柴达木盆地北缘平台地区路乐河组沉积环境与成岩流体特征

综合运用岩石学、矿物学以及元素地球化学等理论，对柴达木盆地北缘构造带中段平台地区古近系路乐河组（E₁₊₂）储集岩的岩石学特征、碳酸盐胶结物、黏土矿物以及沉积成岩环境进行分析研究。结果表明:平台地区路乐河组储集岩主要为岩屑砂岩和长石岩屑砂岩，储集岩内颗粒之间以点-线接触关系为主，少量为线接触关系；孔隙类型以原生粒间孔为主，其次为粒内溶蚀孔隙；黏土矿物含量较高，平均值达到了21.92%，主要以伊-蒙混层、伊利石、蒙脱石和绿泥石为主，具有明显的蒙皂石向伊-蒙混层转化的特征。碳-氧同位素分析结果显示碳同位素值（δ13C）在-8.87‰~-3.91‰之间，平均为-5.66‰，碳来源主要为脱羧作用的有机质碳和碳酸盐岩中的无机碳，淋滤作用也造成胶结物中δ13C值变低；氧同位素值（δ18O）介于-12.97‰~-8.75‰，平均值为10.68‰，推算其古盐度Z值分布范围在103.79~114.27之间，平均为110.38，碳酸盐沉淀温度分布范围在62.89℃~90.53℃之间，平均为75.20℃；通过全岩的微量元素分析表明，U/Th、V/Cr、Ni/Co以及Cu/Zn均反映出温暖、湿润的成岩环境，古盐度、Sr/Ba与U值也指向淡水沉积环境，说明平台地区路乐河组储集岩主要形成于温暖湿润的淡水沉积环境之中。

Sedimentary Diagenetic Environment of the Lulehe Formation in the Pingtai Region,Northern Qaidam Basin

Detailed analysis of the petrologic characteristics,carbonate cements and clay minerals of the reservoir rocks of the Paleogene Lulehe Formation in the central Pingtai region at the northern margin of the Qaidam Basin showed that the reservoir rocks in the platform area are mainly lithic sandstone and feldspar lithic sandstone,commonly with point contact and/or line contact between the clastic particles. As a result there are few primary interstices,only individual intragranular secondary pores. The clay mineral content is relatively high,up to 21.92% on average,mainly in mixed layers of illite/montmorillonite and illite. Montmorillonite and chlorite are predominant,and show obvious montmorillonite transformation into illite-montmorillonite. The pore fluid is alkaline water,which promotes the decarboxylation of organic matter and reflects hydrocarbon generation during compaction. The average paleotemperature at the time of carbonate cement formation was 75.20℃,mainly at the early diagenetic stage B. Elemental geochemical analyses showed carbon isotope values(δ13C) from -8.77‰ to -3.91‰(average -5.66‰);the carbon source was mainly decarboxylated organic carbon and inorganic carbon in carbonate rocks. Leaching also reduced the amount of the δ13C isotope in cements. The oxygen isotope(δ18O) ranged from -12.97‰ to -8.75‰(average 10.68‰). The paleosalinity(Z) was in the range 103.79 to 114.27(average 110.38). The carbonate precipitation temperature ranged from 62.89℃ to 90.53℃(average 75.20℃). Trace element determination gave average values for Sr,Cu,Th,U,Ba,Ni,Co,V and Cr of 195.25 μg/g,19.48 μg/g,7.32 μg/g,21.8 μg/g,735.52 μg/g,22.41 μg/g,9.40 μg/g,59.11 μg/g and 85.55 μg/g,in that order. The average values of the U/Th,V/Cr,Ni/Co and Cu/Zn ratios were 0.25,0.72,2.26 and 0.42,indicating that the Lulehe Formation(E₁₊₂) is currently in a sedimentary oxidizing environment with high redox potential. Since the combined Sr/Ba and U averages 0.30 and 0.85 are both less than 1,together with the C and O isotope content in the carbonate cement,the ancient salinity was estimated to be 110.38 wt%; this indicates a freshwater sedimentation paleoenvironment with a brackish water phase.