Re-Os chronology and REE analysis of paleo-oil reservoir in high evolution areas: a case study of Banjie paleo-oil reservoir, Nanpanjiang Basin

The traditional organic index can be greatly affected in the areas with high thermal evolution degree whilst combination of REE for oil-source correlation will improve the results. In addition, Re-Os dating is a quantitative method of geochronology, but the significance of the test results is quite controversial. In this paper, REE geochemical analysis and Re-Os dating of bitumen in the Banjie paleo-oil reservoir is carried out. The REE distribution pattern of bitumen was compared with that of the main source rocks in the area using the method of grey correlation. Results show that the ΣREE of bitumen in the Banjie paleo-oil reservoir is low, and the fractionation of light and heavy REEs is obvious, showing a weak negative Ce anomaly and an obvious negative Eu anomaly, which indicates that the bitumen was formed in a reducing environment and has not experienced any strong weathering or leaching. The REE distribution mode belongs to the right-leaning mode of LREE enrichment. The REE distribution pattern curve is similar to that of Devonian source rocks in this area, indicating a genetic relationship. The results of Re-Os dating show that the Re-Os model age of bitumen in the Banjie paleo-oil reservoir ranges mainly from (203.6±1.1) Ma to (238.5±4.2) Ma, with an average of (224.8±3.3) Ma (n=7). The last factor which disturbed the Re-Os system of the bitumen is the process of bitumen generation. The model age represents the age of bitumen formation in the Banjie paleo-oil reservoir, when oil was cracked into gas.