Relation of Heterogeneity and Gas-Bearing Capacity of Tight Sandstone: A Case Study of the Upper Paleozoic Tight Gas Sandstone Reservoir in the Southeast of the Ordos Basin.

Abstract

Compared to conventional reservoirs, only a few studies were carried out on the heterogeneity of unconventional tight sandstone reservoirs. This paper focuses on the Upper Paleozoic tight gas sandstone reservoir in the southeast of the Ordos Basin. The reservoir heterogeneity is studied through thin section and scanning electron microscope observations, cathode luminescence, mercury intrusion, and logging data analysis. The results show that the dissolution pore and microfracture is the dominant pathway for the migration of natural gas. The distribution of gas and water within the sand body is affected by the rhythmic change of sandstone, and this rhythmicity is variable with the changing of particle size. It shows “water wrapping gas” for the positive rhythm, “gas wrapping water” for the reverse rhythm, and both of these features for the compound rhythm. Interlayers act as a cap rock or carrier bed on gas distribution. Along with the variation of breakthrough pressure of the interlayer and saturation pressure of the reservoir, the single sand body shows different distribution features of gas and water. The vertical differentiation of natural gas is caused by the barrier layer, and the more barrier layers exist, the worse the capacity of the reservoir to store natural gas. However, the existence of the barrier layer will make the reservoir close to the source area to be the favorable zone for oil and gas accumulation. In this study, the relationship between heterogeneity and gas as well as water distribution of tight sandstone is identified, which can provide guidance to the exploration and exploitation of tight gas in the future.