The Exquisite Comparison of Shale Mineralogical-Geochemical Characteristics between Chang 7 Member and Chang 9 Member in Yanchang Formation, Ordos Basin

Abstract

Significant amounts of unconventional oil and gas resources have been discovered in the Yanchang Formation of Ordos Basin. Shale layers deposited in Chang 7 member (divided into Chang 7-2 submember (C7-2SM) and Chang 7-3 submember (C7-3SM) and Chang 9 member (C9M)) are the main source rocks. Based on the comparison of mineralogical and geochemical characteristics, it is concluded that (1) in terms of mineralogical characteristics, the C7-3SM shale possesses the largest content of illite/smectite mixed layer and reducing minerals and the least quantity of quartz. The C9M shale shows the highest percentage of quartz and illite and the least amount of K-feldspar and Kaolinite. In C7-2SM and C9M shale, amorphous silica surrounded tightly by clay minerals is easily observed by the scanning electron microscopy. Besides the drilling orientation, the small content of quartz contributed to the lowest porosity for the C7-3SM shale. (2) In terms of geochemical characteristics, the C7-3SM shale exhibits high productivity due to type II1 kerogen. The organic matter in the C7-2SM and C9M shale contains mainly type II2 and possibly type III kerogen. The C9M shale exhibits the highest organic thermal maturity. The C7-3SM shale was formed in a relatively higher salinity of sedimentary water.