Origin Mechanism of Tightness from the He 8 Section Sandstone Reservoir in Gaoqiao Area of Ordos Basin, China

Key words: 
• porosity evolution /
• origin mechanism of tightness /
• sandstone reservoirs /
• He 8 section /
• Gaoqiao area /
• Ordos Basin

Abstract: Through conventional section, cast section, SEM samples and constant-rate mercury injection technology, characteristics of He 8 reservoir in Gaoqiao area of Ordos Basin were studied, and its origin mechanism of tightness was analysed based on pore evolution. The following conclusions were obtained. The rock types of He 8 reservoir were mainly lithic quartz sandstone, quartz sandstone and lithic sandstone, the compositional maturity and the textural maturity of sandstone were high, but its reservoir property was low. The selectivity of sandstone was good, the initial porosity overall was higher, about 34.7% on average. On the one hand, due to the effects of rock components, buried depth, acidic fluids of coal measure hydrocarbon source rock, and the compaction was strong, 20.4% primary porosity was destructed. On the other hand, cementation destroyed 13.9% of the remaining primary porosity, leading to the complete loss of most primary pore. The compaction was destructive for the pore formed by the early dissolution, at the same time, the later dissolution was weak, only increased by 3.4% secondary porosity, it was hard to improve the reservoir property remarkably. So, in the diagenesis stage, the complete loss of most primary pore and the less formation of secondary pore common lead to the tight reservoir.