Carbon and oxygen isotope characteristics and paleoenvironmental significance of deep karst fracture-cave fillings in Huanjiang sag, Guangxi
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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。
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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Key words:
- carbon and oxygen isotopes /
- deep karst /
- ancient environment /
- karst period /
- Huanjiang sag
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表 1 岩溶缝洞充填物分类及碳氧同位素测试结果
Table 1. Classification of filling materials in karst fracture-cave and test results of carbon and oxygen isotopes
分类 序号 样品编号 地层 岩性/产出 采样地点/HD1-4井深/m δ13C（V-PDB）/‰ δ18O（V-PDB）/‰ 大浦组、黄龙组基岩 1 HD015-1 C2d 白云岩 环江猜峒断裂 3.48 −11.51 2 HD015-4 C2h 白云质灰岩 环江猜峒断裂 2.51 −9.13 3 HD016-1 C2h 灰岩 环江猜峒断裂 2.50 −8.72 4 HD1-3B C2d 孔壁白云岩 38 3.24 −8.53 5 HD1-8B C2d 孔壁白云岩 120 3.34 −7.80 6 HD1-14 C2d 洞壁白云岩 419 2.48 −11.35 7 HD1-24A C2d 花边状白云岩 484 3.35 −5.30 8 HD1-45B C2d 灰质白云岩 1 088 3.02 −8.22 9 HD1-47B C2d 灰质白云岩 1 131~1 137 1.72 −6.28 溶蚀孔洞中的白云石 10 HD1-3A C2d 溶孔白云石 38 4.21 −8.90 11 HD1-8A C2d 溶孔充填方解石 120 −0.94 −12.62 12 HD1-13 C2d 花斑状溶孔白云石半充填 415 −4.93 −8.80 13 HD1-25 C2d 白云石半充填溶孔 497 −0.81 −13.50 断层或裂缝充填白云石 14 HD015-2 C2d 白云石脉 环江猜峒断裂 0.56 −16.36 15 HD015-3 C2d 白云石脉 环江猜峒断裂 −1.76 −12.03 16 HD016-2 C2h 裂缝方解石 环江猜峒断裂 −3.11 −10.76 17 HD1-56 C2d 断层砾石及网状缝方解石充填 1 721 −4.10 −13.94 18 HD1-57 C2d 断裂充填方解石 1 762 −1.04 −14.29 19 HD1-58 C2d 方解石 1 840 −2.15 −15.76 角砾间白云石胶结物 20 HD1-20 C2d 角砾白云石胶结 448 −5.04 −8.34 孔洞边缘花边1期白云石 21 HD1-24B C2d 花边1期白云石 484 4.10 −7.53 孔洞边缘花边2期白云石 22 HD1-24C C2d 花边2期白云石 484 2.45 −12.15 溶洞充填钙质泥混合物 23 HD1-46 C2d 溶洞充填钙质泥 1 091~1 094 −5.20 −11.56 溶孔充填方解石 24 HD1-45A C2d 溶孔充填方解石 1 088 −4.55 −12.19 25 HD1-47A C2d 方解石充填 1 131~1 137 −2.93 −16.78 26 HD1-50 C2d 溶孔方解石充填 1 220 −3.65 −15.81 27 HD1-51 C2d 溶孔方解石充填 1 224~1 225 −3.80 −15.87 28 HD1-53 C2d 溶孔方解石 1 258 −1.81 −13.99 29 HD1-54 C2d 溶孔方解石 1 457 −0.81 −13.93 
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表 2 环江地区晚石炭基岩及缝洞充填物碳氧同位素指示环境
Table 2. Carbon and oxygen isotope indicator environment of late Carboniferous bedrock and fracture-cavity fillings in Huanjiang area
发育期次 形成环境 同位素特征 δ13C(PDB)/‰ δ18O(PDB)/‰ Ⅰ 同生期或准同生期岩溶环境 1.8~4.2 −4.0~−9.0 Ⅱ 表生期大气淡水岩溶环境 −2.5~−6.0 −8.2~−13.0 Ⅲ 中浅埋藏岩溶环境 −2.0~2.5 −9.0~−13.0 Ⅳ 深埋藏或热液岩溶环境 −4.0~2.0 ＜−13.0
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