DIAGENESIS OF TIGHT SANDSTONE AND ITS INFLUENCE ON RESERVOIR QUALITY: A CASE STUDY OF THE TRIASSIC CHANG 8 MEMBER IN THE XIASIWAN AREA, ORDOS BASIN, CHINA

The diagenesis of tight sandstone reservoirs controls the evolution of reservoir petrophysical properties and the distribution of high-quality reservoirs. The average porosity of the Triassic Chang 8 Member sandstone reservoir in the Xiasiwan area of the Ordos Basin in China is 10.15%, and the average permeability is 0.73 x10(-3)mu m(2), which is a typical tight sandstone reservoir. Through a large number of thin sections and scanning electron microscopy (SEM) analysis, the main diagenesis of the study Chang 8 Member tight sandstone reservoir includes compaction, cementation and dissolution. By comparing the degree of compaction and cementation, it is shown that compaction is the main cause of reservoir densification, followed by cementation. When the content of calcite cement is less than 10%, it is negatively correlated with the petrophysical properties of the reservoir; however, when the content of calcite cement is more than 10%, it is positively correlated with the petrophysical properties of the reservoir due to the influence of dissolution. Chlorite cement has an inhibitory effect on the compaction of tight sandstone. When the content of chlorite cement is less than 3%, it is positively correlated with the petrophysical properties of the reservoir; however, when the content of chlorite cement is more than 3%, it is negatively correlated with the petrophysical properties of the reservoir. Compaction and cementation are the main factors controlling the quality of the Chang 8 Member tight sandstone reservoir.