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须四段砂岩中碳酸盐矿物包括了矿物岩屑和胶结物两类。在矿物组成上,碳酸盐矿物岩屑由方解石和白云石组成,而碳酸盐胶结物均为方解石。
砂岩中的碎屑颗粒和胶结物可以通过其在偏光显微镜图像和电子探针BSE图像中的灰度差异进行区分(图2a、图3a)。方解石胶结物充填原生孔隙和次生孔隙,次生孔隙由长石粒内溶孔(图2a、图3)和少量岩屑粒内溶孔(图2a)组成。长石颗粒溶解后,可见残余的长石和自生石英同时分布于充填粒内溶孔的方解石胶结物中,但根据能谱分析(图2a)和OM-CL发光特征(图2b)可以区分自生石英及残余长石矿物。钾长石和奥长石等长石矿物在OM-CL发光图像中呈蓝色发光,而自生石英胶结物则不发光或被方解石胶结物覆盖(图2b)。在SEM-CL图像中(图2c,d、图3d),长石的阴极发光强度也强于石英。然而,方解石胶结物在OM-CL和SEM-CL下的发光特征却截然不同——在OM-CL下方解石胶结物呈明亮的橙色至红色发光,但在SEM-CL下却不发光(图2c,d、图3d)。结合BSE图像,进一步观察到方解石胶结物在SEM-CL下显示较高亮度的区域是未被完全填充的孔隙或矿物表面的沟槽(图2a,c、图3c,d)。
图 2 CG561样品的BSE(a)、OM⁃CL(b)和SEM⁃CL(c,d)图像特征,深度4 009.01 m
Figure 2. BSE, OM⁃CL and SEM⁃CL images of sample CG561, depth 4 009.01 m
图 3 GH2样品的反射光(a)、OM⁃CL(b)、BSE(c)和SEM⁃CL(d)图像,深度3 921.65 m
Figure 3. Optical microscope, OM⁃CL, BSE and SEM⁃CL images of sample GH2, depth 3 921.65 m
碳酸盐岩屑与方解石胶结物在BSE图像中通常显示出类似的颜色及对比度,因而难以区分(图4a)。在OM-CL图像中,部分方解石岩屑和方解石胶结物同样具有相似的发光特征而难以辨认(图4b)。与之相比,方解石岩屑和白云石岩屑在SEM-CL分析中均表现出显著低于石英碎屑的发光强度,因而在样品的SEM-CL图像中清楚地勾勒出碳酸盐岩屑和方解石胶结物的不同轮廓(图4c)。以上结果表明,SEM-CL技术对于区分在矿物结构和元素含量上相似的碳酸盐矿物时,比OM-CL更为有效。
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应用电子探针对样品中42个碳酸盐矿物(包括8个碳酸盐岩岩屑和34个方解石胶结物)进行了Na、K、Mn、Mg、Ca、Fe、Ca和Al等元素的分析,结果以氧化物质量分数表示(表1)。除CaO外,方解石胶结物的MnO含量最高,其次是FeO、Al2O3、K2O、MgO和Na2O(图5)。在方解石岩屑中,除CaO外,FeO是最高的元素,其次是MgO、Al2O3、MnO、Na2O和K2O。在白云石岩屑中,除CaO和MgO外,FeO是最高的元素,其次是Al2O3、MnO、Na2O和K2O(图5)。
表 1 不同类型碳酸盐矿物的Fe和Mn含量及OM⁃CL、SEM⁃CL发光强度
样品 碳酸盐矿物类型 Fe/% Mn/% Mn/Fe OM-CL发光强度 SEM-CL 发光强度 CH139 方解石岩屑 0.065 0.008 0.109 I II 白云石岩屑 1.504 0.098 0.066 II II 方解石胶结物 0.812 0.139 0.169 II I GH2 方解石胶结物 0.572 1.403 2.420 V I GM3 方解石胶结物 0.400 0.375 0.953 III I CG561 方解石胶结物 0.470 0.553 1.190 III I DY1 方解石胶结物 0.519 0.825 1.594 IV I 注: 阴极发光强度从I到V增强。 -
元素面扫描分析结果表明,方解石胶结物中常量元素的分布较为均一(图6,7),充填粒内溶孔和粒间孔的方解石胶结物在成分组成上不存在明显差异。方解石胶结物的Mn元素分布与Ca元素分布较为匹配。方解石岩屑与胶结物相比,通常具有相对较低的Fe和Mn元素含量(图7)。
SEM-CL and EMPA Mineralogical Characterization of Authigenic Calcite Cement in Tight Gas Sandstone and Implications for Diagenesis: Case study of Xujiahe Formation in western Sichuan Basin, China
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摘要: 扫描电镜—阴极发光(SEM-CL)可通过同时分析矿物的微观结构和阴极发光特性来反映成岩机制及流体来源,但碳酸盐矿物在SEM-CL下的发光特性及其影响因素尚不明确。应用光学显微镜阴极发光(OM-CL)、电子探针(EPMA)和SEM-CL等技术手段,以川西坳陷须家河组致密砂岩中碳酸盐矿物为研究对象,分析碳酸盐矿物在OM-CL和SEM-CL下发光特性的差异,评估影响碳酸盐矿物SEM-CL发光特性的因素,探讨砂岩中方解石胶结物的成岩意义。结果表明,相比OM-CL,SEM-CL在区分碳酸盐岩屑和碳酸盐胶结物方面具有更高的灵敏度。当Mn/Fe值低于0.1或Mn/Fe值介于0.1~0.5且Fe含量低于0.1%时,碳酸盐矿物具有中低强度的SEM-CL发光强度;当Mn/Fe值高于0.5且Fe含量高于0.1%时,碳酸盐矿物在SEM-CL照射下几乎不发光。Mn/Fe值和Fe含量是影响SEM-CL照射下碳酸盐矿物发光强度的主要因素。阴极发光分析和EPMA元素组成面扫描分析表明,方解石胶结物的元素分布是均质的,形成于同一成岩世代,晚于长石等矿物的溶解。岩屑砂岩和岩屑石英砂岩的方解石胶结物具有相同的来源,表现为高Mn/Fe特征;钙屑砂岩的方解石胶结物具低Mn高Fe特征,成分组成与白云石岩屑相似,推测流体可能受到白云石岩屑溶解的影响。Abstract: Scanning electron microscope-cathodoluminescence (SEM-CL) analysis is an effective tool for tracing the diagenesis associated with fluid flow in buried material by determining both the micro-texture and the CL response of authigenic minerals. However, neither the luminescence of carbonate minerals nor how their elemental compositions influence CL response is not clear. In this study, the CL response of carbonate minerals and its correlation with element compositions were observed when subjected to optical microscope-CL (OM-CL) and SEM-CL illumination, electron probe microanalysis (EPMA) and SEM-CL were used to analyze tight gas sandstone samples from the Xujiahe Formation in the western Sichuan Basin, China. The results suggest that SEM-CL distinguishes carbonate grains from carbonate cements with greater sensitivity than OM-CL. SEM-CL images generally showed that carbonate minerals with Mn/Fe values lower than 0.1 have low- to moderate intensity luminescence. This was also apparent in Mn/Fe values of 0.1 to 0.5 with < 0.1% Fe content, but carbonate minerals with Mn/Fe > 0.5 and Fe content < 0.1 barely showed a visible CL response. The Mn/Fe ratio and Fe content were found to have the greatest effect on CL. The integration of SEM-CL and OM-CL with element mapping showed that the calcite cement is homogeneous, suggesting a single episode of calcite precipitation after the dissolution of feldspar. Sublitharenitic and litharenitic calcite cements have the same/similar source of diagenetic fluid that is indicated by high Mn/Fe values. The calcite cement in calcarenaceous sandstone showed low Mn and high Fe contents similar to dolomite rock fragments, suggesting that the fluid may have been influenced by the dissolution of dolomite rock fragments.
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图 2 CG561样品的BSE(a)、OM⁃CL(b)和SEM⁃CL(c,d)图像特征,深度4 009.01 m
(a)石英颗粒(Q)、方解石胶结物(CC)和石英、方解石及奥长石(Ogc)的能谱图,方解石胶结物充填了粒间孔和长石粒内溶孔,在被溶解的长石颗粒边缘可见少量残余长石和自生石英,BSE图像;(b)图a相同视域,钾长石发强烈蓝光,自生石英不发光,方解石胶结物具橙色发光特征,OM⁃CL图像;(c)钾长石(Kfs)和残余奥长石(Ogc)具强烈发光特征,方解石胶结物(CC)无发光,SEM⁃CL图像;(d)石英颗粒(Q)及方解石胶结物(CC)发光特征对比,箭头处是未完全充填的粒间孔,SEM⁃CL图像
Figure 2. BSE, OM⁃CL and SEM⁃CL images of sample CG561, depth 4 009.01 m
(a) quartz grains (Q), calcite cements (CC) and EDS spectrum of quartz, calcite and oligoclase (Ogc), calcite cement fills dissolved pores within the feldspar grain, and quartz cement and a few residual feldspar minerals formed by feldspar dissolution occur at the edge of the grain, BSE; (b) in same area as (a), K⁃feldspar (Kfs) shows strong blue luminescence and non⁃luminescent quartz cement, OM⁃CL; (c) residual feldspar (Kfs, Ogc) with strong luminescence and non⁃luminescent calcite cement (CC), SEM⁃CL; (d) quartz grains (Q) and calcite cements (CC), SEM⁃CL
图 3 GH2样品的反射光(a)、OM⁃CL(b)、BSE(c)和SEM⁃CL(d)图像,深度3 921.65 m
(a)方解石胶结物(CC)充填粒间孔和长石粒内溶孔,反射光;(b)方解石胶结物发橙色光,被方解石充填的长石残晶(DF)仍具较强的蓝色发光,OM⁃CL图像;(c)方解石胶结物充填长石粒内溶孔,可见残余长石和粒内溶孔,BSE图像;(d)方解石胶结物不发光,残余长石和溶孔具较强发光,SEM⁃CL图像
Figure 3. Optical microscope, OM⁃CL, BSE and SEM⁃CL images of sample GH2, depth 3 921.65 m
(a) calcite cements (CC) filled primary pore and feldspar intragranular dissolved pore, reflect light; (b) calcite cement shows orange luminescence; Kfs shows blue luminescence, OM⁃CL; (c) feldspar intragranular dissolved pore partially filled with calcite cements, BSE; (d) strong luminescence under SEM⁃CL in unfilled feldspar intragranular dissolved pore; calcite cement is non⁃luminescent
图 4 CH139样品的BSE、OM⁃CL和SEM⁃CL图像,深度3 781.79 m
(a)方解石岩屑(C)与方解石胶结物(CC)难以区分,下方为能谱图像,BSE;(b)方解石岩屑与胶结物均具橙色阴极发光,OM-CL图像;(c)方解石岩屑(C)和白云石岩屑(D)具相同的发光特征,而方解石胶结物(CC)不发光,SEM⁃CL
Figure 4. BSE, OM⁃CL and SEM⁃CL images of sample CH139, depth 3 781.79 m
(a) calcite detrital grains indistinguishable from calcite cements (EDS spectrum of calcite and dolomite), BSE; (b) calcite detrital grains and cements show orange luminescence and are difficult to distinguish apart, OM⁃CL; (c) calcite and dolomite detrital grains are luminescent; calcite cements are non⁃luminescent, SEM⁃CL
图 7 CH139样品的BSE图像,SEM⁃CL和元素面扫描图像
(d~h)Ca,Mg,Mn,Fe和Fe、Mn元素叠加分布图;黄色箭头所指为白云石岩屑局部溶解;CC.方解石胶结物;C.方解石岩屑;D.白云石岩屑
Figure 7. Calcite cements in sample CH139 for BSE images, SEM⁃CL image and element mapping
(d) Ca; (e) Mg; (f) Mn; (g) Fe; (h) superposition of Mn and Fe in partly dissolved dolomite fragments filled with calcite cements (yellow arrows in (a)); CC.calcite cement; C.calcite rock fragment; D.dolomite rock fragment
表 1 不同类型碳酸盐矿物的Fe和Mn含量及OM⁃CL、SEM⁃CL发光强度
样品 碳酸盐矿物类型 Fe/% Mn/% Mn/Fe OM-CL发光强度 SEM-CL 发光强度 CH139 方解石岩屑 0.065 0.008 0.109 I II 白云石岩屑 1.504 0.098 0.066 II II 方解石胶结物 0.812 0.139 0.169 II I GH2 方解石胶结物 0.572 1.403 2.420 V I GM3 方解石胶结物 0.400 0.375 0.953 III I CG561 方解石胶结物 0.470 0.553 1.190 III I DY1 方解石胶结物 0.519 0.825 1.594 IV I 注: 阴极发光强度从I到V增强。 -
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