Tracing the Charging Directions of Biodegraded Oils: A Case Study of the Large Oil Fields on the Shijiutuo Uplift, Bohai Bay Basin, China

Abstract

It's difficult to trace charging directions for oils with multiple origins, especially for the biodegraded oils. The Shijiutuo uplift is a major petroliferous uplift to the northern of the Bozhong sag and to the southern of the Qinnan sag. The two large oil fields (QHD32 and QHD33), locating on the middle part of the Shijiutuo uplift, are generally suffered mild biodegradation, with oils being contained in sand reservoirs at bury depths lower than 1600m. There occur three main organic-rich source rock intervals (Ed3, Es1 and Es3) in the Bohai Bay Basin, and each of which has a distinct biomarker assemblage. Based on multivariate analysis of the selected biomarker parameters, this study discussed the origin and the charging directions for these two oil fields. Five source-related biomarker parameters, being not significantly influenced by biodegradation, were carefully inspected and selected to conduct this research. In contrast to Ed3-derived oil, oils from the two large oil fields can be identified from Es1 and Es3 source rocks. The QHD32 oil field, containing much more Es3-derived oil, was mainly charged in the north by oil generated from Shahejie Formation source rocks in the Bozhong depression; and three main charging directions can been discriminated apparently. However, oils from the northern of QHD32 field display remarkable difference to the oils in the southern of this field, which may indicate a charging pathway from the QHD33 field exists as well. In addition, the vertical migration in the QHD32 oil field can be discriminated. Compared with QHD32 oil field, the QHD33 oil field shows relatively high contributions from the Es1 source rock. Considering the variations in biomarker compositions on the west-east and south-north sections crossed the QHD33 and QHD32 oil field, it can be deduced that Es3-sourced oil migrated westward to the QHD32 trap and then charged by Es1 oil from the Bozhong sag, resulting in the QHD33 oil field being characterized by the mixture of Es3- and Es1-sourced oil. Moreover, no Es1-derived oil migrated from Qinnan sag was identified, implying that QHD33 are mainly charged from the northeast of Bozhong sag. This research reveals that the analysis of biomarker parameters is an effective method to identify charging directions within oil fields suffered biodegradation.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015