岩盐成因与发育模式——以柴达木盆地英西地区古近系下干柴沟组为例

柴达木盆地英西地区古近系下干柴沟组为寒冷干旱的咸化湖盆沉积,发育多期岩盐(成层石盐),其成因和发育模式是当前待解决的热点问题。岩心资料揭示完整的成盐单期旋回为"蒸发沉积序列",探井岩性序列组合特征表明湖盆经历了半咸化、咸化和盐湖3个演化阶段。通过包裹体岩相学、均一温度测试及成分研究与含盐层系硫、碳、氧同位素分析明确提出岩盐为低温水下浓缩结晶成因。岩盐形成于蒸发作用强烈的陆相闭塞环境,物质来源为陆源地表水携带而来。咸化阶段中后期与广盐湖阶段发育2种成盐模式,每种模式可再分为初始咸化期、咸化期及成盐期3个演化阶段。连井对比分析表明存在5个岩盐集中发育期,盐湖经历了初始、鼎盛与萎缩3个演化过程。盐湖中心岩盐厚度最大,平面分布具局限性,受古地形控制形成多个次级盐洼中心。垂向上多期岩盐的发育缘于陆相湖盆季节性的湖平面频繁升降。

Origin and developing model of rock salt: a case study of Lower Ganchaigou Formation of Paleogene in the west of Yingxiong ridge,Qaidam Basin

The Paleogene Lower Ganchaigou Formation in the west of Yingxiong ridge, Qaidam Basin is originated from saline lacustrine-basin sedimentation with the development of multiphase rock salt (layered halite), of which the origin and development mode is a hot issue to be solved at present. The core data reveal that the complete salt-formation single-phase cycle is an evaporation sedimentary sequence. The lithological sequence assemblage characteristics of prospecting wells show that the lacustrine basin experienced three evolutionary stages, i.e., semi-saline, salinizing and saline lake. Through the petrography analysis, homogenization temperature test and composition research of the inclusions as well as the analyses of the sulfur, carbon and oxygen isotopes of salt-bearing strata, it is explicitly put forward that the rock salt was generated through low-temperature underwater concentration and crystallization, and formed in the confined continental environment with intense evaporation; the material source was carried by terrestrial surface water. Two kinds of salt forming modes were developed in the mid-late saline stage and euryhalinous lake stage, and each mode is divided into three evolutionary stages, i.e., initial saline stage, saline stage and salt forming stage. The comparison and analysis of joint wells indicate that five rock-salt concentration development periods existed, when the saline lake experienced three evolutionary processes, i.e., the initial stage, peak stage and shrinking stage. The saline lake center had the maximum thickness with limitations in plane distribution, and multiple secondary salt depression centers were formed due to the control of paleo-terrain. The vertical development of multiphase salt rock was caused by the frequent seasonal fluctuation of lake level in terrigenous lake basin.