The formation mechanism of multi-stage carbonate cements in tight sandstone: A case study of Chang 63 sub-member of Triassic Yanchang Formation in Huaqing area, Ordos Basin
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摘要:
为明确致密砂岩中碳酸盐胶结物的类型、分布特征及其形成机理，以鄂尔多斯盆地华庆地区延长组长63致密砂岩为例，在铸体薄片、阴极发光、扫描电镜分析的基础上，从岩石学、地球化学的角度进行分析。研究表明，碳酸盐胶结物主要有方解石、铁方解石、白云石和铁白云石，在不同地区分布类型差别较大，西南部含量较高，东北部含量较低，其含量、类型与物源及水介质中钙离子的来源有关，可分为内源及外源成因，由于晚期碳酸盐胶结物发育，较大规模地堵塞了孔隙，且后期溶解作用较差，成为长63砂岩致密化的一个重要因素。在绘制研究区铁方解石和铁白云石胶结物等值线图的基础上，认为研究区碳酸盐胶结物主要为碳酸盐岩屑溶解-再沉淀、长石溶解-再沉淀等形成机理。研究成果可为华庆地区延长组致密油储层有利区预测提供依据。
Abstract:In order to understand tight sandstone characteristics and formation mechanism, the authors analyzed the distribution and formation mechanism of carbonate cements in terms of petrological and geochemical characteristics of the samples by means of casting thin sections, cathodoluminescence and scanning electron microscopy in the samples, take the Chang 63 sub-member reservoir of Triassic Yanchang Formation in Huaqing area of Ordos Basin as the study object. The result shows that the main types of carbonate minerals are calcite, ferrocalcite, dolomite and ferrodolomite, and different areas show great difference in type distribution, being higher in content in southwest and lower in northeast. The content and type are related to the provenance and the source of calcium ions in aqueous medium, which can be divided into authigenic carbonate and exogenous carbonate. As the pore volume was plugged up by the carbonate cement on a large scale and the later dissolution was lacking, the sandstone of Chang 63 sub-member of Yanchang Formation became densified. On the basis of drawing the contour map of ferrocalcite and ferrodolomite cement in study area, the mechanism of carbonate cements was investigated. It is held that the carbonate cements were mainly formed by the dissolution-reprecipitation of carbonate rock debris and feldspar dissolution-precipitation. The results obtained by the authors can provide reference for favorable prediction of tight oil of Yanchang Formation in Huaqing area, and can also provide reference for microscopic characteristics study of tight hydrocarbon reservoir both in China and abroad.
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Key words:
- carbonate cements /
- multi-stages /
- formation mechanism /
- tight sandstone /
- Ordos Basin /
- Huaqing area /
- Yanchang Formation
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表 1 华庆地区长63典型碳酸盐胶结物电子探针分析结果
Table 1. The electron microprobe analyses of carbonate cements of Chang 63 sub- member of Yanchang Formation in Huaqing area
% 样号 岩性 点号 Na2O Al2O3 MgO SrO SiO2 CaO FeO BaO K2O MnO NiO B2O3 63 铁白云石 169 0.135 0.262 11.93 0.119 0.777 28.48 11.55 0 0.031 1.485 0 / 63 方解石 168 0 0.125 0.257 1.021 0.254 54.74 0.886 0.027 0.2 0.634 0.01 / 63 白云石 170 0.009 0.034 21.14 0.013 0 29.87 0.168 0 0.011 0 0.031 / 15 白云石 121 / 0.01 20.12 0 / 29.7 0.339 0 / / 0.019 1.001 15 铁白云石 122 / 0.709 9.908 0.125 / 29.34 13.01 0.068 / / 0 1.039 注：电子探针位置见图版Ⅰ 
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