression lies at the Taricuo and provenance analysis revealed a south and southwest progradation.Hydrocarbon source rocks in Duoba Formation is dominated by limestone of neritic facies,which the thickness and amount of limestone increase toward the Taricuo area with mixed type of organic matter.Coquina of shoal facies,micrite of neritic facies and sandstone of tidal flat facies may constitute the reservoir rocks in which the fractures serve as reservoir space.Mudstone of subtidal zone occurs as favourable caprock.Multicylcic superposition in the vertical profile creates well developed source reservoir seal association.However,the fact that the thickness of hydrocarbon source rock is thin becomes an unfavourable condition for oil accumulation.