Source of Biodegraded Oil in the Upper Jurassic of Fukang Sag in Junggar Basin, Northwest China

Abstract

To define the source of crude oil in the upper Jurassic of Fukang Sag in Junggar Basin, NW China, a combined investigation including oil-source correlation, oil charging history and oil fluorescence analysis was carried out. From gas chromatography maps of oil samples, multiple n-alkanes were selectively removed and the chromatographic baseline hump became prominent. And 25-norhopanes presented in most crude oil samples, which indicated that crude oil had experienced moderate-severe biodegradation. Firstly, a biodegradation index called 25-C29norhopane/C30hopane was introduced to analyze the resistance of biomarkers to biodegradation. The results showed that Pr/Ph and Gammacerane/C30hopane had striking negative and positive correlations respectively with 25-C29norhopane/C30hopane, while Gammacerane /(C31+C32)hopane, ETR((C28tricyclic terpane+C29tricyclic terpane)/(C28tricyclic terpane + C29tricyclic terpane+Ts)), and C19tricyclic terpane/C23tricyclic terpane had no significant correlations with 25-C29norhopane/C30hopane, so these biomarkers were reliable for oil-source correlation. The biomarkers of biodegraded oil were characterized by a wide range of Gammacerane/(C31+C32)hopane (0.15~0.83), moderate ETR values (0.19-0.51), and relatively low C19tricyclic terpane/C23tricyclic terpane (<0.4), which indicated that biodegraded oil was mixture of crude oils generated from Triassic and Middle Permian source rocks. Secondly, oil charging history from analysis of oil-bearing fluid inclusions was carried out to validate the presence of mixed oil from Triassic and Middle Permian source rocks. The homogenization temperatures of the inclusions presented two peaks: 40°C~50°C and 80°C~90°C, which revealed that two phases of oil charging occurred, respectively, in Late Jurassic (152~144 Ma) and Middle Cretaceous (115~88 Ma). And Late Jurassic and Middle Cretaceous were dominant periods for Middle Permian and Triassic source rocks to generate and expel hydrocarbon respectively, which was in accordance with oil charging periods. Thirdly, fluorescence microscopic imaging was used to observe hydrocarbon component and distribution. There existed two distinct hydrocarbon components with blue-white or dark brown fluorescence, which was a response to multiple oil charging. In conclusion, biodegraded oil in the upper Jurassic originated from Triassic and Middle Permian source rocks.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018