| 广西环江凹陷深部岩溶缝洞充填物碳氧同位素特征及古环境意义 |
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| 引用本文： | 董红琪,张庆玉,梁嘉鹏,淡永,梁彬,聂国权,季少聪.广西环江凹陷深部岩溶缝洞充填物碳氧同位素特征及古环境意义[J].中国岩溶,2022,41(5):838. |
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| 作者姓名： | 董红琪 张庆玉 梁嘉鹏 淡永 梁彬 聂国权 季少聪 |
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| 作者单位： | 1.中国地质科学院岩溶地质研究所/自然资源部、广西岩溶动力学重点实验室, 广西 桂林 541004 |
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| 基金项目： | 国家重点研发计划课题（2018YFC0604301）；中国地质调查局项目（DD20190562，DD20190723，DD20221658） |
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| 摘 要： | 深部岩溶是碳酸盐岩地区深部油气资源勘探中不可避免的问题。深部岩溶发育期次的确定是岩溶储层地质的技术难题。广西环江凹陷在页岩气钻井中发现大量深部溶洞且充填物丰富多样，成为深部岩溶发育期次研究的良好素材。文章对HD1-4钻井揭露的深部岩溶缝洞充填物及地表岩溶缝洞充填物进行碳氧同位素分析，揭示出环江地区的4种不同岩溶环境:同生期或准同生期岩溶环境、表生期大气淡水岩溶环境、中浅埋藏岩溶环境、深埋藏或热液岩溶环境，环江地区深部岩溶发育为4期不同岩溶环境叠加的效果，主要受到热液岩溶环境和大气淡水岩溶环境的影响。
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| 关 键 词： | 碳氧同位素 深部岩溶 古环境 岩溶期次 环江凹陷 |
| 收稿时间： | 2022-03-02 |
Carbon and oxygen isotope characteristics and paleoenvironmental significance of deep karst fracture-cave fillings in Huanjiang sag,Guangxi |
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| Institution: | 1.Institute of Karst Geology, CAGS/ Key Laboratory of Karst Dynamics, MNR& GZAR, Guilin, Guangxi 541004, China2.Guangxi Karst Resources and Environment Research Center of Engineering Technology, Guilin , Guangxi 541004, China3.China University of Geosciences(Wuhan), School of Environmental Studies, Wuhan, Hubei 430074, China |
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| Abstract: | In recent years, deep karst caves are commonly met in deep resource exploration and engineering construction. However, the deep-buried caves lead to the insufficient research on the genesis and development mechanism of deep karst caves. As good research samples, a large amount of complete drilling cores in deep karst caves are founded in Huanjiang sag in Guangxi. The karst morphology analysis of the drilling core in Well HD1-4 reveals that the deep karst in Huanjiang area is mainly composed of net cracks and holes expanding along cracks and dolomite honeycomb pores, and large karst caves are also developed, with the maximum height of 20 m. The types of fillings in deep karst caves are diverse. Through the observation of the whole well core, it is found that the deep karst cave fillings are of four characteristics. (1) Calcification growth,the mixed growth pattern of chemical and argillaceous substances reflects periodic growth in the cave, which may be associated with surface hydrological systems. (2) Flower-like growth pattern,white calcite with multi-stage growth can be identified in pores, but the growth direction and growth environment of each stage are different, hence forming flower-like growth pattern. (3) Primary chemical fillings of the hole,the hole is filled with calcite or dolomite, and the crystal form of calcite in part of the hole is good. (4) Argillaceous fillings in the cave, argillaceous fillings can be seen at 230 m, 432 m, 880 m and even 1,132 m in Well Huandi 1-4, which are far lower than the local discharge datum or the sea level. These argillaceous fillings may come from the surface mud seepage along the fracture or the mud beneath the ancient exposed surface. In this paper, the carbon and oxygen isotope analyses of deep karst cave fillings and surface karst cave fillings in Well HD1-4 drilling show the wide distribution of carbon and oxygen isotope of deep karst fracture. δ13C values are between ?5.2‰ and ?2‰ with the average value of ?0.33‰. δ18O values are between ?16.78‰ and ?5.3‰ wtih the average value of ?11.45‰. The values show the general negative skewness. The negative skewness of carbon and oxygen isotope of calcite fillings in the pores is the largest, and that of dolomite is the smallest. The carbon and oxygen isotope values of calcareous mudstone are the closest to those of modern atmospheric freshwater. Based on the analysis of geological conditions, four large-scale paleokarst processes and filling periods are founded in the formation and filling stages of deep karst in the Huanjiang area. (1) In the karst environment in contemporaneous period and penecontemporaneous peiord, the distribution range of δ18O is the same as the background value of the bedrock of carbonate rock, but δ13C changes greatly. The results show that the karst environment is similar to the sedimentary environment of carbonate rocks, which reflects the short-term exposure of karst after the deposition of carbonate rocks in the contemporaneous period, and the filling shows the characteristics of early precipitation. Karst space is mainly characterized by dissolution pores, which provides the basis for further karst development. (2) The karst environment of atmospheric fresh water in the hypergene period is affected by atmospheric fresh water δ13C, and the δ18O value shows a significant negative skewness. The fact that δ13C is less than ?2.5‰ and δ18O is between ?13 and ?8.2‰ indicates the karst environment is an open system, and a large number of argillaceous substances infiltrate with surface water and fill karst caves, making a certain impact on the preservation of deep resources. (3) In the shallow-buried karst environment, the δ18O value of the filling is negative, which is more negatively skewed than that of Type I bedrock. The δ13C value is basically consistent with the bedrock value, and there is no obvious negative skewness. The δ13C value is between ?2.0‰ and 2.5‰, and the δ18O is between ?13.0‰ and ?9.0‰. Due to the increase of temperature and pressure in the closed system during the burial peirod, there gradually precipitates and forms dolomite. (4) In the deep-buried or hydrothermal karst environment, the δ18O value of fillings is obviously negative, and high-temperature fluid flows up along the fault to form the karst dissolution space which becomes the reservoir place of various deposits. The hydrothermal karst with high temperature and the atmospheric freshwater karst are the main periods of the formation of deep karst fracture-cavity filling in Huanjiang area. The research results are of great significance for deep karst reservoir prediction and deep resource exploration in the later stage. |
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