咸化湖盆混积岩岩相类型、沉积旋回及相模式——以柴达木盆地风西地区新近系为例

Lithofacies types,sedimentary cycles,and facies models of saline lacustrine hybrid sedimentary rocks:A case study of Neogene in Fengxi area,Qaidam Basin,NW China

针对咸化湖相混积岩岩性复杂、沉积机制不明确等问题,综合岩心、铸体薄片、扫描电镜、X射线衍射、测井、碳氧同位素组成等多种资料,采用岩心与测井、岩石学与地球化学交叉结合研究手段,对柴达木盆地风西地区新近系上干柴沟组至下油砂山组(N1—N21)混积岩进行了综合分析。结果表明,风西地区混积岩包括陆源碎屑岩和湖相碳酸盐岩,沉积于远离物源的浅湖环境,偶尔出现半深湖环境,发育5种典型岩相及6种微相类型。稳定碳氧同位素组成揭示,沉积旋回的形成是受气候驱动的补偿-欠补偿循环机制所控制。提出一种干旱咸化背景下的混积岩沉积旋回模式:补偿期湖平面急剧上升,发育泥坪、砂泥坪、滩等微相,沉积机制以物理沉降为主;欠补偿期湖平面缓慢下降,发育灰泥坪、灰云坪、藻丘/席微相,以化学-生物沉积机制为主导。在咸化湖盆混积体系中,湖相碳酸盐岩主要形成于湖退期;相变并非根据传统认为的可容纳空间来解释,而是受控于气候变化导致的湖水化学性质短期波动。研究结果为咸化湖盆混积岩高分辨率层序模式及成因提供了新解释,对指导咸化湖相非常规油气勘探具有重要意义。

The saline lacustrine hybrid sedimentary rocks are complex in lithology and unknown for their sedimentary mechanisms.The hybrid sedimentary rocks samples from the Neogene upper Ganchaigou Formation to lower Youshashan Formation(N1–N21)in the Fengxi area Qaidam Basin,were investigated through core-log and petrology-geochemistry cross-analysis by using the core,casting thin section,scanning electron microscope,X-ray diffraction,logging,and carbon/oxygen isotopic data.The hybrid sedimentary rocks in the Fengxi area,including terrigenous clastic rock and lacustrine carbonate rock,were deposited in a shallow lake environment far from the source,or occasionally in a semi-deep lake environment,with 5 lithofacies types and 6 microfacies types recognized.Stable carbon and oxygen isotopic compositions reveal that the formation of sedimentary cycles is controlled by a climate-driven compensation-undercompensation cyclic mechanism.A sedimentary cycle model of hybrid sedimentary rocks in an arid and saline setting is proposed.According to this model,in the compensation period,the lake level rises sharply,and microfacies such as mud flat,sand-mud flat and beach are developed,with physical subsidence as the dominant sedimentary mechanism;in the undercompensation period,the lake level falls slowly,and microfacies such as lime-mud flat,lime-dolomite flat and algal mound/mat are developed,with chemical-biological process as the dominant sedimentary mechanism.In the saline lacustrine sedimentary system,lacustrine carbonate rock is mainly formed along with regression,the facies change is not interpreted by the accommodation believed traditionally,but controlled by the temporary fluctuation of lake water chemistry caused by climate change.The research results update the interpreted high-resolution sequence model and genesis of hybrid sedimentary rocks in the saline lacustrine basin and provide a valuable guidance for exploring unconventional hydrocarbons of saline lacustrine facies.