Combination of basin modelling and carbazoles to investigate secondary oil migration pathways in the Dongying depression of the Bohai Bay Basin, China
Introduction
Secondary petroleum migration is an important process for petroleum accumulation in the subsurface, which comprises a key process that needs to be understood in petroleum exploration (Welte et al., 2012). Basin modelling, which simulates key geological processes in sedimentary basins, is one of the key techniques to reconstruct secondary petroleum migration (Hantschel and Kauerauf, 2009; Welte et al., 2012). As an important application of basin modelling, oil migration modelling has been successfully used to trace secondary migration pathways of oil in various basins (Hindle, 1997; Xiao et al., 2010; Guo et al., 2011; Welte et al., 2012). Oil migration modelling is an efficient way to simulate secondary oil migration pathways. However, oil migration modelling often needs a comprehensive understanding of thermal maturation and burial history of the basin, which are closely related to the input parameters as such (Guo et al., 2012).
Another technique to understand petroleum pathways is to use carbazole, benzocarbazole, and their alkylated homologous as tracers for secondary petroleum migration. Carbazoles often display strong sorption on organic matter and mineral surfaces due to their high polarities. Changes in the distribution of carbazoles are used to trace oil migration direction and distance (Horsfield et al., 1998; Hwang et al., 2002; Bechtel et al., 2013; Duan et al., 2013; Liu et al., 2016). As secondary migration distance increases, a relative depletion of alkylbenzocarbazoles can be observed. At the same time, an increase in the ratio of nitrogen shielded isomers to nitrogen exposed isomers, and enrichment of higher to lower molecular weight alkylcarbazoles coexist (Li et al., 1995). Larter et al. (1996) proposed a formula as benzo[a]carbazole/(benzo[a]carbazole + benzo[c]carbazole) ([a]/[a] + [c]) to indicate secondary migration distances, which values often decrease as secondary migration distance increases. Thermal maturity (Li et al., 1997; Clegg et al., 1998; Horsfield et al., 1998), source facies (Clegg et al., 1997; Bakr and Wilkes, 2002; Zhang et al., 2008), and biodegradation (Huang et al., 2002, 2003) can significantly affect carbazole-type compounds, that special precautions should be taken when using the carbazole parameters. In combination with petroleum migration modelling technique, carbazole-based geochemical method can also be used to indicate the direction of oil migration.
The Bohai Bay Basin is one of the most oil-rich basins in China, accounting for nearly one-third of the country's total oil production (Hao et al., 2009a). As an important depression in the Bohai Bay Basin, more than thirty oilfields have been discovered in the Dongying Depression (Guo et al., 2010). Although the general oil charging history and episodes have been extensively studied in the Dongying Depression (Guo et al., 2010; Wang et al., 2020), the specific migration pathways of oil are still unclear. A previous study on oil migration pathways in the Dongying Depression was limited in small areas (Wang et al., 2020), which lacks of significance to understand oil migration pathways in the entire depression that needs to be addressed. In this study, integrated approaches including basin modelling and carbazole analysis were undertaken to trace pathways for oil accumulation from source-rocks to reservoirs. The objectives of this research are to:
- (1)
classify the crude oils and carry out an oil-to-source correlation in the Dongying Depression.
- (2)
determine the oil migration pathways with modelling work for the Es3 and Es4 members during the main oil migration period.
- (3)
trace the secondary migration pathways of oil based on basin modelling and carbazole analysis.
Section snippets
Geological setting
The Bohai Bay Basin is one of the most prolific basins along the eastern coast of China (Fig. 1A), which covers an area of approximately 200,000 km2 and stretches approximately 1000 km in a north-south direction (Chang, 1991; Allen et al., 1997; Zheng et al., 2005). The Bohai Bay Basin is bound by the Luxi Uplift in the southeast, the Yan Mountains in the north, the Taihang Mountains in the west, and the Jiaodong and Liaodong Uplifts in the east (Zheng et al., 2005). The basin is a complex
Basin modelling
The secondary migration pathway of oil migration is modelled using the BasinFlow and BasinView softwares to calculate the hydrocarbon head, which equals to the sum of the buoyancy, hydrodynamic force, and capillary threshold pressure (Guo et al., 2011). Migration pathways were perpendicular to the contour lines of the hydrocarbon head, which was utilized to represent the value of hydrocarbon potential energy. For simulation of oil migration, driving forces of petroleum migration were originated
Oil migration modelling
Calculated present heat flow values range from 58.27 to 64.45 mW/m2 in the selected wells (Fig. 3). Heat-flow evolution of the Dongying Depression can be divided into two stages as: rapid rising stage and gradual decreasing stage. During syn-rift stage I (65–51.8 Ma), heat flow risen rapidly, and reached its maximum value at 51.8 Ma due to intense magma activities in the study area (Guo et al., 2012). At 51.8 Ma, maximum heat flow values as calculated from the eight selected wells range from
Origin of oil
Biomarker parameters on the origin and depositional environments of organic matters, such as Pr/Ph, G/H, 4-MSI, C35/C34, ETR, C27 diasteranes/C27 steranes, and C27 to C29 steranes were used in this study to determine origins of oils in the Dongying Depression (Fig. 14, Fig. 15).
Based on the locations of the oil samples used in the study area, the oil samples were divided into three groups as: 1) oil samples near the centre of the sags (CS oils), 2) oil samples near the edge of the sags (ES
Conclusions
Based on basin modelling, oil-source correlation, and carbazole analysis, the secondary oil migration pathways in the Es3 and Es4 Formations were investigated in the Dongying Depression. This research reaches the following conclusions:
- 1.
Crude oils in the Dongying Depression were expelled from source rocks with similar maturity, but with different organic matter compositions. Oils were categorized into three groups based on the depositional environment parameters of Pr/Ph, G/H, 4-MSI, C35/C34,
Author contribution
Jiaxu Chen: Writing – original draft, Investigation, Visualization; Xiaowen Guo: Conceptualization, Writing – review & editing; Yuanjia Han: Writing – review & editing; Sheng He: Supervision; Yongshi Wang: Resources; Xuejun Wang: Resources. Wen Zhao: Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study is supported by the National Major Science and Technology Projects of China (No. 2016ZX05006-003) and the National Natural Science Foundation of China (No. 41872139). The Shengli Oilfield Research Institute, SINOPEC, is thanked for providing background geological data and the permission to publish the results. We thank Dr. Ze Tao for helping to polish the paper. The authors would like as well to thank three anonymous reviewers and associate editors Harry Dembicki, Jr. and Friedeman
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