Characteristics Of Faults And Fractures Of The Xujiahe Formation And Their Relationships With Natural Gas Accumulation In Northeastern Sichuan Basin

The distribution of natural gas of Xuejiahe Formation is significantly controlled by the faults and fractures in northeastern Sichuan Basin.Thus it is of great significance to study the relationships between the faults and fractures and the migration and accumulation of natural gas in Xujiahe Formation.Combined with the core,logging,imaging logging,outcrop and 3D seismic data of study area,systematically studied the relationships between the faults and fractures of Xujiahe Formation and the accumulation of the natural gas in Xujiahe Formation.Structural analysis of seismic data identified two types of high-angle reverse faults in northeastern Sichuan Basin.They comprise type ? and type ? faults,the type ? faults mainly offset the upper Permian and extend to the middle Jurassic,and the type ? faults mainly offset the middle to upper Triassic and extend to the lower Jurassic.And the type ? faults only appeared in Tongnanba area,however,the type ? faults distributed over the whole study area.Additionally,the growth index and geometric features of type ? and type ? faults indicate that they are formed in the kink band of fault-propagation or fault-bend fold during the intense tectonic compression stage,and the homogenization temperature of saline aqueous inclusion of the fault zone proved that both type ? and type ? faults form or activate during Late Jurassic to Early Cretaceous and Miocene to Pliocene.Fractures of Xujiahe formation can be divided into four types based on their occurrence and location,and they are the network fractures of the mudstone and coal stone,the low-angle fractures of mudstone and silty mudstone,and the middle to high-angle fractures of the sandstone or sandy conglomerate.The network and low-angle fractures are diagenetic and overpressure-forming fractures formed in the large-scale hydrocarbon generation and expulsion stage of source rocks of the Late Jurassic to Early Cretaceous,and their distribution is mainly affected by lithology.The middle and high-angle fractures are fractures formed during folding and faulting during the Late Jurassic to Early Cretaceous and Miocene to Pliocene,and their distribution is affected by lithology,faults,stratigraphic curvature and maximum principal strain.The time that the type ? and type ? faults and the fractures of Xujiahe Formation formed or activated exactly matched the time that source rocks of Xujiahe and Longtan Formation generating and expulsing hydrocarbon,and faults and fractures significantly impacted the migration and accumulation of natural gas in Xujiahe Formation.Firstly,the fractures caused by folding and faulting of strata help improve the porosity and permeability of the sandstone of Xujiahe Formation,which provided sufficient space for natural gas to accumulate.Secondly,the middle to high angle fractures also served as the tunnel for the migration of natural gas.Thirdly,some type ? faults which offset the Longtan Formation can provide gas that was generated by marine source rocks for sandstone reservoir of Xujiahe Formation.Fourthly,both type ? and type ? faults reduced the loss of natural gas for their excellent sealing performance in Zilujing Formation.Besides,some faults that activated during Miocene to Pliocene may have destroyed some sandstone gas reservoirs of Xujiahe Formation.