准噶尔盆地乌—夏地区风城组云质岩类沉积环境及成因探讨

Origin of Dolomitic Tuff in Permian Fengcheng Formation in Wu-Xia Area of Junggar Basin

准噶尔盆地乌—夏地区二叠系风城组发育一套特殊的云质岩类,其沉积环境及成因复杂,目前仍存在较多争议。本文基于岩心观察(13口井)与系统取样测试分析(64个样品),揭示了目标层位云质岩的岩石学和地球化学特征,探讨了不同类型云质岩的沉积环境及成因机制。研究结果表明:根据云质岩宏观结构特征,风城组可识别纹层状、星散状、丝絮状、团块状和不规则层状5种云质岩类型;高δ^(13)C_(PDB)值(平均4.0‰)、高古盐度值(平均13.0%)和深水沉积构造等特征表明目标层位沉积环境具有陆相低能、深水还原和咸水高矿化度等特点;纹层状云质岩为"高山深盆"盐湖卤水分层形成的准同生白云岩,团块状、不规则层状、丝絮状和星散状云质岩为埋藏交代白云岩。

The dolomitic rocks are developed in the Fengcheng Formation in Wu-Xia area,Junggar basin and are considered to be the important reservoirs for further petroleum exploration and exploitation.Based on the core observation of 13 wells as well as the core sample tests,the dolomitic rocks petrological and geochemical characteristics were systematically analyzed,and the origin of dolomitic rocks with different occurrences were discussed.In this study,five types of rock textures were distinguished according to their macroscopic occurrence,including rippled,diffuse,flocculent,random bedded,and massive dolomitic rocks.They are characterized by high δ^(13)C_(PDB) values(average 4.0‰) and high paleosalinity values(average 13.0%).Their geochemical characteristics and deep-water originated sedimentary structures together with evaporation indicate a low-energy,reduction in deep-water,dry-hot climate and high salinity deposition environment.The formation of rippled dolomitic rocks is considered to be of the brine delamination in salted lakes.In contrast,the single crystals of random bedded,massive,flocculent,and diffuse dolomitic rocks have fogged center and clear margin,which may be resulted from the metasomatism after burial.This study provides a scientific basis for further oil and gas exploration in the Fengcheng Formation of Wu-Xia area and other dolomite-rich areas.