The black shale of the Mesoproterozoic Cuizhuang Formation in the Changcheng System in Yongji city,North China Craton,is a potential source rock.Understanding the organic matter enrichment mechanism is crucial for eva...The black shale of the Mesoproterozoic Cuizhuang Formation in the Changcheng System in Yongji city,North China Craton,is a potential source rock.Understanding the organic matter enrichment mechanism is crucial for evaluating source rock resources and understanding oil and gas accumulation mechanisms.In this study,we evaluated the sedimentary paleoenvironment and organic matter enrichment mechanisms of shale using thin section observations,mineral composition analysis,organic geochemistry,and elemental geochemistry.We found significant differences in the sedimentary paleoenvironment and organic matter enrichment mechanisms between the lower Cuizhuang Formation and the Beidajian Formation shale.The Cuizhuang Formation was deposited in a late-stage,restricted basin environment during the rift phase,and elemental and geochemical indicators showed that the Cuizhuang Formation was in a suboxic-anoxic water environment,that was influenced by a warm and humid paleoclimate and submarine hydrothermal activities,which promoted the accumulation of organic matter.However,the enrichment of organic matter in the Cuizhuang Formation was mainly controlled by redox conditions.The formation of suboxic-anoxic water environments may be closely related to the warm and humid paleoclimate and submarine hydrothermal activities.Warm conditions promote continental weathering and increase marine productivity,thereby consuming oxygen in the bottom water.Moreover,acidic hydrothermal activity also helps to establish an anoxic environment.Our results reveal the effects controlling various coupled mechanisms dominated by redox conditions,which may explain the development of source rocks in the Cuizhuang Formation.展开更多
Lithologically, two kinds of chert can be recognized in the Middle-Upper Permian from the Tieqiao section in Laibin area, Guangxi, i.e., calcic chert occurring mainly in the Maokou Formation and pure chert mainly in t...Lithologically, two kinds of chert can be recognized in the Middle-Upper Permian from the Tieqiao section in Laibin area, Guangxi, i.e., calcic chert occurring mainly in the Maokou Formation and pure chert mainly in the Wujiaping Formation. Geochemical data show that both kinds of chert contain very low A1203 （0-0.23%） and TiO2 （0.001%-0.024%） and low ZREE （0.55-19.94 ppm, averaging 9.97 ppm）, as well as high ratio of Fe2O3/TiO2 （17-443, averaging 111） and low ratio of A1203/（Al2O3＋Fe203） （0-0.26, averaging 0.09）. Given that the average value Ce anomalies in chert deposited in the ridge-proximal environment is about 0.29, and that in the pelagic environment is about 0.60, the low Ce anomalies in the Tieqiao chert （0.24-0.46, averaging 0.35） imply that they were deposited in an ocean basin with influence of submarine hydrothermal fluid and no input of terrigenous materials. The vertical variation of the silica abundance in strata （SAIS） in the Middle-Upper Permian strata, together with the Eu anomalies and the ratios of ∑REE/Fe, indicates a relationship between the hydrothermal activity and the Emeishan basalt eruption, and that submarine hydrothermal activity was stronger in the Upper Permian （the Wujiapingian Stage） than in the Middle Permian （the Maokou Stage）.展开更多
基金supported by the National Natural Science Foundation of China (Grant U19B6003-01-02,42102150,42372163)。
文摘The black shale of the Mesoproterozoic Cuizhuang Formation in the Changcheng System in Yongji city,North China Craton,is a potential source rock.Understanding the organic matter enrichment mechanism is crucial for evaluating source rock resources and understanding oil and gas accumulation mechanisms.In this study,we evaluated the sedimentary paleoenvironment and organic matter enrichment mechanisms of shale using thin section observations,mineral composition analysis,organic geochemistry,and elemental geochemistry.We found significant differences in the sedimentary paleoenvironment and organic matter enrichment mechanisms between the lower Cuizhuang Formation and the Beidajian Formation shale.The Cuizhuang Formation was deposited in a late-stage,restricted basin environment during the rift phase,and elemental and geochemical indicators showed that the Cuizhuang Formation was in a suboxic-anoxic water environment,that was influenced by a warm and humid paleoclimate and submarine hydrothermal activities,which promoted the accumulation of organic matter.However,the enrichment of organic matter in the Cuizhuang Formation was mainly controlled by redox conditions.The formation of suboxic-anoxic water environments may be closely related to the warm and humid paleoclimate and submarine hydrothermal activities.Warm conditions promote continental weathering and increase marine productivity,thereby consuming oxygen in the bottom water.Moreover,acidic hydrothermal activity also helps to establish an anoxic environment.Our results reveal the effects controlling various coupled mechanisms dominated by redox conditions,which may explain the development of source rocks in the Cuizhuang Formation.
基金supported by National Basic Research Program of China (Grant No. 2005CB422101)
文摘Lithologically, two kinds of chert can be recognized in the Middle-Upper Permian from the Tieqiao section in Laibin area, Guangxi, i.e., calcic chert occurring mainly in the Maokou Formation and pure chert mainly in the Wujiaping Formation. Geochemical data show that both kinds of chert contain very low A1203 （0-0.23%） and TiO2 （0.001%-0.024%） and low ZREE （0.55-19.94 ppm, averaging 9.97 ppm）, as well as high ratio of Fe2O3/TiO2 （17-443, averaging 111） and low ratio of A1203/（Al2O3＋Fe203） （0-0.26, averaging 0.09）. Given that the average value Ce anomalies in chert deposited in the ridge-proximal environment is about 0.29, and that in the pelagic environment is about 0.60, the low Ce anomalies in the Tieqiao chert （0.24-0.46, averaging 0.35） imply that they were deposited in an ocean basin with influence of submarine hydrothermal fluid and no input of terrigenous materials. The vertical variation of the silica abundance in strata （SAIS） in the Middle-Upper Permian strata, together with the Eu anomalies and the ratios of ∑REE/Fe, indicates a relationship between the hydrothermal activity and the Emeishan basalt eruption, and that submarine hydrothermal activity was stronger in the Upper Permian （the Wujiapingian Stage） than in the Middle Permian （the Maokou Stage）.
