| Thermodynamic conditions of framework grain dissolution of clastic rocks and its application in Kela 2 gas field |
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| 作者姓名： | LAI Xingyun YU Bingsong CHEN Junyuan CHEN Xiaolin LIU Jianqing MEI Mingxiang JING Weiguang & CHENG Suhua School of Earth Sciences and Mineral Resources China University of Geosciences Beijing China Key Laboratory of Lithospheric Tectonics Deep-level Process and Exploration of Ministry of Education Beijing China |
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| 作者单位： | LAI Xingyun，YU Bingsong，CHEN Junyuan，CHEN Xiaolin，LIU Jianqing，MEI Mingxiang，JING Weiguang & CHENG Suhua School of Earth Sciences and Mineral Resources，China University of Geosciences，Beijing 100083，China; Key Laboratory of Lithospheric Tectonics，Deep-level Process and Exploration of Ministry of Education，Beijing 100083，China |
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| 基金项目： | Much content and data of this study came from the Fundamental Research Special Support Project of Ministry of Science and Technology of China，国家自然科学基金 |
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| 摘 要： | Feldspar and clastic debris are the most important constituent framework grains of sedimentary clastic rocks and their chemical dissolution plays an essential role in the formation and evolution of the secondary pore in the reservoir rocks. On the basis of thermodynamic phase equilibrium, this study investigates the chemical equilibrium relationships between fluid and various plagioclase and K-feldspar in diagenesis of the sediments, particularly, the impact of temperature and fluid compositions (pH, activity of K+, Na+, Ca2+ and so on) on precipitation and dissolution equilibria of feldspars. Feldspar is extremely easily dissolved in the acid pore water with a low salinity when temperature decreases. The dissolution of anorthite end-member of plagioclase is related to the Ca content of the mineral and the fluid, higher Ca either in the mineral or in the fluid, easier dissolution of the feldspar. Moreover, the dissolution of albite end-member of plagioclase is related to Na of both the mineral and fluid,
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| 收稿时间： | 30 July 2003 |
Thermodynamic conditions of framework grain dissolution of clastic rocks and its application in Kela 2 gas field |
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| LAI Xingyun,YU Bingsong,CHEN Junyuan,CHEN Xiaolin,LIU Jianqing,MEI Mingxiang,JING Weiguang & CHENG Suhua School of Earth Sciences and Mineral Resources,China University of Geosciences,Beijing ,China, Key Laboratory of Lithospheric Tectonics,Deep-level Process and Exploration of Ministry of Education,Beijing ,China.Thermodynamic conditions of framework grain dissolution of clastic rocks and its application in Kela 2 gas field[J].Science in China(Earth Sciences),2005,48(1):21-31. |
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| Authors: | Xingyun Lai Bingsong Yu Junyuan Chen Xiaolin Chen Jianqing Liu Mingxiang Mei Weiguang Jing Suhua Cheng |
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| Institution: | 1. School of Earth Sciences and Mineral Resources, China University of Geosciences, Beijing 100083, China;Key Laboratory of Lithospheric Tectonics, Deep-level Process and Exploration of Ministry of Education, Beijing 100083, China
2. School of Earth Sciences and Mineral Resources, China University of Geosciences, Beijing 100083, China |
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| Abstract: | Feldspar and clastic debris are the most important constituent framework grains of sedimentary clastic rocks and their chemical dissolution plays an essential role in the formation and evolution of the secondary pore in the reservoir rocks. On the basis of thermodynamic phase equilibrium, this study investigates the chemical equilibrium relationships between fluid and various plagioclase and K-feldspar in diagenesis of the sediments, particularly, the impact of temperature and fluid compositions (pH, activity of K+, Na+, Ca2+ and so on) on precipitation and dissolution equilibria of feldspars. Feldspar is extremely easily dissolved in the acid pore water with a low salinity when temperature decreases. The dissolution of anorthite end-member of plagioclase is related to the Ca content of the mineral and the fluid, higher Ca either in the mineral or in the fluid, easier dissolution of the feldspar. Moreover, the dissolution of albite end-member of plagioclase is related to Na of both the mineral and fluid, lower Na out of both the mineral and fluid, easier dissolution of the mineral. Similarly, lower-K fluid is more powerful to dissolve K-feldspar than the higher. The anorthite component of plagioclase is most easily dissolved in ground water-rock system, the albite is the secondary, and K-feldspar is the most stable. Selective dissolution of plagioclase occurs in diagenesis because of the plagioclase solid solution that is mainly composed of albite and anorthite end-members, Ca-rich part of which is preferentially dissolved by the pore water, in contrast to the Na-rich parts. Based on investigation of reservoir quality, secondary pore, dissolution structures of feldspar, clay minerals and ground water chemistry of the Kela 2 gas field of Kuqa Depression in the Tarim basin of Western China, we discovered that the secondary pore is very well developed in the highest quality section of the reservoir, and the plagioclase of which was obviously selectively dissolved, in contrast to the overgrowth of K-feldspar. Chemistry of the ground water out of the highest quality section of the reservoir is consistent with the complete dissolution field of anorthite, partial dissolution field of albite component of plagioclase, and the precipitation of K-feldspar on the temperature-logK phase equilibrium diagram drawn from this study. The thermodynamic modeling calculation gave us an insight into formation of the secondary pore within the reservoir, and the dissolution of plagioclase an important cause of the secondary pore. It is assumed that the solvent to dissolve the plagioclase frame grains originated from the origin-enriched compacted fluid acids out of the mudstone buried beneath the top of geopressured zones in the Kuqa Depression. |
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| Keywords: | dissolution of feldspar thermodynamic conditions groundwater pore chemistry Kela 2 gas field |
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