Forming condition and mechanism of highly effective deep tight sandstone gas reservoir in Kuqa foreland basin

Considering the high cost of deep oil and gas exploration, the pre-drill prediction on gas-bearing properties is particularly important for deep tight sandstone gas reservoir. Deep tight sandstone gas fields in Kuqa foreland basin have high abundance, high gas saturation, high pressure as well as high and stable yield, indicating the highly effective tight gas reservoir. Based on the theoretical analysis of influence factors and mechanisms of gas-bearing properties for tight sandstone gas reservoir, taking the tight sandstone gas fields with high effectiveness such as Dabei, Keshen and Dibei gas fields in Kuqa foreland basin as examples, this study conducts a comparative analysis on typical tight sandstone gas reservoir in Sichuan Basin and Ordos Basin, and explores the forming condition and mechanism of highly effective tight sandstone gas reservoir in Kuqa area. The results show that its forming condition can be concluded as four aspects, including overpressure charging, fracture development, "early-oil and late-gas" accumulation process and favorable preservation condition. Among them, overpressure charging and fracture improvement are the most important factors for the formation of highly effective tight gas reservoirs in Kuqa foreland basin. High-quality source rock conditions and the high source-reservoir pressure difference and overpressure charging induced thereby are preconditions for the formation of tight sandstone with high gas saturation. Fracture development controls the accumulation, enrichment and high yield of tight sandstone gas reservoir. The reservoir wettability changed by early oil charging is beneficial to late natural gas charging, and the preservation condition of high-quality gypsum cap rock is the key for gas reservoir to maintain overpressure and high gas saturation. The matching of above four favorable factors leads to the tight sandstone gas reservoir with high abundance, high gas saturation and high gas production in Kuqa foreland basin, which is very different from other basins. Under the condition of a little difference in physical property of tight sandstone reservoir, excessive source-reservoir pressure difference, facture development, preservation conditions and formation overpressure are the most significant factors to be considered in deep tight sandstone gas exploration and evaluation. © 2018, Editorial Office of ACTA PETROLEI SINICA. All right reserved.