Characteristics of nitrogen compounds and migration of crude oil in Chang 8 subsection of Huaqing region in Ordos basin, China

FU Jinhua; DUAN Mingchen; DUAN Yi; LUO Anxiang; DENG Xiuqin; WU Yingzhong; TAN Jiankang; WU Baoxiang; XU Li; ZHANG Xiaoli

Abstract：To understand the hydrocarbon accumulation process in Chang 8 subsection of the study area,based on the features of fluid inclusions and residual pressure in Chang 7 and Chang 8 subsections,this paper used the compositions of neutral nitrogen compounds for the first time to study crude oil charging sites and migration directions,crude oil charging dynamic characteristics,time and stages.The results show that the migration fractionation effect of carbazole compounds in Chang 8 subsection is significant.The homogenization temperature distribution of fluid inclusions demonstrates that the crude oil has experienced three stages of charging and migration.Horizontally,the absolute content of carbazole compound in crude oil decreases from south to north and east to west,but the isomer ratio shows an opposite increasing trend,indicating that there are two charging sites and migration directions in Chang 8 subsection,namely charging in the south and east of the study area and then migrating toward north and west,respectively.Vertically,the absolute content of carbazole compound reduces from Chang 7 to Chang 8 and Chang 10 subsections,but the isomer ratio gradually increases,indicating that the oil migrates top down from Chang 7 to Chang 8 subsection.The charging sites and migration directions of Chang 8 subsection are consistent with the residual pressure difference between Chang 7 and Chang 8 subsections and the residual pressure distribution in the Chang 8 subsection.The homogenization temperature of fluid inclusions in Chang 8 subsection exhibits three peak temperatures,which are 80,100 and 120 to 130 ℃,reflecting the three stages of oil charging and migration.According to the thermal history and burial history of the basin,it is inferred that the geological time of crude oil charging into Chang 8 subsection is from late Jurassic to late period of early Cretaceous.