Keywords: 
• clay minerals /
• K factors /
• XRD analysis /
• shale in Chang 7 oil-bearing formation /
• Ordos Basin

Abstract: The shale in Chang 7 oil-bearing formation of the upper Triassic Yanchang Formation, Ordos Basin, have always been considered as high quality hydrocarbon-source rock in past, but recent exploration shows this oil-bearing formation is a possible shale reservoir and rich in oil and gas resources. However, the research of its characteristics of lithology and reservoir is still limited, and the traditional research methods of petrological characteristics of sandstone are disappointment for the shale which are composed of fine grains. Consequently, it is extraordinarily important to confirm the lithology and reservoir characteristics of Chang 7 oil-bearing formation by quantitatively calculate the content of clay minerals in the rocks of it. In the quantitative analysis of XRD, the diffraction peak intensity of each mineral is related to the content of the mineral, but it is not a simple proportional relationship, so we need rectify the result by the K factor. Because the special structure and the different crystallization degree of clay minerals are the main factors which influence the K factor, instead of using the results of the former researches, we need calculate the K factors of the clay minerals in Chang7 oil-bearing formation. Therefore, based on the internal standard method of XRD analysis, we carried out the clay mineral separation step to deal with the samples for increasing the proportion of clay minerals, and used quartz as the internal standard material. Depending on the change of quartz content, the diffraction intensity of minerals in two XRD analysis of with and without the internal standard material, we can calculate the K factors. Then, using the internal standard method, the content of each mineral in the rock can be calculated by the K factors. In order to verify the K factor, we choose monomineralic balancing method based on XRF and thermogravimetric analysis to get quantitative and semi-quantitative result, respectively. The analysis result has good consistency with mineral content that calculated based on the K value. The consistency shows the K factor is reliable. In practice, the mineral content calculated by K factor analysis method is useful in judging lithology and comparing mineral content and variation relationship of different kind rocks.