不同赋存状态方沸石的特征与成因——以辽河西部凹陷雷家地区古近系沙四段为例

Characteristics and genesis of analcites in different occurrence states: A case study of the fourth member of Shahejie Formation in the Leijia area of the Western Liaohe Depression

不同赋存状态的方沸石对认识沉积环境、成岩流体类型及储层等均具有重要意义。论文以辽河西部凹陷雷家地区古近系沙四段为例,在大量岩芯、薄片观察的基础上,对不同赋存状态方沸石的阴极发光、微量元素、同位素分析、流体包裹体等进行了系统测试分析,共识别出纹层状、裂缝充填状及结核状3种方沸石赋存状态。纹层状方沸石主要呈纹层状与白云岩、泥岩间互出现,常具有不同程度揉皱变形特征,产状凌乱破碎。高Si/Al值、轻稀土富集、重稀土亏损、偏高Sr同位素、低δ18O、高δ13C、主量元素富SiO2、Al2O3、Na2O、TFe2O3、贫TiO2等特征表明该类方沸石系深部火山热液流体与较高盐碱度水介质相互作用形成;裂缝充填方沸石通常晶粒较大、半自形-自晶型,自裂缝边缘向中部晶粒逐渐增大、自形程度变好,具多期性,并常具沥青质残余,形成时间最晚。低Si/Al值、元素组成特征、流体包裹体温度呈多峰值等特征表明该类方沸石可能具有2种来源,其一为固结成岩后纹层状方沸石溶解并在裂缝边缘胶结形成;其二为来自深部的热液流体沿裂缝缓慢冷却、胶结成因,结核状方沸石多呈分散状,偶见串珠状顺层定向分布,赋存状态特征等表明其为纹层状方沸石成岩分异、物质重新分配形成。

The study of analcites in different occurrence states is of great significance for understanding sedimentary environment, diagenetic fluid type and reservoir. Based on the observation of a large number of cores and their thin sections of rocks in the fourth member of Paleogene Shahejie Formation in the Leijia area of the Western Liaohe Depression, the cathodoluminescence imaging, and geochemical studies of trace elements, isotopes, and fluid inclusions of analcites in different occurrence states from those rocks have been systematically carried out in this paper. Three occurrence states, namely, lamellar, fissure filled, and nodular, of analcites have been identified. The lamellar analcite mainly occurred as lamellar layer in dolomite and mudstone, often with various degrees of crumpled deformation characteristics and disordered and broken occurrence. Its characteristics of high Si/Al values, LREE enrichment, HREE depletion, relatively high 87Sr/86Sr values, low δ18O values, high δ13C values, rich in SiO2, Al2O3, Na2O, TFe2O3, and poor in TiO2 indicate that this kind of analcite was formed by the interaction between the deep volcanic hydrothermal fluid and the high salinity and alkalinity water. The fissure filled analcite usually has relatively large semi-euhedral to euhedral crystals, whose sizes are gradually increased and whose crystalline degrees are getting better from the edge to the center of fissure It has various phases and often contains asphaltene residue. It was formed in the latest time among 3 kinds of analcites. The characteristics of its low Si/Al values and elemental compositions, and various peaks of homognization temperatures of its fluid inclusions indicate that it could be formed by the crystallization of analcite from the infilled fluid, which contains dissolved lamellar analcite from the consolidated rocks, at the fissure edge, or by the crystallization of analcite from the slow cooling deep sourced hydrothermal fluid infilled into fissures of rocks. The nodular analcite is mostly scattered in the host rocks, with occasionally bead-like distributed along the layer of rocks. The characteristics of its distribution and occurrence state indicate that it could be formed by the differentiation and redistribution of lamellar analcite in the diagenetic process.