Abstract:

The hydrocarbon accumulation stages of the Chang 8 Group, Yangchang Formation, Upper Triassic in the Middle West area
of Ordos Basin were studied by analyzing and testing technology of fluid inclusions. The Chang 8 reservoir shows two periods of
hydrocarbon-bearing inclusions according to the analysis of the petrography, homogenization temperature, salinity, density and
chemical composition of hydrocarbon-bearing inclusions and the associated brine inclusions. The first stage hydrocarbon-bearing
inclusions are mainly hosted in the early calcite cement, quartz grain surfaces and their syntaxial quartz overgrowths, and the
cracks healed during the early diagenetic phase that do not cut through the quartz grains. The homogenization temperature peak value
of brine inclusions associated with hydrocarbon-bearing inclusions of the first stage is mainly 80-90℃. The second stage hydrocarbonbearing
inclusions are mainly distributed in the quartz grain surfaces and the cracks healed in the late diagenetic phase that cut

diagenesis and hydrocarbon charging under microscope, diagenesis-hydrocarbon charging evolution sequence study and burial-thermal
history analysis show that three periods of hydrocarbon accumulation occurred in the study area. The first hydrocarbon charging stage
occurred from 169 Ma to 161 Ma, nearly no hydrocarbon-bearing inclusions suitable for homogenization temperature and chemical
component measurement captured in the Chang 8 sandstones due to its limited proportion. Thus inclusions’temperature and
composition data of this phase are lacking. The second hydrocarbon accumulation stage with larger dimension and broader distribution
occurred during 148-135 Ma, which corresponds to the first stage of the hydrocarbon-bearing inclusions with homogenization
temperature and composition measurement data. The third hydrocarbon charging stage occurred in the late Early Cretaceous
(125.2-105.7 Ma) and is the major hydrocarbon accumulation stage in the study area which corresponds to the second stage
hydrocarbon-bearing inclusions with homogenization temperature and composition measurement data in the Chang 8 sandstones.

Key words: fluid inclusions, physical property and chemical composition, hydrocarbon accumulation periods, the Chang 8 Group, Ordos Basin