Characteristics and main controlling factors of volatile oil in the surrounding areas of Bozhong Sag

To investigate the characteristics, main controlling factors, and favorable zones of volatile crude oil in the Bozhong Sag and surrounding areas, we analyzed statistically the data of crude oil composition characteristics and physical properties of many oilfields and oil-bearing structures in Bozhong Sag and its surrounding areas by means of crude oil group composition analysis, crude oil physical property analysis, experimental analysis of PVT high-pressure physical property and other technical means. Then, the characteristics of "five high and three low" of volatile oil in Bozhong Sag and the numerical range of each characteristic are defined. Based on the mechanism of multicomponent phase change, we determined three main controlling conditions on the development of volatile crude oil in Bozhong Sag. The first, after high thermal evolution, high-quality source rocks could provide light-hydrocarbon components for volatile crude oil. The second, the buried depth of Bozhong Sag is large, and the high-temperature and -pressure conditions could satisfy the storage conditions of volatile crude oil. And the third, the regional mudstone caprock formed by rapid subsidence in Bozhong Sag was an important condition favorable for the preservation of volatile crude oil. Based on the temperature pressure gradient of Bozhong Sag, we suggested that the strata in burial depth of 2.5～4.3 km with regional mudstone capping in Bozhong Sag and its surrounding areas shall be targeted for volatile oil reservoirs.