根据作为川东北三叠系飞仙关组作为重要储层的结晶白云岩（包括用以对比的原始结构保存的粒屑白云岩）的结构、元素构成、包裹体均一化温度和碳、氧、锶同位素特征，以及结晶白云岩在时间上和空间上的分布，讨论了这类白云岩形成的流体、温度和白云化机制，解释了结晶白云岩具有较好物性的原因。川东北飞仙关组结晶白云岩主要由细-中晶的平直晶面半自形-自形晶白云石构成，缺乏可进行均一化温度分析的气-液两相包裹体，具有低的锰、铁含量和较高的锶含量，显示较低温条件下海水或海源流体在白云化过程中的控制作用。结晶白云岩的碳、氧、锶同位素组成说明先驱鲕粒灰岩的碳是其主要碳源，白云化流体主要是早三叠世印度阶晚期海水，其次是奥伦尼克阶海水，白云化作用发生时鲕粒灰岩处于埋藏深度＜1 km的埋藏阶段，40～60℃的温度主要由地热增温提供，台地内部埋藏环境的热对流模式可以解释川东北飞仙关组结晶白云岩的形成机制。开江-梁平台棚以东的孤立（或半孤立）碳酸盐台地具有陡峻的边缘，两侧同时对海水开放，鲕粒岩分布层位较低（飞1-飞2段）都有利于热对流作用，台地内较为发育的蒸发盐造成纵向热传导率的差异并加强台内热对流。在白云岩分布样式的控制机理上，由于高渗透率鲕粒岩仅分布在台地边缘，使得海水进入台地后的垂向对流大于水平对流，白云化作用发生在台地边缘，飞3段鲕粒岩没有足够时间完成白云化，台地边缘结晶白云岩的体积向上减少。另外，川东北还存在回流白云化作用和塞卜哈白云化作用，由此形成的白云岩主要分布在台地上部或顶部，保留有很好的原始粒屑灰岩与泥微晶灰岩的结构，具有与同期海水类似的锶同位素组成，稍低的δ13 C值和偏正的δ18 O值以及稍高的锰、铁含量，显示白云化流体是同期海水，白云化系统对大气环境相对开放，白云化流体盐度高于同期海水。由于结晶白云岩是在不对大气水环境开放、但对非同期海水开放的条件下进行的，白云化反应的外来CO2-3较少，白云化过程岩石孔隙相对增加，储层质量较好。原始结构保存的粒屑白云岩是在对大气水环境相对开放的条件下进行的，白云化反应的外来CO2-3较多，白云化过程岩石孔隙增加有限甚至减少，白云岩为差储层或致密层。开江-梁平台棚以西的镶边陆棚飞仙关组虽然也发育高能环境的鲕粒灰岩或其他高能环境的粒屑灰岩，但形成优质储层结晶白云岩的条件差于台棚以东的孤立（或半孤立）碳酸盐台地。%This paper discusses the characteristics of fluids, temperature and mechanism of dolomitization based on the fabrics,elemental composition,cathodoluminescence,homogenization temperature,carbon/oxygen/strontium isotopes of the crystalline dolomite (and also the fabric-preserved grain dolomite phases for comparison)and combined with the space-time distribution of the crystalline dolomites in Northeast Sichuan Basin.The crystalline dolomite has low manganese and iron contents and higher strontium content,indicating the controlling of the sea water or marine-derived fluid on the dolomitization.The crystalline dolomite consists of dolomite crystals with small size and planar crystal boundaries and the gas-liquid fluid inclusions are absent,showing the low temperature of dolomitization.The carbon/oxygen/strontium isotope compositions of the crystalline dolomite indicate that the primary carbon source of the crystalline dolomite is the precursor oolitic limestone, that the sea waters of late Indo-Chinese stage (major)and Olenekian stage (minor)are the fluid for dolomitization of the crystalline dolomite,and that the dolomitization occurs in relatively shallow burial stage with depth less than 1000 m.The thermal convection of the burial environment within the platform is the essential mechanism of forming the Feixianguan crystalline dolomites.The carbonate platform to the east of Kaijiang-Liangping Trough has the steep margin opening to sea water and the oolitic rocks develop in the lower horizon (1st to 2nd Member of Feixianguan Formation).These benefit the thermal convection.The thick evaporite developed in the platform enhances the thermal convection of internal fluids in the platform.The oolitic rocks with high permeability are distributed over the edge of the platform,the impacts of the vertical convection exceed that of the horizontal convection and the dolomitization is restricted within the rim of the platform.There is not enough time for oolitic rocks of Member 3 of Feixianguan Formation to finish the dolomitization,so,the volume of the crystalline dolomite in the upper horizon is less than that in the lower horizon.In addition,the reflux and Sabkha dolomitization model responsible for the fabric-preserved dolomites also exist in the upper part and the top of the platform.These dolomites have the 87 Sr/86 Sr ratios similar to the contemporary sea water.Relatively high manganese and iron contents and relatively negativeδ13 C and positiveδ18 O indicate that the dolomitization fluids responsible for these dolomites are the contemporary sea water and they may develop in the environment open to meteoric water,the salinity of the dolomitization fluids is higher than the contemporary sea water. The crystalline dolomites develop in the environment not open to meteoric water but open to non-contemporary sea water,and consequently,have little external CO2-3 and increase the porosity of rocks and thereby have the higher reservoir quality,whereas the fabric-preserved dolomites have more external CO2-3 and limitedly increase the porosity (or even decrease the porosity )and thereby have the lower quality reservoirs.In the Feixianguan Formation with the rimmed shelf to the west of Kaijiang-Liangping platform-shelf,there also develops the oolitic limestone of high energy environment,however,the crystalline dolomite conditions of forming good quality reservoirs are more than that of the carbonate platform to the east of the platform-shelf.
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