THE CONTROLS AND THE ASSESSMENT METHOD FOR THE EFFECTIVENESS OF NATURAL GAS MIGRATION AND ACCUMULATION PROCESS
天然气成藏过程有效性的主控因素与评价方法

The migration and accumulation process of natural gas included the whole geological processes happened after the natural gas expelled from source rock, such as the secondary migration through permeable bed, faults and unconformity surface, the charging to and accumulation in traps and the dissipation through caprock. The effectiveness of this process could be expressed quantitatively by the net velocity of natural gas accumulation, i.e. the difference between velocities of the gas charging into trap and gas losing through coprock in the accumulation and preservation period of a gas accumulation. The natural gas migration and accumulation process could be classified into high, medium and low effectiveness according to its net velocity of natural gas accumulation. The studies of some typical gas fields with high and low net velocities and the statistic analysis of main large and medium sized gas fields in China had shown that the effectiveness of natural gas migration and accumulation process was mainly controlled by the difference of excess pressures between source and reservoir, the type and the conductibility of conduit system, and the thickness and displacement pressure of the caprock of a reservoir. In the geological environment of main gas-prone basin in China, the migration and accumulation process of natural gas with high and medium effectiveness would take place in the areas with the difference of excess pressures between source and reservoir over 25 MPa, the thickness of caprock over 40 m, the displace pressure of caprock over 15MPa, and with converging conduit system. The above parameters could be used in the prediction of high effective gas field distribution in a basin.