QUARTZ CEMENT ORIGINS AND ITS IMPACTS ON STORAGE PERFORMANCE IN TIGHT SANDSTONES RESERVOIR

Authigenic quartz is an important cementing material which can degrade the capacity of storage whereas the effect of quartz cement on pores and throats on the microscale in tight sandstones is controversial, therefore, it is crucial to verify the sources of quartz cement as well as the controls on microscopic storage performance. Toward this end, a variety of tests were performed on the sandstones of the Permian Upper Shihezi formation, which are the vital exploration and development interval for gas in the northern Ordos basin. We found that the Upper Shihezi formation tight sandstones are mostly litharenites and sublitharenites and share common traits as abundant volcanic rock fragments. Quartz cement is the most abundant interstitial minerals in Upper Shihezi formation tight sandstones which lack of aluminium ion compared to detrital quartz. Homogenization temperatures results show that the quartz cement form around 55℃ to 188℃ with a continuous process. Pore size, throat size and pore throat radius ratio distribution curves measured by RCP tests had significant differences, and the samples can be divided to pore dominant and throat dominant types. In NMR-derived pore-throat size distribution, there is an increase in micropores and a decrease in macropores as the quartz cement content decreases. Chemical compaction and transformation of clay minerals were the main sources of silica, while replacement of silicate mineral by carbonate and feldspar dissolution were only minor amount. The poor relationships between amounts of quartz cement and single or conventional parameters attested that it is essential to combine the pore and throat threshold radius to evaluate the content of quartz cement. This work shows that although quartz cement would occupy the void space and lead to the loss of porosity, it could retard the compaction and preserve the pores which attributed to the limited compressibility of quartz cement supported rocks, thus, diminished the difference of pore radius and throat radius and result in the decrease of pore-throat size heterogeneity in tight sandstones.