膏盐影响烃源岩T_(max)的实质——以柴达木盆地西部古近系下干柴沟组为例

The essence of gypsum’s influence on T_(max) of source rocks:a case study of the Lower Ganchaigou Formation of Paleogene in western Qaidam Basin

沉积盆地中的地质观察和热模拟实验研究均揭示了含膏泥岩生烃高峰提前的现象,而生烃高峰的提前量与膏岩含量之间的关系却尚未明确。选取柴达木盆地西部地区S49-1井中不同膏岩含量的下干柴沟组烃源岩粉末样品,对原样进行抽提,除去可溶有机质(SOM);取部分抽提残余物进行酸洗,除去碳酸盐;对原样和两步前处理后的固体残余物分别进行岩石热解分析。与原样的T_(max)相比,不含SOM的残余物的T_(max)更高,而去除SOM和碳酸盐矿物的样品Tmax更低,这表明含膏泥页岩中的SOM和硫酸盐矿物(石膏、硬石膏)均可促进有机质热演化。虽然后者的促进效果更为显著,但却与其硫酸盐矿物含量不成正比。这表明硫酸盐矿物与有机质的接触面积对T_(max)的影响更为重要。该结论进一步通过不同比例的干酪根与硫酸镁粉末混合物的Tmax值进行了验证。因此,硫酸盐矿物的赋存状态会直接影响有机质与硫酸盐的接触关系,进而对热演化生烃进程产生影响。

Geological observation and thermal simulation experiments have revealed that the peak period of hydro⁃carbon generation of gypsum⁃bearing mudstone is often advanced in sedimentary basins,but the correlation between the amount of advance and the content of gypsum is still not clear.In this study,the source rock powder samples with different gypsum content in the Lower Ganchaigou Formation from well S49⁃1 in the western Qaidam Basin were selected and extracted to remove soluble organic matter(SOM).Part of the extracted residue was pickled to remove carbonate.Rock pyrolysis analysis was performed on the original samples and the solid residue after two⁃step pretreatment.Compared to the T_(max) of original samples,the T_(max) was higher in the SOM⁃free residue but lower in the SOM⁃free and carbonate⁃free residue,showing that SOM and sulfate minerals(gypsum and anhydrite)in gypsum⁃bearing mudstone can promote the thermal evolution of organic matter.Although the latter has a more significant effect,it is not proportional to its sulfate content,indicating that the contact area of sulfate minerals and organic matter is more important to T_(max).The hypothesis was verified by the T_(max) values of different proportions of kerogen and magnesium sulfate powder mixtures.Therefore,the occurrence state of sulfate minerals will directly affect the contact relationship between organic matter and sulfate,which in turn will affect the process of hydrocar⁃bon generation in thermal evolution.