川东北飞仙关组白云岩的主要类型、地球化学特征和白云化机制

Petrography, geochemistry and dolomitization mechanisms of Feixianguan dolomites in Triassic, NE Sichuan, China.

四川盆地东北部蕴藏有丰富的烃类资源,三叠系飞仙关组的白云岩是最主要的储层和产层,这使得飞仙关组白云岩成为人们近年来高度关注的对象。按照结构,我们可以将飞仙关组白云岩分为三种端元类型:微晶白云岩、具原始结构的粒屑白云岩和结晶白云岩,在一个向上变浅的剖面旋回中,这三种白云岩分别位于旋回的顶部、上部和中下部。微晶白云岩和具原始结构的粒屑白云岩具有较高的锰含量和较低的包裹体均一化温度,白云化流体的δ^(18)O值(从白云石的包裹体均一化温度和δ^(18)O值反演,以下同)介于大气水和海水之间,反映这类白云岩形成于有大气水参与的相对开放的沉积-成岩环境中,蒸发泵作用(对于微晶白云岩)和混合水作用(对于具原始结构的粒屑白云岩)可能是其主要的形成机制;结晶白云岩(表现为好储层)具有很低的锰含量和较高的包裹体均一化温度,白云化流体的δ^(18)O值显著高于早三叠世海水,表明这类白云岩形成于相对封闭的埋藏成岩环境,白云化流体具较高的温度和盐度,但到目前为止,还没有一个已有模式可以较好地解释这类白云岩的成因。

There are abundant hydrocarbon resources in the northeast of Sichuan Basin in China, and the reservoir and producer are mainly dolomites of Feixianguan Formation in Triassic. This area and the Feixianguan dolomite are receiving increased attention in recent years. These Feixianguan dolomites of NE Sichuan can be divided into the following three end-member types according to their textures: micrite dolomite, grainy dolomite with original texture and crystalline dolomite, which occurred in the upper most, upper and middle-lower part, respectively in a shallowing-upward succession. The micrite dolomite and the grainy dolomite with original texture exhibit high Mn content and low fluid-inclusion homogenization temperature (Th) , and the δ^(18)O of dolomitization fluid (obtained from the 5 0 and fluid-inclusion homogenization temperature of the dolomite) is between that of meteoric water and seawater, which suggest that the two kinds of dolomite are originate from relatively open sedimentary-diagenetic environment influenced by meteoric water to some degree. Thus, the evaporative pumping model (for micrite dolomite) and mixing-zone model (for grainy dolomite with original texture) might be used to explain their formation mechanisms. The crystalline dolomite (important reservoir and producer) shows very low Mn content, and high fluid-inclusion homogenization temperature (Th) , and the δ^(18)O of dolomitization fluid (also obtained from the δ^(18)O and fluid-inclusion homogenization temperature of this kind of dolomite) is much higher than that of sea water in early Triassic, which indicate that this kind of dolomite should be formed in a relatively closed deep buried diagenetic environment and the fluid should be of high temperature and salinity. However, up to now, we do not find a good model that could fully explain the cause formation of crystalline dolomite.