Formation condition of deep gas reservoirs in tight sandstones in Kuqa Foreland Basin

Abstract

Due to the high expense of deep oil and gas exploration, prediction of gas-bearing properties before drilling is crucial for deep gas reservoir of tight sandstone. Deep tight sandstone gas fields in Kuqa Foreland Basin are characterized by high abundance, high gas saturation, high pressure, high and stable yield, which belong to high-efficiency tight gas reservoir. Based on theoretical analysis of controlling factors and mechanisms of gas-bearing properties for tight sandstone gas reservoir, and taking tight sandstone gas fields with high effectiveness such as Dibei, Keshen and Dibei gas fields in Kuqa Foreland Basin as examples, formation condition and mechanism of high-efficiency tight sandstone gas reservoir in Kuqa area are studied through a comparative analysis of typical tight sandstone gas reservoir in Sichuan Basin and Ordos Basin. The results show that the formation condition of deep gas reservoir of tight sandstone in Kuqa foreland basin includes four factors: i.e., overpressure gas charging, fracture development, “early-oil and late-gas” accumulation process and favorable preservation condition. The overpressure gas charging and fracture development are the most important factors for formation of high-efficiency tight gas reservoirs in Kuqa Foreland Basin. High-quality source rocks, high source-reservoir pressure difference, and overpressure filling induced thereby are preconditions for formation of tight sandstone with high gas saturation. The fracture development controls gas migration, accumulation, and high yield of tight sandstone gas reservoir. The reservoir wettability changed by the early oil charging is beneficial to late natural gas charging, and the preservation condition of high-quality gypsum cap rocks is the key factor for gas reservoirs to maintain overpressure and high gas saturation. 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 little difference in physical property of tight sandstone reservoir, excessive source-reservoir pressure difference, facture development, preservation condition and current formation overpressure are the most significant factors to be considered in exploration and evaluation of deep tight sandstone gas.