鄂尔多斯盆地姬塬地区上三叠统延长组长6油层组沉积环境:来自地球化学的证据

Sedimentary environment of the Chang 6 oil-bearing interval of Upper Triassic Yanchang Formation in Jiyuan area,Ordos Basin:Evidences from geochemical data

姬塬地区位于鄂尔多斯盆地中西部,主要含油层位为上三叠统延长组。延长组长6油层组是重要的含油层系,其沉积期跨越2个三级层序,从底至顶又再分为长63、长62、长61共3个小层。依据元素地球化学资料,对姬塬地区长6油层组氧化还原环境、古盐度、古水深、古气候进行研究。结果表明,姬塬地区长6油层组沉积时期处于温暖湿润的气候条件下,整体为低盐度的淡水湖泊沉积;氧化和还原环境共存,还原环境范围逐步减小并有所迁移;湖盆有逐渐缩小和变浅的趋势,后期趋于稳定。长63沉积时期,湖泊范围较大,砂体主要为河口坝和远砂坝沉积,半深湖发育浊积砂体。随着湖平面的下降,长62沉积时期湖盆范围明显缩小,半深湖砂体基本不发育,以河口坝砂体为主。长61沉积时期,湖盆范围相比长62略有缩小,砂体厚度明显增大,在湖盆范围内发育多期河道叠加及河道河口坝叠加成因的砂体。该研究成果有助于明确该地区的沉积演化规律,对进一步勘探开发提供参考。

The Upper Triassic Yangchang Formation is the main oil-bearing strata in Jiyuan area,middle-western Ordos Basin.The Chang 6 oil-bearing interval is one of the most important oil-bearing horizons,which can be subdivided into 3 layers,i.e.,6 3,6 2,and 6 1 layers in ascending order.The Chang 6 oil-bearing interval spans two three-order sequences.The redox environment,salinity,water depth and paleoclimate are reconstructed based on element geochemical data.In the warm and humid climate,the lake was generally low in paleo salinity.In addition,both oxidizing and reducing conditions co-existed.The reducing environment gradually decreased and migrated.While the water depth tended to decrease and become stable in the latter.In Jiyuan area,the sedimentary characteristics of Chang 6 1,6 2 and 6 3 layers are different.At the beginning of Chang 6,it inherited the sedimentary characteristics of the lacustrine basin in the end of Chang 7 oil-bearing interval.The lake range of Chang 6 3 was large and its sand bodies were mainly composed of the mouth bars and distal bars.Turbidite sand bodies developed in semi-deep lake.Along with the declining lake level,the length of the shoreline of Chang 6 2 was obviously reducing,and the-semi deep lake did not develop.The lake shoreline length of the Chang 6 1 was slightly smaller than that of the Chang 6 2,and the sand body thickness increased dramatically.In the range of the lakeshore line,there were many sand bodies formed by superposition of river channels and mouth bars.The results can help to clarify the sedimentary evolution laws of the Jiyuan area and provide reference for further exploration and development.