Deep Reservoir Development And Preservation Of Changxing Formation In Yuanba Gas Field,NE Sichuan Basin,china

Reef-shoal facies carbonate rock is the main reservoir of the Permian Changxing Formation in Yuanba gas field,Sichuan Basin,south of China.The reservoirs were buried to about 6600 m with average porosity of 8%,maximum up to 23.96%,which is different to the commom sense of carbonate reservoir development.So the deveopment and preservation mechanism of deep high quality carbonate reservoirs is very important in the demand of theory and practice as the deep carbonate reservoirs exploration has become a new hot spot now.The Changxing Formation,a high quality reservoir in Yuanba gas field,is the main research object in this paper.Based on the core observation,description and sample collection,according to the theories of petrology,sequence stratigraphy and geochemistry,the development and preservation mechanism of deep high quality carbonate reservoirs is studied.Dolostone is the main lithology of reservoir in the Changxing Formation.Intercrystalline pores,biological cavity pores and caves make up the principle part of the reservoir space.The reservoir with good porosity mainly develops in the upper part of the second third-order sequence of Changxing Farmation.Reef cap,inter-reef shoal and back reef shoal are the three main favorable microfacies.Statistics demonstrate that reservoir properties have minor relationship with the depth.The succession of Changxing Formation carbonate platform is evolved from the ramp type to the rimmed shelf.Reef mainly develops in the platform margin near the slope,and the biodetritus bank widely distributes in the gentle slope.Facies controlles the distribution of primary pores.High-energy sedimentary facies rocks usually exhibit the character of high primary porosity,including intergranular pores and framework pores.In addition,controlled by the relative sea level fluctuation,reef,reef cap and back-reef bank are more easily to suffer from the meteoric water dissolution in the HST stage.The diagenetic environment is divided according to the fluid environment.In the early diagenetic stage,moldic pores and intergranular dissolution-enlarged pores improved the reservoir property in an open system.Dolomitization fluid mainly came from the late Changxing stage seawater.Massive dolomitization completed under the depth of about 400 m,even more shallow.Dolomitization changed the rock structure and pore space,improving the rock compressive resistance.The combination of dolomitization,meteoric water dissolution and cementation is vital to high quality reservoirs formation.The Changxing Formation were quickly burial to about 1500 m in the early Triassic,and the reservoirs is in a semi-closed to closed system.Early hydrocarbon filling prohibited a large scale water-rock reaction.Overpressure reduced the damage to the reservoir quality caused by compaction and pressure solution.Diagenesis have a minor impact on the porosity as the reservoir getting into a closed system,which is an important condition to preserve the deep high quality reservoirs.