Mechanism of tight sandstone reservoirs with shallow burial in southwest of Ordos Basin

The densification of fine sandstone occurred under relatively shallow burial conditions in the 8th member of the TrassicYanchang Formation, deposited in the Zhenyuan area of the southwest Ordos Basin. The pore-throat structure and diagenesis of Chang 8 member tight reservoir were analyzed through core analysis, casting thin sections, scanning electron microscope, fluorescence thin section with picture analysis, and other methods. The results indicate that both compositional and structural maturity are relatively low,with the dominant pore-throat structure being fine pores with minimal throats. Diagenesis is characterized by strong compaction, multi-period cementation, and dissolution accompanied by variations in the diagenetic environment. The quantitative characterization of densification process was carried out under the comprehensive control of " burial history-oil and gas filling history-diagenetic evolution sequence-pore-throat structure" . It reveals that Chang 8 member contains a large amount of fine grains and plastic fragments with a high matrix content, leading to significant reservoir space loss under rapid burial after deposition. This was influenced by a stable tectonic setting combined with a braided river delta sedimentary system, resulting in strong compaction contributing to 85% of the densification. Additionally, pore fluids saturated with various salts led to intense cementation, affected seriously by multiple changes in the diagenetic environment, contributing 40. 8% to the densification. The early-formed chlorite film protected clastic particles from being dissolved. Cement layers limited the exchange of pore fluid with surrounding rock under the stable tectonic background, which were formed at the roof and floor of Chang 8 member by strong siliceous cementation and carbonate cementation. Early entry into a low-permeability stage, coupled with distance from the hydrocarbon-generating center of the basin, hindered continuous acid fluid injection, reducing the dissolution intensity. Consequently the inhibiting effect of dissolution on densification was only 26. 1%. Besides, the secondary pores generated by dissolution were easily filled by later-period cements. In summary, the tight sandstone reservoirs in the Chang 8 member were buried at shallow depths, controlled by multiple factors, including tectonic setting, sedimentary environment, diagenesis, and their coupled relationship with periods of hydrocarbon generation and expulsion. © 2024 University of Petroleum, China. All rights reserved.