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Coupling relationship between hydrocarbon charging events and palaeopressure cycles in the Es3 Member of the Shahejie Formation, Minfeng Sag, Bohai Bay Basin, East China
Journal of Geochemical Exploration 101 (2009) 59
Contents lists available at ScienceDirect
Journal of Geochemical Exploration j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p
Coupling relationship between hydrocarbon charging events and palaeopressure cycles in the Es3 Member of the Shahejie Formation, Minfeng Sag, Bohai Bay Basin, East China Chunquan Li a,⁎, Huimin Liu b, Honghan Chen a a b
Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China Geological Research Institute of Shengli Oilfield Branch Co. SinoPec, Dongying 257015, China
Minfeng Sag is a secondary tectonic unit of the Dongying depression, Bohai Bay Basin, East China. The upper Es4 Member and the lower Es3 Member of the Shahejie Formation comprised the main source rocks for the hydrocarbon migration and accumulation in and around the Minfeng Sag, and the entire Es3 Member, especially the middle section, comprised the main reservoir. In this study fluid inclusion analysis was employed to investigate the formation pressures and the hydrocarbon charging events, and the relationship between them in the Es3 Member of the Shahejie Formation; hence 22 core samples from 8 wells were collected and prepared for fluorescence microscopy and spectral analysis of oil inclusions, for petrographic characterization and microthermometry of oil and aqueous inclusions, and for thermodynamic modelling of palaeopressure with the VTflinc software. The fluorescence colour and spectra of oil inclusions indicate that reservoirs in the Es3 Member trapped at least one lower maturity oils with yellow fluorescence colour as well as a higher maturity oils with white bluish fluorescence colour. The petrographic characterization and microthermometry results suggest that the Es3 Member entrapped three generations of oil inclusions with homogenization temperature ranges of 70–105 °C,105–130 °C and 130–160 °C, and with most values at 90 °C, 115 °C, and 150 °C, respectively. Their coeval aqueous inclusions have homogenization temperature ranges of 75–115 °C, 115–160 °C, and 160–180 °C, and with most values at 100 °C, 140 °C, and 170 °C, respectively. The integrated results made it clear that the Es3 Member of the Minfeng Sag underwent three hydrocarbon chargings during its evolution. Using homogenization temperatures of oil inclusions and their coeval aqueous inclusions, gas to liquid ratios and compositions of oil
⁎ Corresponding author. E-mail address: [email protected] (C. Li). 0375-6742/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2008.12.023
inclusions as input data of VTflinc software, allows the determination of trapping pressures and thus the palaeopressures of the fluids. The thermodynamic modelling results demonstrate 3 cycles of pressure increase and decrease in the Es3 Member of the Minfeng Sag. Combined with the burial history curve, the first cycle is from 30 Ma to 24 Ma, the second is from 8 Ma to 2 Ma, and the third is from 2 Ma to present. These three palaeopressure cycles very well corresponded to the three hydrocarbon charging events that were determined as the early sedimentary period of the Ed Formation, the late sedimentary period of the Ng Formation and the entire sedimentary period of the Nm Formation as well as the sedimentary period of the Qp Formation to present. Using a value of 1.2 as the threshold coefficient for abnormally high pressure, as inferred for the first hydrocarbon charging, the formation pressure is still at a normal or slightly elevated state. During subsequent uplift, formation pressure decreased, possibly to the extent of underpressuring. This was followed by a continuous pressure increase during burial, until the second hydrocarbon charging event when the pressure coefficient exceeded 1.2. At the end of the second hydrocarbon charging the formation pressure dropped to a normal pressure. During the third hydrocarbon charging the formation pressure again increased to the present abnormal pressure state. Thus, episodic hydrocarbon charging was coupled to the hydrocarbon charging events and the palaeopressure cycles in the Es3 Member. This study was supported by NSFC Project No. 40702022.
Contents lists available at ScienceDirect
Journal of Geochemical Exploration j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p
Coupling relationship between hydrocarbon charging events and palaeopressure cycles in the Es3 Member of the Shahejie Formation, Minfeng Sag, Bohai Bay Basin, East China Chunquan Li a,⁎, Huimin Liu b, Honghan Chen a a b
Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China Geological Research Institute of Shengli Oilfield Branch Co. SinoPec, Dongying 257015, China
Minfeng Sag is a secondary tectonic unit of the Dongying depression, Bohai Bay Basin, East China. The upper Es4 Member and the lower Es3 Member of the Shahejie Formation comprised the main source rocks for the hydrocarbon migration and accumulation in and around the Minfeng Sag, and the entire Es3 Member, especially the middle section, comprised the main reservoir. In this study fluid inclusion analysis was employed to investigate the formation pressures and the hydrocarbon charging events, and the relationship between them in the Es3 Member of the Shahejie Formation; hence 22 core samples from 8 wells were collected and prepared for fluorescence microscopy and spectral analysis of oil inclusions, for petrographic characterization and microthermometry of oil and aqueous inclusions, and for thermodynamic modelling of palaeopressure with the VTflinc software. The fluorescence colour and spectra of oil inclusions indicate that reservoirs in the Es3 Member trapped at least one lower maturity oils with yellow fluorescence colour as well as a higher maturity oils with white bluish fluorescence colour. The petrographic characterization and microthermometry results suggest that the Es3 Member entrapped three generations of oil inclusions with homogenization temperature ranges of 70–105 °C,105–130 °C and 130–160 °C, and with most values at 90 °C, 115 °C, and 150 °C, respectively. Their coeval aqueous inclusions have homogenization temperature ranges of 75–115 °C, 115–160 °C, and 160–180 °C, and with most values at 100 °C, 140 °C, and 170 °C, respectively. The integrated results made it clear that the Es3 Member of the Minfeng Sag underwent three hydrocarbon chargings during its evolution. Using homogenization temperatures of oil inclusions and their coeval aqueous inclusions, gas to liquid ratios and compositions of oil
⁎ Corresponding author. E-mail address: [email protected] (C. Li). 0375-6742/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2008.12.023
inclusions as input data of VTflinc software, allows the determination of trapping pressures and thus the palaeopressures of the fluids. The thermodynamic modelling results demonstrate 3 cycles of pressure increase and decrease in the Es3 Member of the Minfeng Sag. Combined with the burial history curve, the first cycle is from 30 Ma to 24 Ma, the second is from 8 Ma to 2 Ma, and the third is from 2 Ma to present. These three palaeopressure cycles very well corresponded to the three hydrocarbon charging events that were determined as the early sedimentary period of the Ed Formation, the late sedimentary period of the Ng Formation and the entire sedimentary period of the Nm Formation as well as the sedimentary period of the Qp Formation to present. Using a value of 1.2 as the threshold coefficient for abnormally high pressure, as inferred for the first hydrocarbon charging, the formation pressure is still at a normal or slightly elevated state. During subsequent uplift, formation pressure decreased, possibly to the extent of underpressuring. This was followed by a continuous pressure increase during burial, until the second hydrocarbon charging event when the pressure coefficient exceeded 1.2. At the end of the second hydrocarbon charging the formation pressure dropped to a normal pressure. During the third hydrocarbon charging the formation pressure again increased to the present abnormal pressure state. Thus, episodic hydrocarbon charging was coupled to the hydrocarbon charging events and the palaeopressure cycles in the Es3 Member. This study was supported by NSFC Project No. 40702022.