鄂尔多斯盆地延长组凝灰岩夹层展布特征及其地质意义

Distribution Characteristics and Geological Significances of Tuff Interlayers in Yanchang Formation of Ordos Basin

鄂尔多斯盆地延长组凝灰岩夹层分布广泛,通过众多凝灰岩夹层的实际岩心观察,归纳总结出凝灰岩夹层的常规测井曲线表现为具有相对低电位、高自然伽马、高电阻率、高声波时差值的特征,并且高值常略低于致密泥岩段;单井分析凝灰岩夹层与上、下部岩石整合接触,较少发育同沉积构造,以典型的火山尘灰大气降落沉积产物为主,但也偶见凝灰岩与砂泥岩一起发育扰动构造,明显经受了水动力改造;连井剖面分析说明延长组长1—长9皆有凝灰岩分布,横向上可对比性凝灰岩主要有4套,分别为长91、长73底、长72、长71期,以盆地西南部长73期最发育;凝灰岩平面展布特征整体呈北西向展布,由南西—北东向凝灰岩厚度逐渐变薄,并且与烃源岩、放射性异常展布形态一致,因此认为凝灰岩的沉积作用所引起的元素迁移和古环境变化,可能对延长组优质烃源岩的发育具有重要意义。

Tuff interlayers distributing widely in Yanchang Formation in Ordos Basin, lots of cores which containing tuff interlayer were observed, then summarized the log curves characteristics of tuff, which represent by relatively low SP, high GR, R and AC, and usually lower than compacted mudstone ; Single cores analysis of tuff interlayers suggest that the tufts conformity contact with up and down rocks, which were typical sediments of volcanic ash carried by atmosphere then fall off, but there were perturbation structures that tuff accompanied with sandstone or mudstone, which underwent hydrodynamic reformation evidently ; Well-tie profiles analysis indicate that tuff occun＇ed in form Chang 1 to Chang 9, there are four layers can be contrasted in lateral, which are Chang91 , bottom of Chang 73, Chang 72 and Chang 71, and the tuff in Chang 73 is most; Tuff plane distributing characteristics is northwest trending, and from southwest to northeast the tuff gradually became thinner, which distributing characteristics are similar with source rock and radioactivity anomaly, So, the tufts sediment process may induced elements migration and paleoenvironment change, which may play an important role in developing excellent source rock in Yanehang Formation.