鄂尔多斯盆地延长组长73页岩轻烃散失规律及主控因素OA

Owing to evaporative loss of light hydrocarbons,accurate restoration of retained hydrocarbon content in shale under in-situ formation conditions remains a prominent challenge,which restricts the objective evaluation of shale oil resource volumes.Taking the Chang 73 shale of the Ordos Basin as the research target,we collected sealed frozen fresh core samples and long-term exposed core samples.Adopting sealed thermal desorption analysis based on low-temperature sealed crushing technology,as well as conventional pyrolysis technology,this paper investigated the loss pattern and controlling factors of light hydrocarbons,calculated the in-situ retained free hydrocarbon content,and clarified its distribution characteristics.The results showed that C7-light hydrocarbons were the main components lost during coring and tripping of Chang 73 shale.As the air exposure duration of core samples increased,the amount of lost light hydrocarbons decreased significantly,showing two attenuation patterns:slow decline and rapid decline,separated by a TOC threshold of 7.0%.The loss of light hydrocarbons basically stabilized after 30 days,with an average light hydrocarbon loss rate of 20.13%.The sealed thermal desorption S1 of fresh cores was 2.66 times the conventional pyrolysis S1 of samples stored for 30 days.The total loss of light hydrocarbons during sample preservation and experimental testing averaged 3.23 mg/g,with an over-all average loss rate of 62.90%.Under similar thermal maturity conditions,TOC content,hydrocarbon fractions,felsic mineral content,and carbonate content constitute the primary factors controlling the loss amount of light hydrocarbons.Using the restoration equations of in-situ retained free hydrocarbons and residual free hydrocarbons obtained by pyrolysis,we estimated that the in-situ retained free hydrocarbon resources of Chang 73 shale can reach 3.9×109 t.These resources are mainly distributed in the Jiyuan-Huachi-Heshui-Zhengning belt,which serves as a favorable zone for shale oil exploration.This study provides new ideas and insights for resource evalua-tion of Chang 73 shale oil in the Ordos Basin,and the predicted favorable zones can offer a geological basis for exploration and target selection of Chang 73 shale oil.