塔里木盆地永安坝剖面蓬莱坝组白云岩成因与形成过程--来自有序度和晶胞参数的证据

Origin and formation mechanism of dolomite in Penglaiba Formation of Yonganba outcrop,Tarim Basin:Evidence from ordering degree and unit cell parameters

塔里木盆地巴楚地区永安坝剖面蓬莱坝组发育粉晶白云岩、细中晶白云岩和粗晶白云岩3类白云岩。结合研究区沉积背景、白云岩特征和XRD测试结果认为3种白云岩代表不同的形成环境和过程:粉晶白云岩具中等有序度,平均有序度为0.75,晶胞参数偏离理想白云石,为灰泥经准同生快速云化,在浅埋藏期发生重结晶而成;细中晶白云岩具中等有序度,平均有序度为0.79,晶胞参数接近理想白云石,原岩为孔隙型颗粒灰岩,在准同生期发生白云石化作用,后经埋藏期白云石化作用调整具有埋藏白云岩的特征;粗晶白云岩具高有序度,晶胞参数接近理想白云石,原岩为颗粒灰岩,在中埋藏期受持续、充足的云化流体供给,结晶速度较慢,云化时间较长。受云化发生时期差异的控制,细中晶白云岩最有利于孔隙保存,构成有效储层。

There are three types of dolomites in Penglaiba Formation of Yonganba outcrop in the Bachu area of the Tarim Basin.Based on the analysis of the sedimentary context of this area,and the characteristics of dolo⁃mite and XRD results,it could be suggested that three types of dolomites represent different sedimentary envi⁃ronments and process.The average ordering degree of powder crystal dolomite is 0.75 and their unit cell parame⁃ters deviate from that of ideal dolomite,which means they are the products of lime-muds by both rapid dolomiti⁃zation in penecontemporaneous stage and recrystallization in shallow burial stage.The average ordering degree of the fine-middle crystal dolomite is 0.79 and their unit cell parameters are close to ideal dolomite,indicating the effects from porous grain limestones with both dolomitization in penecontemporaneous stage and adjustment to burial dolomites during the burial stage.The average ordering degree of coarse crystal dolomite is 0.90 whose unit cell parameters are close to the ideal dolomite,revealing their original rocks are granular limestones influ⁃enced by the persistent and sufficient dolomization fluids,with the slower crystallization speed and longer dolo⁃mization period.Preservation of pores and formation of effective reservoirs in the fine-middle crystal dolomites were more common than in other two types,because of the disequilibrium dolomitization.