Origin and significance of "sweet spots" of analcites in shale oil reservoirs in Permian Lucaogou Formation, Jimsar Sag, Junggar Basin
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摘要: 准噶尔盆地吉木萨尔凹陷二叠系芦草沟组页岩油储层中普遍发育方沸石,且与甜点段对应好,展示出“示烃矿物意义”。为全面深入了解方沸石对页岩油甜点的意义,采用偏光显微镜、全岩X衍射成分分析、电子探针BSE图像分析及矿物能谱分析等手段,对方沸石的岩石学和矿物学特征进行了综合研究,再结合地质背景探讨了成因及意义。吉木萨尔凹陷芦草沟组方沸石主要赋存于沉凝灰岩、碳酸盐岩质凝灰岩中,个别出现在凝灰质白云岩中,而方沸石岩极为少见。方沸石在岩石中的赋存状态包括纹层状、球粒状、块状、脉状。方沸石形成于高盐度、碱性、低温的热液条件下,埋藏过程中碱性长石及部分蒙脱石类黏土成岩转化形成自生方沸石;火山喷发活跃期火山物质经过热反应与蚀变形成次生方沸石。成岩早期形成的大量方沸石胶结物起到了抗压实作用进而保护原生孔,也为成岩中后期次生溶孔的形成提供物质基础,大幅提升了页岩油层的储集性能,因而方沸石富集段与页岩油甜点段对应良好。Abstract: As a hydrocarbon indicating mineral, analcite was always found in shale oil reservoirs and has good correspondence with "sweet spots" in the Lucaogou Formation, Jimsar Sag, Junggar Basin. The petrographic and mineralogical characteristics as well as geologic origins of analcite were studied using a polarized light microscope, whole rock XRD analysis, electron probe BSE image analysis and mineral energy spectrum, in order to discuss its significance for shale oil "sweet spots". The analcite is mainly found in sedimentory tuff and carbonate tuff with some in tuff dolomite, and the analcite rock is very rare. The occurrence of analcite in rocks is lamellar, granular, massive and vein-shaped. Analcite was formed in high salinity, alkaline, low temperature hydrothermal conditions. The diagenetic transformation of non-volcanic material during burial forms authigenic analcite. Secondary analcite was formed by reaction and dissolution of volcanic material and hydrothermal during volcanic jetting. Analcite cements in the early diagenesis protected native porosity, and afforded the formation of secondary solution pores during mid and late diagenesis which has a good correspondence with shale oil "sweet spots".
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Key words:
- shale oil /
- analcite /
- secondary dissolved pore /
- Lucaogou Formation /
- Jimsar Sag /
- Junggar Basin
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图 2 准噶尔盆地吉木萨尔凹陷芦草沟组含油段岩心照片及显微镜下岩石学特征
a.吉35井,P2l1,白云岩夹凝灰岩纹层,岩心照片;b.吉174井,P2l1,含凝灰质泥晶白云岩中富含藻类纹层,荧光显微镜下照片;c.吉302井,P2l1,碳酸盐岩质凝灰岩中方解石脉清晰可见;d.吉301井,P2l1,沉凝灰岩岩心照片;e.为d对应的荧光显微照片,显示凝灰物质含油性好;f.吉176井,P2l2,安山质凝灰岩中晶屑和基质见强烈的溶蚀港湾,铸体薄片显微镜下照片
Figure 2. Photographs of cores and photomicrographs of petrographic features in the oil-bearing section in Lucaogou Formation, Jimsar Sag, Junggar Basin
图 3 准噶尔盆地吉木萨尔凹陷芦草沟组方沸石显微岩石学特征
a.吉37井,2 852.13 m,方沸石呈纹层状富集,与白云石互层,单偏光;b.a对应的正交偏光图像;c.吉174井,3 203.85 m,方沸石质岩,岩石中球粒方沸石被白云石环绕,单偏光;d.吉31井,2 717.14 m,方沸石质白云岩中方沸石近乎悬浮于白云石中,正交偏光下加入石膏试板;e.吉174井,3 202.3 m,球粒方沸石与黏土矿物,单偏光;f.为e中黄色线框区域放大图像,电子探针背散射图像;g.为f中黄色线框区域放大图像,方沸石内部可见“交代残余”的正长石,方沸石悬浮于蒙脱石基底中,电子探针背散射图像;h.吉31井,2 894.28 m,方沸石被溶蚀后的溶孔,铸体薄片显微镜下图像;i.吉31井,2 721.5 m,云质沉凝灰岩中见方沸石脉体,脉体边缘见共生白云石,荧光显微镜下图像。Anl.方沸石,Dol.白云石,Clay.黏土矿物,Mnt.蒙脱石,Or.正长石
Figure 3. Photomicrographic features of analcite from Lucaogou Formation, Jimsar Sag, Junggar Basin
表 1 准噶尔盆地吉木萨尔凹陷芦草沟组方沸石电子探针化学组分
Table 1. Chemical components of analcite by electronic-probe, Lucaogou Formation, Jimsar Sag, Junggar Basin
样号 化学组分含量/% Si/Al K2O CaO TiO2 P2O5 Na2O Al2O3 MgO SiO2 MnO FeO 总 36-Anl-05 0.04 0.01 0.00 0.03 6.78 20.20 0.01 65.02 0.00 0.11 92.19 2.73 36-Anl-08 0.22 0.03 0.00 0.00 8.54 20.93 0.04 62.00 0.04 0.11 91.91 2.51 36-Anl-09 0.63 0.03 0.02 0.01 7.44 21.59 0.18 59.67 0.00 0.30 91.91 2.34 38-An-04 0.05 0.01 0.01 0.00 7.12 20.80 0.00 65.04 0.00 0.11 93.13 2.65 38-Anl-05 0.02 0.00 0.01 0.02 8.26 20.88 0.00 63.47 0.00 0.19 92.85 2.58 平均 0.19 0.02 0.01 0.01 7.63 20.88 0.05 63.04 0.01 0.16 92.40 2.56 -
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