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Reservoir Forming Mechanism and Main Controlling Factors of Different Types of Large Tight Sandstone Gas Fields in China


Tight sandstone gas has become hotspot of natural gas industry in China. From latest statistic of resource assessment, total reserve of tight sandstone gas is 300 trillion cubic meters, about 88 – 121 trillion of which could be recoverable. By 2012, China has found sixteen large tight sandstone gas fields such as Sulige, Daniudi and Anyue gas fields. Proved reserve of these gas fields accounts for 49.5% of total proved natural gas resource in China. These gas fields could be divided into three types: large-area tight sandstone gas in gently Craton basin, large-size structural-lithological tight sandstone gas in foreland basin and structural-lithological tight sandstone (conglomerate) gas in deep rift basin. Tight sandstone gas in upper Paleozoic of Ordos basin and Xu Jiahe formation of lower Jurassic of Sichuan basin, Jurassic tight sandstone gas in Kuqa depression and deep buried tight sandy conglomerate in Songliao basin are typical representatives of the three types respectively. Based on components and isotope of natural gas and maturity of source rock, we suggest that, for the first type, gas mainly filled and accumulated in near-source reservoir rocks driven by overpressure resulting from hydrocarbon generation. Physical simulation shows that gas migrates in the way of low-velocity non-Darcy seepage and diffusion in tight sandstone. The gradient of overpressure is higher, the range of the trap would be larger, and hydrocarbon saturation would be higher. Fluid inclusions show a wide scope of homogenization temperature with a single peak, which reflects one-period accumulation. Calculated proportion of gas migrated and accumulated accounted for 5.2% of total gas generated. Accumulation and enrichment of these reservoirs are mainly controlled by four factors: structure controls migration direction and accumulation degree, reservoir rock controls scale of the gas pool, effective source rock controls the saturation of gas pool and fractures control enrichment and high yield. For the second type, development of micro-fracture is the key to improve storage space of tight reservoir rock and micro-fracture generates from active tectonic movement. So, tectonic movement controls accumulation and enrichment of gas pool. Besides, preservation condition is equally important for this kind of gas reservoirs. As to the last type, natural gas accumulates in the near-source reservoir. Quality of reservoir rock controls accumulation and enrichment of natural gas.