白云石有序度对湖相混积碳酸盐岩储集层物性的控制作用:以柴达木盆地英西地区渐新统为例

对英西地区渐新统湖相混积碳酸盐岩储集层的物性特征及主控因素进行了分析。系统地开展了岩石矿物学特征、物性分析、孔隙结构、白云石有序度测试、无序白云石成因机理及其对储层物性的影响研究,提出英西地区渐新统储集层主要为"低孔-特低渗"型储层,其孔隙发育程度主要受控于岩石中的白云石含量,其中白云石有序度对岩石物性具有重要的控制作用。研究结果表明:(1)测井孔隙度与岩石中的白云石含量呈明显正相关关系,而与泥质含量呈负相关关系;(2)理想模式下,当岩石中白云石含量为100%时,有序白云石的分子结构决定了其孔隙度应为13%,研究区氦气法实测孔隙度与白云石含量相关关系投点图中的数据点大多分布在该理想模式线附近;(3)理想模式线上方的样品,其晶间孔多被扩大改造,有的被碎屑颗粒支撑扩大,有的被微弱溶蚀扩大,理想模式线下方的样品,其晶间孔多被黄铁矿或其他物质半充填而变小;(4)少数岩石样品的白云石含量较高但其孔隙度值较小,主要为有序度较低的白云石或者无序白云石,在白云石含量相同的条件下,有序度越高,其孔隙越发育。

Controls of the Degree of Order of Dolomite on the Physical Properties of Lacustrine-facies Diamictitic Carbonate Reservoirs:A Case Study of the Yingxi Area in the Qaidam Basin

To study the physical properties and main controlling factors of the Oligocene lacustrine-facies diamictitic carbonate reservoirs in Yingxi area, we have conducted systemic analyses on the petrology and mineralogy, physical properties, pore structure, degree of order of dolomite, formation mechanism of the disordered dolomite and its influence on the reservoir physical properties. It is suggested that the Oligocene reservoirs in the Yingxi area is mainly of low porosity and ultra-low permeability, and its pore development is mainly controlled by the dolomite content in the rocks. The degree of order of dolomite likely plays an important role in controlling the physical properties of the rocks. Our results also show that: (1) the well-log porosity is positively correlated with the dolomite content of the rocks, but negatively correlated with the clay content; (2) when the dolomite content is 100%, the molecular structure of the ordered dolomite suggests that its porosity should be 13% in the ideal model. In the correlation diagram of the measured porosity (via the helium gas method) versus the dolomite content, most of the data points are distributed near the ideal pattern line; (3) the intercrystal pores above the ideal pattern line are expanded, some are enlarged by the support of clastic particles, and some by weak dissolution; whereas the intercrystal pores below the ideal pattern line are mostly filled with pyrite or other material; (4) a few rock samples have relatively high dolomite content but smaller porosity. They are mainly composed of dolomite with low-/zero-degree of order. With the same dolomite content, the higher the degree of order is, the better developed are the pores.