Study On Pore Pressure Of The Sinian-Paleozoic In The Central And Western Sichuan Basin

The deep marine carbonate reservoirs have become the focus area for natural gas exploration,which has made a major breakthrough in the Sinian-Cambrian of Central Paleo-Uplift and the mid-Permian of northwestern fault-fold belt in Sichuan Basin.The temperature and pressure field is the key point for studying on the mechanism of hydrocarbon accumulation in the petroliferous basin,especially the reconstruction of paleo-pressure plays an important role in the study of hydrocarbon migration and accumulation,and even diagenesis in reservoirs and hydrocarbon generation for source rocks.However,the study on pore pressure is often lack of effective methods for the ancient marine stratum with complex evolution.In this paper,based on the quantitative analysis of overpressure controlling factors by using pressurization model for continuous hydrocarbon generation and tectonic compression model,with pressure pressure distribution as constraint,the paleo-pressure were reconstructed in the Sinian-Cambrian and the middle-Permian of Central and Northwestern Sichuan Basin respectively,by adopting a comprehensive method including inclusions pressure-volume-temperature simulation,inclusions in situ Raman shifts,and basin modeling.Through comparative analysis,the differences of pressure evolution and causes for overpressure between the central paleo-uplift and northwestern fault-fold belt of Sichuan Basin have been furtherd discussed.A quantitative model for evaluating overpressure caused by continuous hydrocarbon generation in Qiongzhusi Formaiton has been established.The results showed that,in the central paleo-uplift,the source rock of Qiongzhusi Formation was in the stage of oil generation pressurization from the early Jurassic to the middle Jurassic and in the stage of oil cracking pressurization from the late Jurassic to the end of early Cretaceous.The overpressure developed in source rocks was transferred to adjacent reservoirs during the oil generation phase,while in the oil cracking stage,the cumulative overpressure was even greater than the fracture pressure which made the episodic hydrocarbon expulsion.And the pressure evolution in the Longwangmiao Formation and the Dengying Formation have experienced normal pressure,weak overpressure,strong overpressure and pressure relief successively.Especially since late Cretaceous,the excess pressure decreased,the Dengying Formation has been restored to normal pressure during the Neogene,while Longwangmiao Formation still remains moderately overpressurized.By the analysis of the structural background and preservation conditions,the primary reasons for the different pressure states in Z₂dn and?₁l during the last stages are the types of sedimentary facies,the preservation condition of the traps,and the differential weathering process.In northwest fault-fold belt of Sichuan Basin,the middle Permian developed two stages of overpressure,especially from the late Jurassic to the end of early Cretaceous,the overpressure accumulated rapidly in reservoirs under the effect of compressive stress which varied greatly on different structural units.The area near the orogenic belt developed weak overpressure,which recovered to normal pressure after late Cretaceous;but the strong overpressure developed in the fold and foredeep zone far from the orogenic belt even after the structure uplift.By comparing the differences of overpressure mechanism between the central and northwest Sichuan Basin,it suggested that the overpressure developed in the Sinian-Cambrian of Central Sichuan Basin was mainly controlled by inner factors,such as fluid phase and temperature.Although the overpressure of middle Permian in the northwestern Sichuan fault-fold belt was also affected by hydrocarbon generation,cooling and other internal factors,it was mainly controlled by the external compressive stress from northwest.Especially since the late Yanshanian tectonic uplift,the temperature of Permian decreased sharply which gave rise to rapid reduction of overpressure,however,affected by lateral compressive stress,the excess pressure could be preserved in the northwest fold belt.It was the basement property that made the pressure mechanism different from the central to the northwest Sichuan Basin,which affected the conditions of hydrocarbon accumulation furtherly.Therefore,the results of pore pressure reconstruction and causes analysis for overpressure could not only improve the overpressure mechanism and provide parameters for reservoir diagenetic simulation experiment in deep marine carbonate rocks,but also could provide a basis for hydrocarbon accumulation analysis combined with the temperature or other factors,which could support the further discussion about the favorable conditions for the formation of large-scale gas field in the deep marine carbonate rocks.