Abstract
Exploring the tightness mechanism through a quantitative analysis of the pore evolution process is the research hotspot of tight oil reservoirs. The physical characteristics of Chang 6 (Ch-6) sandstones in the western Jiyuan area have the typical features of a tight oil reservoir. Based on the reservoir physical property, lithological characteristics, diagenetic types and sequence, and burial and thermal evolution history, this study analyzes the factors leading to reservoir tightness and establishes the model of the pore evolution process. The results show that the sedimentary microfacies type controls the reservoir detrital material and further affects its physical properties. The high content of feldspar and rock fragments and the fine grain size are the material cause for the reservoir tightness. The sandstones of the main underwater distributary channel are the dominant sedimentary bodies for the development of a high-quality reservoir. In terms of diagenesis, compaction is the primary cause for reservoir tightness, and the porosity reduction by cementation is weaker than that by compaction. Meanwhile, the quantitative calculation results indicate that the porosity losses by compaction, carbonate cementation, kaolinite cementation, chlorite coatings, and siliceous cementation are 23.5, 3.1, 3.8, 3.0, and 0.8%, respectively. In addition, dissolution is significant to improve the reservoir physical property, and the increase of dissolved porosity is around 3.2%. More significantly, this study uses a detailed and systematic method for analyzing the tightness mechanism and the pore evolution process of the Ch-6 sandstones in the western Jiyuan area, Ordos Basin, China.