四川盆地中南部筇竹寺组页岩储层压力形成机制及其地质意义OA

Overpressure is widely observed in the shale of the Lower Cambrian Qiongzhusi Formation in the central-southern Sichuan Basin,and significant pressure differences exist among different types of shale reservoirs.However,the formation mechanisms and controlling factors of shale overpressure remain poorly understood.Based on well-log data of the Qiongzhusi shale reservoirs in the central-southern Sichuan Basin,this paper systematically investigates the genesis,evolutionary process and controlling factors of overpressure across different shale lithologies by means of multiple overpressure identification methods including combined logging method,porosity comparison method,and acoustic velocity-density crossplot method.The following results are obtained.First,two types of overpressure systems are identified in the study area.Specifically,the overpressure in the organic-rich shale is mainly caused by hydrocarbon generation,while that in the silty shale is jointly controlled by fluid charging and differential compaction.Second,the dominant timing of overpressure development varies with overpressure types.Hydrocarbon generation overpressure mainly occurs during the oil window and crude oil cracking stages,whereas fluid charging overpressure is closely related to fault activity during critical tectonic episodes and to pore preservation.Differences in pressurization timing and main controlling factors are the principal reasons for pressure differentiation among different types of shale reservoirs.Third,the evolution of overpressure is directly responsive to the natural gas charging process.The'pressure-resistant pore preservation'effect of overpressure reservoirs results in better preservation conditions and higher gas content.In conclusion,two overpressure development models are established,and the pressure coefficient is determined to be a key indicator for selecting sweet spots of Qiongzhusi shale gas.The research findings provide guidance for the sweet spot prediction and well placement in the shale gas development in the Sichuan Basin.