天然气成藏过程有效性的主控因素与评价方法

THE CONTROLS AND THE ASSESSMENT METHOD FOR THE EFFECTIVENESS OF NATURAL GAS MIGRATION AND ACCUMULATION PROCESS

天然气的成藏过程是指天然气从源岩排出后的运移、聚集和散失过程,这一过程的有效性可以用气藏成藏保存阶段单位圈闭面积内的充注速率与散失速率的差值,即天然气净聚集速率定量表征。根据净聚集速率的高低,可将天然气成藏过程的有效性划分为高效、中效和低效3个级别。根据对典型气藏的解剖和大量气田的统计,天然气成藏过程的有效性主要受气藏成藏期源储剩余压力差、输导体系的类型和输导效率、盖层的厚度和排替压力3方面因素的控制。在我国含油气盆地的地质条件下,中、高效成藏过程的的主要地质条件是天然气成藏期的源储剩余压力差大于25MPa,具有汇聚型输导体系,气藏盖层厚度大于40m和排替压力大于15MPa。利用上述指标可以对盆地范围内天然气成藏过程的有效性进行定量或半定量评价和预测高效气藏的分布。

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.