Reservoir Characteristics And Seismic Response Of The Longwangmiao Formation In The Longnüsi Area, Sichuan Basin

Longnvsi area is located in the central part of Sichuan Basin.The Cambrian Longwangmiao Formation in the study area is a carbonate platform deposit inherited from the Leshan-Longnvsi paleouplift.According to the characteristics of logging curves,the Longwangmiao Formation can be divided into Longl and Long2 sections,which corresponding to two transgressive-regressive sedimentary cycles respectively.The Longwangmiao Formation in Longnvsi area is in the southwest-northeast direction as a whole.The strata on the East and west sides of the study area are thick and thin in the middle,showing the characteristics of "two thicknesses and one thin".The sedimentary period of Longwangmiao in study area is a restricted platform sedimentary environment.Based on core,thin section and logging data,the sedimentary subfacies of restricted lagoon,intertidal and granular beach can be further identified.Granular beach subfacies can be divided into beach margin and beach core.The beach core mainly develops in high energy sedimentary palaeogeomorphology.Due to the influence of sea level changes,granular beaches overlap vertically,and restricted lagoons are generally confined between different beaches.In order to further characterize the plane distribution characteristics of sedimentary facies,the palaeogeomorphology of Longwangmiao Formation in the study area was restored.However,the traditional palaeogeomorphology restoration method is not effective in the study area due to the restriction of the quality of seismic data and less well distribution.Based on the change of seismic physical attributes caused by the lithologic combination of Longwangmiao Formation and its underlying strata Canglangpu Formation,this paper puts forward for the first time the viewpoint that the amplitude of bottom boundary valley of Longwangmiao Formation is proportional to the thickness of strata,and restores the palaeogeomorphology in sedimentary period.Compared with traditional methods,the bottom trough of Longwangmiao Formation is continuous and traceable in the study area,which greatly improves the accuracy of palaeogeomorphological restoration.The scientific nature of this theory is verified by forward simulation.The detailed description of the ancient landforms of Longwangmiao period provides a geological basis for the study of the distribution of granular beaches and sedimentary facies in the area without well control.The thickness of granular beaches in the study area is positively correlated with the thickness of strata,mainly developed in Moxi 16 well area where on the east side and Gaoshi 16 well area where on the West side.The Longwangmiao Formation reservoirs in Longnvsi area are mainly developed in the middle and upper parts of Longl and Long2 members.The reservoir rock types include sandstone dolomite,oolitic dolomite,bioclastic sandstone dolomite,residual sandstone powder fine-grained dolomite and mud-powder dolomite with karst caves and model holes.Among them,the residual sand powder fine-grained dolomite,oolitic dolomite and mud-powder dolomite with karst caves,gypsum moulding holes and moulding holes have better reservoir performance.The reservoir space type includes pore,karst cave and fracture,among which pore can be further divided into primary pore and secondary pore,karst cave is mainly secondary cave,fracture is structural fracture.Reservoirs generally show the characteristics of low porosity and low permeability.In some high porosity and high permeability sections,reservoirs show a certain degree of heterogeneity.Combining with the reservoir development model in the study area,it is considered that the seismic response characteristics of reservoirs are affected by the resolution of seismic data,formation combination model,reservoir thickness and reservoir development location.Three types of reservoir development seismic response models are divided by forward simulation.Through waveform clustering analysis of Longwangmiao Formation in the study area,the favorable zones for reservoir development are predicted.The results are in good agreement with the sedimentary facies zones.It is considered that the most favorable zones for Longwangmiao Formation reservoirs in Longnusi area are the granular beach subfacies developed on both sides of the East and west of the study area,which have "bright spot"reflection on seismic reflection profiles.