柴达木盆地西部古近系石膏及其硫同位素分布特征

石膏是柴达木盆地油气圈闭的盖层之一。根据石膏层含量统计和石膏~(34)S_(V-CDT)分析,发现柴达木盆地古近系石膏主要分布于靠近阿尔金山的狮子沟凹陷和小梁山凹陷,石膏含量在垂向上具有旋回性。石膏的~(34)S_(V-CDT)值分布范围很大,为-23.3～71.4%,属国内外罕见。其原因是来自阿尔金山的硫酸盐~(34)S值很低,而由盆缘向盆内随着还原环境的强化,细菌的分馏作用逐渐强烈,导致~(34)S_(V-CDT)值的巨大差异。石膏~(34)S_(V-CDT)值的主控因素为硫酸盐~(34)S_(V-CDT)初始值、硫酸盐补给速率和还原菌分馏作用的强度。形成石膏的硫酸盐来自阿尔金山,经由狮子沟凹陷进入盆地,然后向盆地其它方向扩散。

Distrbution of gypsum and sulfur isotopes in the Palaeogene strata, western Qaidam Basin, Qinghai

The gypsoliths occur as part of the cap rocks of the oil and gas traps in the Qaidam Basin, Qinghai. In the light of the statistics of gypsum contents and ³⁴S_V-CDT values, the gypsum in the Palaeogene strata mainly occurs in the Shizigou depression and Xiaoliangshan depression near the Altun Mountains. Vertically, the gypsum contents display the cyclic distribution. The ³⁴S_V-CDT values show a rarely wide range of -23.3% to 71.4%. The initial ³⁴S values are very low in the Altun Mountains. The increase of the reducing conditions and the fractionation of bacteria towards the Qaidam Basin leads to the great differences in the ³⁴S_V-CDT values. The main controlling factors include the initial ³⁴S_V-CDT values, supply rates of sulfates, and intensity of bacteria fractionation. It follows that the sulfates are derived from the Altun Mountains, and then enter into the Qaidam Basin through the Shizigou depression. The results of research in this study may be helpful to the study of the gypsum distribution, sedimentary environments of the Palaeogene salt lakes, and sulfate origins in the study area.