Diagenetic evolution and densification mechanism of the Upper Paleozoic tight sandstones in the Ordos Basin, Northern China

Abstract

Abstract The Upper Paleozoic sedimentary succession in the Ordos Basin, Norther China, contains typical tight gas sandstones. Densification process and diagenetic evolution of these sandstones are poorly known but are important for exploration of reservoirs. Herein, petrographic features of these rocks are described by using scanning electron microscopy (SEM) and cathodeluminescence. In addition, inclusions in quartz overgrowth and calcite cementation are examined with homogenization temperature measurements, and Raman spectroscopy. The sandstones are mainly quartz sandstone, lithic quartz sandstone and lithic sandstone, with a medium to coarse grain size and good sorting. The reservoir properties are quite poor, with low porosity and permeability. Densification of the Upper Paleozoic sandstones are attributed to long-term mechanical compaction and multi-stages of siliceous and calcite cementations. Moreover, the quartz dissolution due to the presence of alkaline fluid forms intragranular and intergranular pores, thus providing the main space for the sandstone reservoirs. During the diagenetic evolution, tight sandstone reservoirs have experienced various stages of rapid burial and strong compaction, alternate acidic and alkaline actions, formation of reservoirs, uplifting and rework, with great differences between these stages in mineral assemblage and reservoir physical property. Massive hydrocarbons were generated following Stage Ⅱ of quartz overgrowth in which the number of quartz dissolved pores substantially increased because of more intensive alkaline dissolution. The hydrocarbons preferably flow into the reservoir strata with better properties, thus forming tight sandstone gas reservoirs.