羌塘盆地南部地区布曲组砂糖状白云岩埋藏成因的新证据

New evidence for deep burial origin of sucrosic dolomites from Middle Jurrasic Buqu Formation in southern Qiangtang basin

羌塘盆地南部中侏罗统布曲组广泛发育东西向延伸长达100余千米的层状白云岩,它们具有典型的晶粒镶嵌结构,孔隙度最高达17.48%,且有油浸显示,是目前寻找优质储层的主要勘探目标。本文采用铸体薄片、阴极发光、包裹体测温、碳氧同位素等分析手段对这套含油白云岩进行了研究,结果表明该套白云岩为高温埋藏成因。白云岩包裹体均一温度分布在100～175℃之间,这一温度范围超过了近地表和浅埋藏阶段混合水成因白云岩范围,且雾心亮边结构的白云石在高温深埋藏阶段白云岩化过程中也可以形成,且亮边与核心光性方位一致,是空隙流体直接沉淀形成的共轴增生加大边。根据包裹体盐度的测量计算结果表明白云石化流体具有高盐度卤水特征。

The massive dolomites from the Middle Jurrasic Buqu Formation in the southern Qiangtang basin extend more than 100kin in length to the east-west strike direction. These dolomites typically have crystal grain mosaic textures with a top porosity rate of 17.48%. Because the sucrosic dolomites in this region have extensive oil-immersed impressions, the dolomite reservoir is considered a good target for petroleum exploration. After studying the sucrosic dolomites based on casting thin sections, inclusion temperature, and carbon and oxygen isotope analysis, this paper suggests that the most likely origin of these dolomites is replacement by formation water during high temperature deep burial stage. The homogenization temperatures in dolomite from fluid inclusion analysis range between 100℃ and 175℃, which are higher than the temperatures of mixed seawater and the freshwater dolomitization formation mechanism model in near-surface and shallow burial stage. The dolomites with a fogged center and clear margin structure can be formed in the high temperature deep burial stage. Having the same optic orientation as the fogged center dolomite, the clear margin is considered as the secondary enlarged fringes of dolomites which are directly deposited by formation water. After measuring and calculating the salinity of the fluid inclusion, it is concluded that the digenesis pore fluid is similar to saline or brine water.