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综合运用岩石学、矿物学和地球化学等方法,对柴达木盆地北缘中段九龙山地区侏罗系砂岩中碳酸盐胶结物的成岩温度、形成环境和物质来源等进行了系统研究。结果表明,九龙山地区侏罗系砂岩中存在4种自生的碳酸盐胶结物类型:方解石、含铁方解石、白云石和铁白云石。测定其碳同位素(δ13C)值分布范围在-15.1‰~-1.3‰之间,平均值为-6.01‰;氧同位素(δ18 O)值分布范围在-17.8‰~-11.2‰之间,平均值为-15.06‰,推算古盐度(Z)值分布范围为87.71~117.77,平均值为107.5。说明九龙山地区侏罗系砂岩在成岩阶段早期有少量有机碳的加入,且成岩流体主要是来自碎屑析水和大气中的淋滤淡水。碳酸盐胶结物的形成温度在78.5~126.55℃之间,平均值为105.86℃。根据成岩特征及形成温度说明侏罗系砂岩的埋藏深度应该大于2 500m,结合目前侏罗系的埋藏深度在1 600m左右,说明九龙山地区侏罗系砂岩在成岩过程中早期被深埋藏后,又经历了后期构造运动的强烈改造,被抬升1 000m左右。
Based on the comprehensive application of petrology, mineralogy and geochemistry methods, the diagenesis temperature, forming environment and material sources of carbonate cements in Jurassic sandstones in the Jiulongshan sandstone in the middle margin of the northern margin of the Qaidam Basin were systematically studied. The results show that there are four kinds of spontaneous carbonate cements in the Jurassic sandstone in Jiulongshan: calcite, calcite, dolomite and iron dolomite. The δ13C value ranged from -15.1 ‰ to -1.3 ‰, and the average value was -6.01 ‰. The δ18O value ranged from -17.8 ‰ to -11.2 ‰, with the average value Is -15.06 ‰, the ancient salinity (Z) value is estimated to range from 87.71 to 117.77 with an average of 107.5. This indicates that a small amount of organic carbon was added to the Jurassic sandstone in the Jiulongshan area early in the diagenetic stage and the diagenetic fluid mainly came from detrital water and leachate in the atmosphere. The formation temperature of carbonate cement was between 78.5 and 126.55 ° C with an average of 105.86 ° C. According to diagenetic characteristics and tectonic temperature, it indicates that the buried depth of Jurassic sandstone should be greater than 2 500 m. Combined with the current buried depth of Jurassic is about 1600 m, indicating that the Jurassic sandstone in the Jiulongshan area was deeply buried during its diagenesis, It also underwent a strong transformation of the tectonic movement in the late period and was lifted up to 1 000 m.