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

Key words: 
• Ordos Basin

Abstract: 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 tuffs 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 occurred in form Chang 1 to Chang 9, there are four layers can be contrasted in lateral, which are Chang91, bottom of Chang 73, Chang 72 and Chang 71, and the tuff in Chang 73 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 tuffs sediment process may induced elements migration and paleoenvironment change, which may play an important role in developing excellent source rock in Yanchang Formation.