柴北缘西段侏罗系沉积特征*

柴达木盆地北缘中小盆地(包括苏干湖盆地)是中国西部潜在的油气资源战略接替区,前期工作揭示了在柴北缘西段发育具有一定生油潜力的中生界烃源岩。沉积特征和沉积环境分析对于研究烃源岩及储集层分布具有重要意义。但有关柴北缘西段沉积特征和沉积环境的认识相对较少,制约了油气勘探工作的进一步开展。通过野外露头观测、钻测井资料分析以及地球化学测试等技术手段,初步落实了柴北缘西段侏罗系的沉积体系和沉积环境,为今后的油气勘探提供了依据。研究表明,苏干湖盆地早中侏罗世发育扇三角洲—湖泊沉积,晚侏罗世发育辫状河,物源可能主要来自北部祁连山;赛什腾山南缘侏罗纪发育沼泽—三角洲—湖泊沉积体系,富含煤层,物源来自西北和东北;冷湖一带早侏罗世主要发育湖西山组深湖—半深湖沉积,物源可能来自西北部阿尔金山附近。泥岩样品沉积地化分析显示研究区整体为还原环境,不同采样点样品的稀土元素配分模式具有相似性。构造-沉积演化分析表明,早侏罗世柴达木盆地北缘西段初始断陷,湖盆沉积中心位于冷湖一带;中侏罗世中期(对应于大煤沟组四段)苏干湖盆地内部裂陷成盆,沉积中心向东转移,后期(对应于七段)湖泛形成大湖盆。侏罗系烃源岩主要发育于大煤沟组五段至七段湖平面上升时期。

Sedimentary characteristics of the Jurassic in western north margin of Qaidam Basin

The medium-small basins including Suhai Hu Basin,in north margin of Qaidam Basin,are potential strategic replacement fields of oil and gas resources in western China,and the preliminary work has revealed that Mesozoic source rocks developed in western north margin of Qaidam Basin have a certain exploration potential. It is essential that the analysis of sedimentary characteristics and environments is of great significance to the study of the distribution of source rocks and effective reservoirs. However,up to now,the little cognition of the sedimentary characteristics and environments has restricted the further oil-gas exploration work in this area. Through a series of technical means,such as outcrop observation,drilling data and geochemical data analysis,it is the sedimentary systems and environments in Jurassic that have been studied preliminarily. All of those will support the further oil-gas exploration. In the study,fan delta and lake depositions were developed in Suhai Hu Basin in the Early and Middle Jurassic,and the dominant braided river facies were developed in the Late Jurassic. Meanwhile,during Jurassic Period the sedimentary provenance probably came from the northern Qilian Mountain. In the Jurassic Period,the delta-marsh-lacustrine depositional system,containing rich coal seams,was well developed in the southern margin of Sertengshan Mountain,and the sedimentary provenance came from the northwestern and northeastern areas. In Lenghu area,the deep to semi-deep lacustrine deposits were developed in the Huxishan Formation in the Early Jurassic,and the sedimentary scource provenance probably came from the nearby Altun Mountain. According to the geochemistry analysis of mudstone samples,rocks in the study area mainly form in reduction environments,and different samples contain the similar distribution patterns of REE. The analysis of tectonic-sedimentary evolution shows that the western north margin of Qaidam Basin was on the initial rift stage in the Early Jurassic,and the sedimentary center of the lacustrine basin was located in Lenghu area. In middle age of the Middle Jurassic,corresponding to the Member 4 of Dameigou Formation,the interior of Suhai Hu Basin was rifted,while the sedimentary center was moving toward to east. Afterward, corresponding to Member 7, a big lacustrine basin was formed due to lake flooding in this area. The source rocks of Jurassic were mainly developed during the rising stage of the lake level when the Member 5 to Member 7 of Dameigou Formation deposited.