鄂尔多斯盆地上三叠统延长组典型重矿物在埋藏成岩期间的演化特征及成因

The heavy minerals in the burial diagenetic conditions will suffer the progressive intrastratal dissolution and even form overgrowth and new heavy mineral cements. At present, the provenance by using the method of heavy minerals and the diagenesis of the Upper Triassic Yanchang Formation in Ordos Basin have been extensively considered. However, the evolutions of the heavy minerals during the burial diagenesis have been ignored in the both aspects. The preliminary studies about the Upper Triassic Yanchang Formation in Ordos Basin revealed that the garnets were dissolved and become facet garnets, the authigenic sphene crystals and sphene overgrowth were also seen, and the growth of the titanites has several times, the authigenic epidote crystals and epidote overgrowths were seen. The project will focus on the typical kinds of garnets, titanites and epidotes from the Upper Triassic Yanchang Formation and their distinct epigenetic evolutions of dissolution and authigenic precipitation closely. Their relation to the common cements, associated minerals, occurrence, vertical and horizontal distribution and influence factors will be carried out in detail. Then the corrosion mechanism of the garnets and the formation mechanism of the authigenic titanites and epidotes will be explored. This project will have profound implications, which not only will provide a new approach for further research on the diagenetic fluid and diagenetic environment of the Upper Triassic Yanchang Formation sandstones in Ordos basin, but also will provide new more in-depth understanding of the coupling relationship between the diagenesis of the typical heavy minerals and the ultra and extra low permeability reservoir quality evolution of the Upper Triassic Yanchang Formation sandstones. Eventually, the achievements may further promote the research of clastic rock diagenesis in Ordos basin and our home.

在埋藏成岩期间重矿物可能会发生强烈溶蚀和形成次生加大甚至可独立沉淀生成新的自生重矿物。目前鄂尔多斯盆地上三叠统延长组相关的物源分析和成岩作用研究都未注意到重矿物的成岩演化现象。事实上，延长组砂岩中石榴子石发生溶蚀形成阶梯状石榴子石，见榍石次生加大、自生榍石晶体、绿帘石次生加大、自生绿帘石晶体，且榍石次生加大具有多期次性。项目紧密围绕延长组石榴子石、榍石和绿帘石三种典型重矿物经历的截然不同的溶蚀与沉淀作用，系统研究重矿物与常见胶结物的结构关系、伴生矿物、赋存状态、后生演化的纵横向分布规律及控制因素，探索石榴子石的溶蚀机理、自生榍石和绿帘石的沉淀机理。项目的开展将为进一步研究鄂尔多斯盆地延长组砂岩成岩流体与成岩环境提供新的研究方法和途径，也为更深入了解典型重矿物成岩过程与延长组砂岩超低渗—特低渗储层质量演化的耦合关系提供崭新的资料，并将为促进鄂尔多斯盆地乃至我国碎屑岩成岩作用研究提供重要资料。

砂岩中自生榍石胶结物对成岩作用演化过程研究具有重要研究意义。利用铸体薄片鉴定、电子探针、背散射、流体包裹体等方法研究了鄂尔多斯盆地上三叠统延长组和可与之类比的四川盆地中部中侏罗统上沙溪庙组砂岩自生榍石胶结物的岩石学、地球化学特征，分析了其成因机制及其与优质储层的关系。鄂尔多斯盆地上三叠统延长组自生榍石胶结物具有次生加大边生长和呈晶粒状生长于粒间孔隙中两种赋存模式，电子探针分析结果表明基本上为富钛自生榍石矿物。常见的伴生胶结物有早期绿泥石薄膜、早期和晚期两期方解石胶结物，中期浊沸石充填及晚期石英次生加大等四种。延长组榍石胶结物晶体较小，Ti来源不足，榍石胶结物形成所需要的钛可能来自于砂岩中富含钛的黑云母矿物蚀变；砂岩经历了强烈的压实作用，面孔率低，具有一定的渗透条件，允许含Ti流体流动。榍石胶结物形成于主要发生在中生代晚期的早白垩世(130～100 Ma)的构造热事件中，构造热事件发生时期为地温梯度及热流异常时期。四川盆地中部中侏罗统上沙溪庙组砂岩中发育可与之类比的榍石胶结物。砂岩中榍石胶结物具有围绕碎屑状榍石呈次生加大边，部分可达两期次加大和围绕石英和长石碎屑、不透明岩屑、绿帘石碎屑等生长，呈晶粒状榍石生长于粒间孔隙中等2种产出形式。榍石次生加大边和自生榍石晶体都较大，最大可达0.1m，且不限于在榍石碎屑上成核生长。榍石胶结物发育所需的钛元素可能来自富含钛的黑云母矿物或者不透明铁-钛氧化物岩屑。榍石胶结物的形成共有富钛和贫钛两期次，形成温度分别为80-90℃和105-110℃，与A1期烃源岩早期成熟阶段油气充注带来的大量有机酸、碳酸及中成岩A2期大规模脱羧有机酸的充注对长石、岩浆岩岩屑和黑云母等进行溶蚀有关。含自生榍石胶结物的砂岩需要较好的渗流条件、需要活动流体的流动和扩散，往往其孔渗性较好，往往具有较高的原生粒间孔隙和次生粒内溶孔，部分可以作为较好储层的指示矿物。