Abstract: The formation mechanism of a great deal solid migrabitumens of Carboniferous gas reservoir in the east of Sichuan Basin is discussed.There are high content methane and solid bitumen in the gas reservoir.Solid bitumens mainly distribute system of dissolution pore(intergrain pore,insidegrain pore and intercrystalline pore),fracture(fissure) and dissolution cavity in carbonate reservoir.Compared with other composition in gas reservoir and source rock,such as δ¹³C_saturated,δ¹³C_aromatic,δ¹³C_NOS,δ¹³C_asphaltenes and δ¹³C_kerogen,the value of δ¹³C_bitumen is the heaviest and mainly range from -28.83‰ to -23.99‰.But δ¹³C value of methane is the lightest and about -37.7‰ to -29.2‰.The difference is result of carbon isotope thermal dynamical frationaction.With increasing depth and rising temperation,there is a tendency for crude oils in reservoirs to become specifically low lighter molecular hydrocarbon and high molecular weight constituent.The ultimate stable end products are low molecular methane and insoluble carbon-rich residue of very high molecular weight.Finally crude oils in reservoirs crack into gas and solid bitumen.The formation mechanism of solid migrabitumen is a result of high molecular with aromatic cycles condensation insoluble carbon-rich residue.