Overpressure-generating mechanism and its evolution characteristics of Cretaceous Shahezi Formation in Xujiaweizi Fault Depression,Songliao Basin
Using the data of drilling,logging,pressure testing and analysis test,the current overpressure characteristics and the formation mechanism of overpressure of Cretaceous Shahezi Formation in Xujiaweizi fault depression of Songliao Basin were analyzed The overpressure evolution of source rocks with different structural units and different lithology is quantitatively restored by numerical simulation method,and the contribution of each overpressure to total overpressure is calculated.The results show that:(1)The present pressure of Cretaceous Shahezi Formation in Xujiaweizi fault depression are normal pressure-weak overpressure system,and the overpressure in the uplift is largest in the sag.Overpressure in the source rock is mainly driven by hydrocarbon generation and its second origin is undercompaction,while overpressure in the reservoir is the overpressure transfer effect from source rock overpressure,which is closely related to the formation and development of faults and anticlines.(2)The overpressure of source rocks in the Shahezi Formation in the study area is mainly caused by hydrocarbon generation of coal seam and organic-rich mudstone.Generally,the contribution ratio of hydrocarbon generation to total overpressure is the largest in coal seam,followed by organic-rich mudstone,and organic-poor mudstone is lower,while the contribution ratio of hydrocarbon generation to total overpressure in the organic-rich mudstone is higher than that of coal seam in the sag and its periphery.The contribution ratio of hydrocarbon generation to total overpressure in the sag is higher than that in the slope zone and the uplift.(3)The evolution of overpressure in Shahezi Formation in the study area can be divided into three stages.The early to middle Cretaceous period is a slow pressurization stage.Overpressure was generated by undercompaction and slow hydrocarbon generation of source rocks,and was transmitted to the reservoir through active faults and anticline formation in Shahezi and Yingcheng formations,and overpressure in the reservoir increased slowly.The late Cretaceous period was a stage of rapid pressurization,with a large amount of gas generation in source rocks.In this stage,continuous transfer of overpressure caused by hydrocarbon generation and undercompaction make the overpressure in the reservoir be rapidly increased.The overpressure in the reservoir in this stage accounted for 90％of the total overpressure at present.The early Paleogene to the present is a stable stage.During this period,the tectonic is stable,overpressure induced by the hydrocarbon generatation and undercompaction in the source rock basically unchanged.Otherwise,the overpressure in the reservoir increases steadily.(4)The differential evolution of overpressure in different lithologic strata of Shahezi Formation in the study area controls the distribution of natural gas,overpressure difference between source and reservoir provides dynamic conditions for oil and gas migration,and the overpressure developed by organic-poor mudstone cap layer provides overpressure sealing conditions for the underlying reservoir.
万方数据
维普
