鄂尔多斯盆地三叠系延长组长7段古沉积环境恢复及意义

Paleo-sedimentary environmental restoration and its significance of Chang 7 Member of Triassic Yanchang Formation in Ordos Basin, NW China

通过对鄂尔多斯盆地三叠系延长组长7段周缘露头剖面及盆内钻井取心共289块样品进行元素地球化学测试分析,运用CaO/MgO·Al_2O_3、Sr/Cu、Rb/Sr、B元素含量、Rb/K_2O、Th/U和V/(V+Ni)等一系列古温度、古盐度和古氧化环境判别指标,尝试恢复长7段沉积期的古气候、古盐度和古氧化还原条件等古环境特征。综合分析表明,长7段沉积期为古气温大于15℃的温暖潮湿的温带—亚热带气候,水体为陆相微咸水—淡水环境,沉积物是在强还原条件下形成的。适宜的温度、大面积深水湖盆、强还原性的古沉积环境,导致长73亚段有机质大量发育和富集保存,既形成了一套优质烃源岩,同时也为大规模页岩油的富集创造了有利条件。

Paleo-sedimentary environment of Chang 7 Member of Upper Triassic Yanchang Formation in Ordos Basin,including the paleoclimate,paleo-salinity and paleo-redox conditions were restored through geochemical element analysis of 289 samples collected from the outcrop sections around and wells drilled in the basin and using a series of identification indexes of paleo-climate,paleo-salinity and paleo-redox conditions,such as CaO/MgO?Al2O3,Sr/Cu,Rb/Sr,Rb/K2O,Th/U,V/(V+Ni),the content of element B tested from the mudstone.Comprehensive analysis shows that in sedimentary period of the Chang 7,the paleo-climate was warm temperate to subtropical climate with temperature higher than 15℃,the water body was continental brackish water to freshwater,and the sediments were deposited under strong reduction conditions.Suitable temperature,extensively deep lake basin and strongly reductive paleo-sedimentary environment led to the blooming,enrichment and preservation of organic matter in the submember Chang 73.As a result,a set of high-quality source rock was formed,laying material foundation for large-scale accumulation of shale oil.